This Page

Thanks for being here. I created this page as a service to my community of knife enthusiasts, knife collectors, users, aficionados, and knifemakers. I am certain that after you read this page, you will have a greater understanding of modern, high alloy steels used in the finest knife blades, and how steels are physically processed to achieve the very best knife blades ever made in the history of man. We are lucky to be alive in a time when this is possible, and when knowledge and research are available for free—for the advancement of mankind—in an instant.

What I want you to learn from this page is what modern, high alloy, and stainless steels are, what role they play in the world of fine knives, and how heat treating and processing works in my professional field. There is a right way to heat treat knife blades, and it has taken me decades to achieve the level of understanding I have in this field. There is always more to learn, and—God-willing—I'll continue this journey until I'm finished with this world.

I also want you to know what this particular part of knifemaking is not, and what misleading and erroneous ideas are still prevalent, and how inferior and antiquated processes, ideas, and  steels are being hyped as of some value other than superficial appearance and tradition. I want my clients purchasing knives because the knives are the best they can possibly be, and that starts with the finest, most advanced metals and treatments that bring them to the pinnacle of their performance.

What kind of performance am I writing about? The performance of knives is cutting, cleanly, repeatedly, and continually. Simple enough; any piece of sharpened metal or other hard material will cut. The performance issue is then about durability, longevity, and strength. These characteristics exist not only in the design of the blade, but also in the steel alloy itself, with advanced metallurgy, scientifically treated, for the highest wear resistance, toughness, strength, and corrosion resistance. This is the working end of the knife, the cutting edge, and performance has to be built into the blade alloy and brought to its most effective physical state by processing, typically done by the knifemaker himself.

A knife is not just appearance, it is first about performance, and that starts with an extremely finely-made advanced technology blade. While the other parts of the knife are just as important, this page deals with heat treating and processing modern, high alloy tool and stainless steels, which far surpass traditional lower carbon, lower alloy blade steels by many orders of magnitude and in many distinctive characteristics.

Welcome to what is perhaps the best page about heat treating modern high alloy custom knife blade steels you will find on the internet, and thanks for taking the time to be here.

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Simple Steel?

There is no way to make this simple. Modern media—movies, videogames, and television shows—always tends to show armorers, bladesmiths, and knifemakers heating, forming, quenching, and using blades in highly visual and active procession, with a lot of sparks and fire, and glowing embers scattering around the blade. This may be fine for blades made in the 1800s, but the very best blades are never, ever treated this way. The reality of modern steel and its processing is much more technical. If you just want a brief overview of the process to make superior knife blades, here's a quick step guide:

1. Cut, form, or forge the blade using machines, heat, pressure, or tools.
2. Harden blade using timed heat and quenching with air, water, brine, salts, and/or cryogenic means.
3. Using heat, temper to specific hardness required.
4. Finish blade surfaces.

And that's pretty much it! If you are not interested in the technical nature of blade making, you're done with this page.

That's not really why you're here, is it?

Due to the complexity of the process and material, there is no way—for the sake of brevity—to sum it up with the four steps above. Steel and its crystalline forms are quite complicated, and our understanding of them determines, as knifemakers, knife users, and knife owners the choices and nature of the knife that interests us.

In order to understand the properties of the particular blade, you must first know the steel alloy. There are a vast number of blade steels available in the market today; steel has reached an extremely high level of sophistication and the science will continue to grow. Understanding the nature of this very special material will offer a greater insight into how, why, and where these steels function as they do, and why premium steels are at the forefront of modern technology in nearly every field, not just knifemaking.

This is why it's astounding to discover manufacturers and knifemakers who will not tell you what steel they are using for their knife blades! They offer some undocumented and arbitrarily-assigned name—without disclosing alloy content—when it's clear that the the steel type is absolutely critical to the performance of the alloy and its function, place, and value as a knife! Without demonstrating or revealing even the basic properties of the steel (much less identifying the alloy), the manufacturer or maker of the modern knife is negligent in his service to his customer, or he simply caters to a customer who doesn't care. That's not my client, not my customer, and not my patron.

Another important issue is one of authenticity. Makers and manufacturers are claiming superior blade performance when there is none, and that somehow, a claiming a knife blade made of modern high alloy steel is somehow inferior to the plain carbon steel blade, which is an outright, easily verifiable lie. I do not want to be part of a profession that allows lies to stand for the sake of egos, tradition, or profit, and the best way to eliminate them is with knowledge and scientific facts.

This page is for you—the knife enthusiast, the blade aficionado, and the client, collector, patron, or user who wishes to know why steel is what it is, and how an individual knifemaker can create a superior blade. This page will also make it clear why factory and manufactured knives often cut corners to increase their profit while offering a lower-performance knife overall. There is nothing wrong with cheaper, lower-performance knives, and here you will learn why they are inferior, and what the cheaper knife sacrifices in condition, and thus, function.

Knifemakers will also find this page a useful resource, I'm certain. This page will clarify why modern high alloy tool steels are so special and important to our trade and civilization. It will also clearly show why simple, low alloy carbon steels and hand-forging are crafts based in the romance and antiquated tradition, and high alloy scientifically-processed steel knife blades are the present and future superior performers and premium value.

I'm not here to discard hand-forging, which typically involves lower alloy steels by necessity. If you like a hand-forged or primitive knife, that's a personal preference and I know of thousands of knifemakers who can make this kind of knife for you. The hand-forged knife blade is not the kind of knife I make, for a reason—I've grown beyond 18th-century practices (along with the rest of all modern machinists) for most of my work. I use extremely high alloy hypereutectoid tool steels, and they cannot be hand-forged; they must be treated in specifically controlled processes more like a laboratory than a forge. These are the finest, most state-of-the-art, most advanced tool steels made, and these are the steels I make my knife blades with. More so, these are the steels my clients request, and they are who I make knives for.

This is not a required page for my profession and career—gratefully, I've been successful for decades without having this page on my site. This page is a significant reference and insight into the world of creating effective, superior, and valuable knife blades with some of the finest high alloy steels on our planet. I want to honor you who are reading this with as much viable information as is reasonable, since your time is so very valuable.

The reason I've created this page is because it's my service to the my field, trade, art, and a service to the people who have made it possible for me to do what I do in my career. Improving the performance properties of the highly specialized tool steel blades is critical, and after you finish reading this page, I guarantee you'll know more than most knife owners, most knife manufacturers, and most other knifemakers about this fascinating process.

Thanks for being here!

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What is Steel?

Steel is a tremendously important part of our lives. It's everywhere, from the nails and screws that hold our homes together to the vehicles we drive. From the flatware we eat our meals with to the handles of our doors. Steel is part of just about every device, machine, or object that requires some level of durability, and steel is definitely part of the machine or process that allows us to make every device or object we create.

Steel is, then, a creator's dream, and everything that involves steel in any way in our lives is created, because humanity created steel. Steel does not exist in nature; it is entirely man's realm. Though metals may by chance or God's hand in nature come together in rare and random circumstances, steel—as far as we know—came about only because man was tinkering with other metals: copper and tin, and then moved on to iron. Though meteoritic iron was used throughout ancient times, it's a modification of a rock, in essence, and not a direct creation of man.

Iron trinkets were found with bronze works in ancient Egypt, and it is believed that they were accidental creations discovered when smelting copper and casting bronze, evidenced by findings in Israel in ancient copper smelters. Iron oxide (rust and the powdered rock hematite) was used as a flux, a cap to prevent oxidation of copper melt. Some ancient man skimmed off the iron he had used to protect the copper. But then, the iron formed stringy lumps and had to be discarded. Because the melting temperature of iron was so high, they could not do with iron what had been done with copper. For a true melt, the smith needed 1537°C (2800°F), so he had to settle for working the iron in a spongy mass called a "bloom" and by repeated hammering, the slag could be forced out. This is beaten iron, or wrought iron.

About 2500 years ago, an ancient metalsmith created a dagger, with a wrought meteoritic iron blade and a gold hilt (or handle). It's one of the earliest known artifacts made of man-made iron. It was found in the Hattic royal tombs at Alaca Huyuk, near Hattusa in Turkey.

Near Delhi, India, there is a 13,000 pound tower, carefully constructed of pieces of forged iron. The iron is high in phosphorous, so because of this, has formed a passive protective film of oxide on the surface that inhibits corrosion. Uncounted hands have passed by, touching, petting, and stroking the iron and it is polished and oiled by the human tide. It is the largest ancient piece of manmade iron. It was made in about 300 AD, when Constantine established his Capital at Byzantium, when Galerius convinced Diocletian to persecute the Christians, and when the Ostrogoths were subjected by the nomadic Mongols sweeping in from Asia. Manmade iron alloys are very old.

Incidentally, wrought iron is a very specific type of iron, iron with less than .08% of carbon, and it's creation is described above. Other than in conservatory or historic reproduction practice, there is no wrought iron commercially available today! Does that surprise you? It should. What we see sold as wrought iron today is simply mild steel, or low carbon steel. This is the practice of using an old, respected, and traditional name to help sell the romance of the past. What would you think of the architectural railing, table stands, and garden furniture if they sold it as "mild steel, painted black"? No, "wrought iron" sounds so much more… classic.

In the past, wrought iron was not as malleable and formable as cast and forged bronze, so it languished until the Mediterranean basin was conquered by fierce invaders from the sea, whose identity to this day remains mysterious, and the chaos collapsed the entire bronze age. Bronzes disappeared, and more and more, metalworkers turned to iron. Determined to create better iron, the Hittites of Anatolia, peoples of mysterious origin, created a material that was known in their language as "good iron." It was much more durable and superior to wrought iron, and then the Hittites themselves disappeared, prey to European tribes. They left behind the physical evidence of improved iron, and an iron culture that continued widespread.

In the second millennium BC, iron smiths worked furiously with the material and in order to do this, had to expose the iron to white-hot charcoal and carbon monoxide from the combustion. This they repeated over and over to keep the iron hot enough to forge. This exposure forced carbon into the iron, and, simply as a side effect of working with the iron, steel was born. It was harder, stronger, and tougher than iron.

Just .03% increase of carbon in the iron makes into a steel that is harder than bronze. That was the final blow for the bronze age, and the addition of carbon could be directed by exposure in the forge to just the tip of an iron shaft, turning it to case-hardened steel (c. 1200 BC). During that same time, smiths stumbled onto the amazing discovery of the effects of quenching. Maybe they were just tired of waiting for the steel to cool, and wanted to get their projects done, so they quenched it in water. Then a new property was in play.

The only thing Homer got wrong in his comparison is that quenching is not tempering, at least not during our current times and definition, in this vast history of metals and mankind.

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Steel: More Than Iron

Carbon

Steel, in its basic form, is iron with carbon. Carbon is the number one element in steel that affects its alloy properties. While I'll get deeper into this later, it's important to know that carbon is key. As little as a few hundredths of a percent of carbon in iron makes it steel, and the percentages top out in the standard steel types at about one percent. Remember, I'm mentioning standard steels, not high alloys, tool steels, stainless steels, or specialty steels. I'll start out simply, and we'll get to the really good stuff later!

Standard steels also contain varying amounts of other elements.

Manganese

acts as a deoxidizer since oxygen is harmful in the refining process, and in small amounts, reduces brittleness to improve forgeability, meaning less brittle, more easily forced around: hammered, bent, deformed: forged. Higher amounts increase hardenability, but not by much.

Silicon

is a deoxidizer also, readily attaching to oxygen in the pour. Silicon (sometimes called dirt) is harmful to the surface finish, is hard on cutting tools, and is regulated to low limits by steel manufacturers. It is helpful in creating more shock resistant steels.

Sulfur

is strictly regulated and considered an impurity in steels. It helps to make steel easier to machine, but little else. In high alloy polished stainless steels, high sulfur gives the steel a yellowish cast, not the bright bluish-chrome appearance.

Phosphorus

is a solution strengthener in steels, and will increase both the yield and tensile strength of steels, aid in machinability, and aid somewhat in corrosion resistance. Remember the ancient iron pillar loaded with phosphorus described in the previous topic on this page? Like any element, phosphorus must be limited, because it will also embrittle steel.

There are many other alloy elements in modern tool steels, but just for the carbon steel discussion, these are the important and prevalent players.

High alloy, stainless, or hardenable alloy steels also contain additional elements:

These are the best steels available today: high alloy steels and stainless steels. Unlike carbon steels, most of them cannot be hand-forged and must be machined (with power tools and by hand) and processed in a clean, scientific, and highly controlled environment. The predominant additional alloy elements in these high performance steels are:

Chromium

is probably the most important alloy in these types of steels, next to carbon. It is the hardest metal on the periodic table; it's incredibly corrosion resistant; it forms extremely hard complex carbide particles; it adds toughness and wear resistance; it stabilizes steels in wide-ranging environments; and it works in concert with other alloy elements to increase all strength, corrosion resistance and wear resistance properties.

Tungsten

improves "hot hardness," or the ability to resist softening of the alloy when exposed to elevated temperatures, and forms hard, wear-resistant tungsten carbides

Vanadium

helps refine the carbide structure by creating initialization nuclei for carbide grains to precipitate on, and forms extremely hard vanadium carbides adding to wear resistance

Molybdenum

helps with deep hardening and hot strength (not particularly essential since knife blades are not thick and blocky with deep dimensions and are not exposed to extremely high temperatures) but moly also helps tremendously with toughness and corrosion resistance, particularly in stainless steels. Also contributes to forming extremely hard molybdenum carbides.

Cobalt

is a hot hardness alloy, typically used in high speed tool steels which may work at elevated temperatures. Not fairly common in knife blades, it has been on the increase in use in recent years. It also contributes to wear resistance by working with carbon, chromium, and tungsten to form critical carbides. Toxicity of cobalt and exposure to the metal's dust and sharpening swarf is a concern.

Nickel

is used, particularly with chromium, to increase toughness of the alloy, but is limited since it's an austenite stabilizer, lowing the transformation temperatures, typically necessary in larger sections. Also adds ductility, not a typical desire in a cutting blade, since bending and being ductile is not a desired outcome.

Niobium

this element is used in stainless steels because it stabilizes the steel against intergranular corrosion in heat affected zones, strengthening the metal microstructure which improves toughness. It also contributes by improving wear resistance by creating tremendously hard niobium carbides. Unfortunately, there are solubility issues currently so the niobium content in steels is limited, but who knows what great new materials are on the horizon using this element?

Copernicium

is a laboratory-created element and an extremely volatile metallic compound, adding multi-species erasure capabilities to any knife blade. Since it only has a half-life of 29 seconds, you'll have to be really quick in dispatching your alien invaders. Of course, taking a time to build a handle on the copernicium alloy knife blade is impossible, so it's limited to skeletonized blades. (I wrote this to see if you were paying attention … since only 75 actual atoms of copernicium have been detected as of the time of this writing, you can't have any for your custom blade.)

While I could go on and on in the periodic table of elements to detail each alloy, the important thing to know for at this time is steel's relationship with carbon, and how important carbon is. Carbon is the most important alloy in steel and you'll understand why as you continue to read.

This can be a lot to take in; don't bother trying to remember each specific alloy and its contribution. It's enough to know that the relationship of iron, carbon, and the alloy set is synergistic, with the performance of the whole being greater than the individual elements, in strength, hardness, wear resistance, heat resistance, corrosion resistance, and toughness, when properly processed.

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Steel as a Crystal

Steel and iron are crystals. This doesn't mean you can hold them up in the sunrise light beams and call the forest nymphs to do your bidding with a chanting spell; it simply means that they have a regular, repeating pattern of atomic arrangement. Like quartz crystals and other mineral, rock, and gem crystals, there is a uniformity of structure based on the bonding of molecules. While there are lots and lots of molecules in, say, a knife blade, for clarity it's best to look at the smallest piece of molecular arrangement, the unit cell. Many unit cells together create a lattice.

It's important to understand that nearly all iron has carbon, but it's not considered steel until the percentage is significant enough to affect phase changes. To get an idea of the variability of iron and steel in context:

• Pure iron does not typically exist; all iron contains some carbon, along with small amounts of other elements. If iron has less than .002% carbon, it's considered pure iron. Pure iron is actually rare, and is not obtainable by smelting.
• Steels are iron with from 0.1% carbon to 2.1% carbon, and some steels can be 1000 times harder than pure iron.
  • Mild steels typically contain from 0.105% to 0.3% carbon
  • Carbon steels (or medium carbon steels) contain 0.3% to 0.6% carbon
  • High carbon steels contain over 0.9% to 2.5% carbon
  • Ultra high carbon steels contain 2.5% to 3% carbon
• Cast iron has 2.1% to 4% carbon and 1% to 3% silicon

From this, you can see that iron and steel have a similar makeup, but they are vastly different materials.

The unit cells, and thus, the lattices of metals are many, but let's start with the simplest, iron. As I mentioned before, it's important to understand that all iron contains some carbon. The carbon atom is only 1/30th the size of the iron atom. Iron only has two unit cell arrangements, face-centered cubic and body-centered cubic.

Referring to the illustration of crystal geometry above. we all know what a cube looks like; it's like a box. In the box of your mind, use a red marker to put a red dot on each corner (8 total). These red dots are atoms of iron. Now add an blue dot in the very center of each flat face (6 total). These are also iron atoms. The face-centered cubic lattice has 14 atoms of iron in this arrangement. This is a unit cell. While it's easy to see in a singular fashion, that cell shares its atoms with the adjacent cells, creating a repeating stack of cells which is the lattice.

 The body-centered cubic unit cell has a different arrangement. It's still a cube, but in our box we put one red dot representing iron atoms on each corner (8 total or iron) and one blue dot representing an iron atom in the very center of the inside of the box. So the body-centered cubic unit cell has only nine atoms. Just like the face-centered cell, the unit cell shares its atoms in the corners with adjacent cells in a stack, or repeating arrangement, creating a lattice.

You can visualize these the atoms in the unit cells forming a series of stacked balls. You might consider them visually as ping pong balls, all filling a space, like a bucket. In examining these balls, you'll see that there is space between them. This is where, roughly speaking, many of the carbon atoms reside, in the center of the "holes" between the ping pong balls. Carbon atoms also exist in the interstitial spaces within the unit cells, since the carbon atom is so much smaller than the iron atom.

Those balls touch in relationship to each other, and the arrangement varies. The arrangement is the body packing of the cells, which can be an open or less dense structure of the body-centered arrangement found in alpha-iron or ferrite. The face-centered structure is more dense, and less open structure and it's found in gamma-iron or austenite. More on the phases below.

We humans are are all about heating things. We cook, we bake, we like blowing things up (heating up stuff to a point of massive instantaneous burning or oxidation). Our vehicles combust (burn) fuel, we heat our homes with natural gas (burning) or oil (burning) or electricity (produced by burning fuels). Understanding steel, though, requires a different perspective. We must think of matter and elements cooling or freezing, for that is where the real magic takes place.

When we heat up our iron to become liquid it takes a lot of heat (2790°F/ 1530°C). It looses all of its crystalline structure, just like ice looses its crystalline form when heated to liquid water; everything moves around. It's not the liquefying of the iron that's unusual; it's when it cools, or solidifies into a crystal. As it cools, the atoms lose their energy and bond with nearby atoms to form the crystal lattices. The iron first forms a body-centered unit cell structure and lattice with the the nine atoms. Remember, this is a solid, but it will transform into different crystalline structures while solid. Strangely, as the iron cools to 2550°F/1400°C, the unit cells and lattices change from body-centered to face-centered. Then, in more strangeness, at 1670°F/910°C, the face-centered lattices change again to body-centered unit cells and lattices!

It's fun to visualize this weird structural morphing taking place as the atoms bond, re-bond, move, lose energy, and transform their crystalline form and their molecular arrangement, and the iron and carbon move in form and structure. Remember, all of these crystalline changes happen while the material is solid. The temperatures at which this takes place are critical (important) for us to know, for the next considerations.

The addition of carbon to the iron changes the phases and structure as well as the time, temperature, and characteristics of how the metal reacts and exists. Carbon is introduced into the iron, and occupies the interstitial spaces between the atoms. In essence, the carbon is in a solid solution of the iron. Austenite, ferrite, and iron-carbon alloys are then "interstitial solid solutions."

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From Alpha to Omega: Steel Phases

It is helpful to understand that when steel cools (or is heated), it undergoes a group of changes in the crystalline structure, forming allotropes of iron. Allotropy is the phenomenon that allows a singular substance to have completely different forms; carbon can be both graphite and diamond. In steels, these changes have been called various things over time, and currently, in materials science, Greek system has been adopted that incorporates some of the older terms to describe these various structures and molecular forms. We also use terms based on the name of the person who discovered or researched the phase (austenite and martensite), and we also have names based on the appearance of the phase structure when viewed through a microscope (pearlite). We even name some of the structures for their physical properties (cementite). This confusing mish-mash of terms comes from many directions, from discovery, industry, research, and applications of steel in our modern world. It's important to know what these terms are, and how they interrelate, because steel metallurgy science crosses so many disciplines.

Please understand that steel and transformations that occur are well known, but there are also some things that are not understood! Though we trust our current knowledge of steels (and other metals) we still have a lot to learn, and that's exciting! Here are the terms:

1. Alpha-ferrite (Ferrite) (α-ferrite or α-Fe), also called alpha-iron or just plain ferrite, is a materials science term for pure iron. This, in itself, is a misnomer, since iron is never pure and the molecule always contains some carbon. α-ferrite is the body-centered unit cell with eight iron atoms in the corners and one iron atom in the very middle. It is very soft and ductile. The structure is highly magnetic, and is what gives cast iron and steel their magnetic properties. Mild steel consists mostly of ferrite. Ferrite has a lower ability to dissolve carbon, so much so, that at room temperature ferrite will accommodate almost no carbon.
2. Beta-ferrite (β-ferrite or β-Fe), also called beta-iron, is actually an obsolete term! Okay, so much for scientific regularity of terminology. In any case, it was once thought that this was a different crystalline structure, but is now believed to be the heated, liquidus version of alpha-ferrite. This is the high temperature end of the alpha phase, the point at which paramagnetism occurs. Paramagnetism means there are unpaired electrons attracted to a magnetic field, but not strong magnetism of a solid crystalline structure. So this is the range of alpha-ferrite where it loses notable magnetic attraction.
3. Gamma-ferrite (Austenite) (γ-ferrite or γ-Fe), also called austenite. Austenite is an important phase. When alpha-ferrite is heated above a critical temperature, it changes into austenite (or gamma-ferrite). This is the face-centered crystalline lattice structure which can contain up to 2% carbon in solution. It is non-magnetic. Austenite is created by heating steel above its critical eutectoid temperature and that temperature varies depending on the alloy components of the steel.
4. Delta-ferrite (δ-ferrite or δ-Fe). This is the allotrope of high-temperature iron, formed at temperatures close to the melting point of iron. Delta-ferrite has body-centered structure and can be retained at room temperature. Delta ferrite is not good in knives and other occurrences, since it can lead to banding, segregation, cracking, and precipitation of sigma-phase iron.
5. Sigma phase (σ-Phase). This phase of crystalline structure in FeCr occurs in high chromium stainless steels, and is a complex tetragonal structure (rectangular, and body centered). While hard, it's also brittle and detrimental to stainless steels overall. It occurs due to prolonged heating and welding. Many details of forming of this phase are unknown. Since this phase typically occurs as a precipitate of delta-ferrite in high chromium austenitic stainless steels, it has little bearing currently on knife blade steels.
6. Hexaferrum (ε-ferrite or ε-Fe), also called epsilon-iron. This is a hexagonal crystalline structure, created only at extremely high pressure (13 gigapascals, or over 1 million psi!) applied to alpha-ferrite. Okay, so this has no bearing on current metallurgy, for when the pressure is removed, the epsilon-iron reverts back to alpha-ferrite. But it may have some interest in geology, where immense pressures within the planet core create this structure. Our very earth is built on a core of hexaferrum, or epsilon-iron! I wonder if I can get some for a knife blade...
7. Zeta-ferrite (ζ-ferrite or ζ-Fe), also known as light saber powder. This is a very special ferrite that is created in the intermolecular spaces of matter (air) created by the oscillations of an amplified voltage wave generator. It uses the electrostatic repulsion/attraction between particles as a balance point for rigidly forming a short energy beam from a specialized hand-held emitter. Zeta-ferrite is only made by the Massassi on the red gas giant planet Yavin, from Star Wars Episode IV: A New Hope. Okay, just a humorous diversion; there is no such thing, so don't write me asking me to make your light saber. I do congratulate you, however, on reading this far!

Other important terms

It's okay if you don't completely understand the term descriptions below; there is a lot to absorb. Steel phases and transformations can be quite detailed and complicated. Some of the descriptions are repetitive, since these phase structures all interrelate and are formed in interactions with each other.

1. Cementite: forms many different shapes and arrangements in steel; it's a compound rather than a solid solution. This is a very important component of steel, and is extremely hard and brittle. So hard is cementite—that in it's pure form—it's considered a ceramic. It's iron with a lot of carbon, up to 6.7%! It's formed from austenite during slow (equilibrium) cooling, or from martensite during tempering. In slow cooling of austenite, the cementite precipitates in bands or layers alternating with ferrite, and the layered structure is called pearlite (from the layered appearance of seashells), visible by microscope. In tempering (heating after forming martensite), the martensite is heated, the lattices of the martensite are deformed by displacive transformation. The saturated carbon in the ferrite precipitates into cementite. While cementite is hard, it's also a detriment to steels in large quantities, because it's brittle.
2. Pearlite: This is the layered structure created by large scale movement of iron and carbon atoms that has a "pearly" appearance under the microscope. Pearlite is created by very slow cooling of austenite. Pearlite in the whole is not a phase; it's a combination of two phases: one layer is of the iron carbide cementite, and the other layer of ferrite (alpha-ferrite). The cooling of austenite transforms the face-centered cubic lattice structure to body-centered cubic ferrite. Since the structure looks like the nacre or seashell layers in a microscope, it was named pearlite.
3. Bainite (bainitic ferrite): this is cementite and ferrite, formed in a plate-like structure. To form bainite, austenite is cooled more quickly than the rate which forms pearlite, but more slowly than the rate that forms martensite. At first glance, you might think that bainite is like pearlite, in that it's composed of alpha-ferrite and cementite, but this is not the case. In bainite, the alpha-ferrite is dislocation-rich, making the alpha-ferrite harder than it would ordinarily be. Also, there is no layering present as in pearlite. Bainite is an intermediate of pearlite and martensite, and has characteristics between the two, and forms as sheaves separated by austenite, cementite, martensite. There are also distinctions of upper bainite and lower bainite, but I won't go into them here. In bainite steels, there is an inherent hardness and toughness that does not require complete transformation to martensite, and these steels can be used in the as-quenched condition. Bainite steels are used in large steam turbine rotors, pressure vessels, and nuclear reactor components, as well as other commercial purposes. You might wonder why not simply direct heat treating and composition towards the creation of bainite, and do away with the whole hardening and tempering process. After all, bainitic ferrite can be used "right out of the can" so to speak, with minimal processing. The limitation is that the cementite particles are coarse and reduce toughness in the steel. So for knife blades, which are very thin at the cutting edge, toughness must be improved. There is a way to do this, by adding silicon to the alloy, which inhibits precipitation of the cementite, but there are other problems (like blocks of austenite that transform into martensite upon stress, creating brittleness) that make bainite unsuitable for our purposes of knife blades. Bainite does play a role in the overall structure of knife blade steels though, as it is formed by decomposition of retained austenite during the second temper. This increases the toughness of knife blade steels, so it's critical that the second temper is accomplished for increased toughness in some types of blade steels.
4. Martensite: This is a big one, so much so that most of the high alloy tool steels I use and many tool steels used in premium material knives are classified as "martensitic stainless steels." Martensite is formed as a direct transformation from austenite. Heat the steel to its austenitizing temperature, and then cool it quickly, and martensite is formed. The key here is "cool it quickly." More on the distinctions of cooling later. Martensite is extremely hard, because carbon is trapped in the solid solution. While cementite and ferrite are formed in slow cooling and as the atoms diffuse into these two crystalline forms, the formation of martensite is sudden and drastic. The face-centered austenite lattice transforms into a body centered tetragonal ferrite, highly stressed, super saturated with carbon. This happens by a shear stresses and dislocations. Dislocations are simply areas where atoms are not in position in the crystal structure. Martensite is, clearly, the primary strengthening structure in tool steels, particularly knife steels. When it is formed, it creates both lath-like structures and plate like structures, microscopically visible. Martensite is deteriorated or destroyed by the application of heat, and in knifemaking, we call this tempering. Typically, knife and tool steels are processed for an overabundance of martensite, to leave as little austenite as possible, and then, in tempering, a controlled amount of martensite remains, giving a specific hardness and planned martensite volume with martensite's distinctive properties.
5. Ledeburite: This requires mention because you may see this material listed on phase diagrams and in the discussion of cast irons. Ledeburite is a 4.3% carbon-containing eutectic mixture of austenite and cementite. Though technically, it is not steel, it is formed in phase transformations as a constituent of high alloy steels. It's also an equilibrium phase constituent, so is less important to knifemaking as we are not about equilibrium, but rapid phase changes in heat treating!
6. Trootsite: this is a now rarely used term that originated when microscopy was not detailed enough to define specific structures. Trootsite referred not specifically to a material with a determinable hardness, which is standard when giving the -ite nomenclature, but referred instead to a transition pattern seen in fine pearlite (i.e. trootsitic structure). There is no delineation between trootsite and sorbite (below)
7. Sorbite: this is a now rarely used term that originated when microscopy was not detailed enough to define specific structures. Sorbite referred not specifically to a material with a determinable hardness, which is standard when giving the -ite nomenclature, but referred instead to a transition pattern seen in unsegregated pearlite. (i.e. sorbitic structure). There is no delineation between sorbite and trootsite (above)
8. Lemmingite: this is a tiny little crystalline structure that just follows the others off the cliff and into the sea. I was surprised to learn that Disney—in order to add real drama to the story—took to tossing our little creatures to their death for more dramatic film footage, thus creating the myth of the lemmings. Don't believe me? You're on the internet; look it up! (there is no actual lemmingite; I just knew you would want to know the truth about a large media corporation focused on dollars and a small, inoffensive grass-eating mouse. The abuse and slaughter is legendary. Mickey was also forced into slavery by Walt...

Crystal Unit Cells and Internal Interstice Geometry

Picturing the geometry of the crystalline structures of unit cells and how they stack, repeat, and are arranged is a textbook in itself. If you are interested in these structures, there are some great reference books identified below. Courses on metallurgy are a great educational tool, if you have the time and money.

The short version in our particular material—steel or Fe-C (iron with carbon)—is this: there are definite geometric spaces created within the structure of a unit cell of both austenite and ferrite. These spaces are identified as either octahedral or tetrahedral. This is a result of the arrangement of the iron atoms.

In these spaces, the carbon in the solution can be accommodated. These spaces are defined as interstitial voids. The voids in austenite are at a different location than in ferrite. These voids are larger than the voids in ferrite. Also, in austenite, the crystalline lattice expands somewhat to accommodate the carbon. In ferrite, the voids are much smaller. The tetrahedral interstices are also not symmetrical as in austenite, and a carbon atom there would displace the iron atoms, so the carbon tends to locate in the octahedral interstices. This means that ferrite will accommodate much less carbon than austenite.

This may seem a bit counter to logic. A unit cell structure that has 14 iron atoms has more available space for carbon in it that a unit cell structure containing 9 iron atoms. You would think that less iron atoms would allow more space for the carbon, but it is not the case because of the octahedral and tetrahedral interstices and their arrangement and locations.

An extremely simplistic way of perceiving this is with balls, often used in molecular and crystalline arrangement illustration. If you stack balls in neat, straight and axis-aligned arrangement, there are large gaps between them. Stacking them this way represents a face-centered structure, with five balls forming a side, there is a lot of room in the "cube." This represents austenite.

If you allow balls to simply locate themselves in a container, they will not stack; they will be offset between the layers, and less space will exist between them. There will be less room between them in the "cube." This represents ferrite.

I hope this simple idea helps. Please don't attack me for my generalization; I'm trying to keep it simple, and I know it's not! So, simply put, austenite accommodates more carbon atoms in solution than ferrite.

The predominate allotropes, constituents, and crystalline structures for our specific discussion of fine knife blade steels are :

• Ferrite
• Cementite
• Pearlite (Ferrite + Cementite)
• Bainite
• Austenite
• Martensite

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Phase Diagrams

An important part of understanding steel phase transformations is the phase diagram. While scientists and metallurgists diagram many things, the phase diagram illustrates precisely under what circumstances individual phases form.

The key to understanding heat treating is understanding transformations, and our induction of these transformations. This is another reason I believe the knifemaker should do all his own heat treating, so he can precisely control the structures of his creations. This may not matter much if he's using plain carbon steels and hammering out his blades in an open air forge—rarely do bladesmiths who work this way use any method other than looking at a relative and generalized color of heated steel for their control. They look at the color of the heated steel and make a judgment. There have been attempts to substantiate this process by giving it a technical sounding name like "thermo-optical emission viewing" (whoa!), but it's dependent on the skill, references, background lighting, color sensitivity, and the material itself, and is simply a guess. This is not how the best blades made of modern high tech alloys are created; there are instruments called pyrometers that can measure temperatures to a fraction of a degree.

The knifemaker who works with the highest alloy tool and stainless steels should be part scientist, or at the least, a laboratory technician, able to produce specific, controlled, and regulated environments and exposures for accurate and repeatable results in his steels. While I do make some forged blades and use pattern-welded damascus steels from time to time, these are chosen for one reason only; the patterned appearance. The very best performers are, of course, high alloy stainless and specialized tool steels.

The diagrams that detail these specific phase and eutectoid transformations specify points at which the eutectoid transformation occurs, which are the points at which one solid transforms into two different solids with different properties and compositions, important to understanding the whole process and how critical temperature is.

While at first glance, the chart seems intimidating, for fine modern high alloy tool steel blades, we are only concerned with the  narrow band of hyper-eutectoid steels. The first thing to remember is that these are equilibrium charts, and that the phases show here occur in slow temperature changes. Start at the very top of the vertical dashed line labeled "B." B is in the middle of the  hyper-eutectoid band of the steel area listed on the bottom of the chart.

1. At the very top of B, the temperature is 3000° F, and our hyper-eutectoid steel is liquid.
2. Moving vertically down, when the dashed line of B crosses the first solid line, at about 2500° F, the primary austenite begins to form. You can see our composition is austenite (γ-ferrite) and liquid, this is the slushy stage of welding.
3. The austenite is liquid until about 2200° F, and then the steel solidifies, becoming a solid solution of carbon and gamma iron (austenite). Going down the vertical B line, you'll see a black dot near the bottom of the austenite solid solution phase. This is a general indicator just to signify that at this point, most of the solid solution is austenite, or gamma ferrite.
4. Further down, the B line crosses the A_CM line. A_CM means Austenite to Cementite, and the cementite starts to coalesce in the solid solution at about 1600° F. It continues to form down the temperature line. The next black dot is a location of coalescing cementite (FeC₃) with austenite.
5. Down the line further, at 1333° F, formation of cementite is complete and pearlite starts to form. The cementite coalesces in layers alternating with ferrite, forming the pearlite structure in combination with cementite. Cementite is formed in hypereutectoid steels at this point because these steels contain so much carbon, that simple pearlite cannot contain enough of the carbon, so points of FeC₃ (cementite) coalesce within the pearlite.
6. Finally, at 410° F, the cementite in the solution reaches the Curie point of magnetic transformation (designated by A_O At this temperature, the cementite in the solution changes from non-magnetic to magnetic.

With this basic understanding of the phase diagram, I encourage you to look over and examine different areas and indicators on this and other diagrams. They are a rather simplistic way of showing how materials transform from liquid to solids, the eutectic points of steel and iron, and the temperatures at which all of this occurs. There are charts showing sublimation, deposition, melting, freezing, condensation, vaporization, and the crystalline structure of all kinds of materials.

Note that martensite is not on this chart, anywhere. This is the really important point here. This chart is describing material transformations at equilibrium, which for steel and iron, means very slow temperature changes. By sudden and deep cooling, we alter this slow migration of carbon and iron, and form astounding structures that drastically affect the steels performance and arrangement.

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Understanding the Eutectic

When water freezes to ice, it happens at exactly 32°F/0°C. At this same temperature, ice turns to water. It's so accurate that thermometers are calibrated in a bath of crushed ice and water because it's exactly 32°F/0°C. This is not true with steel (iron with more carbon), and when we add more carbon, the weirdness factor increases. Steel starts freezing (solidifying) at one temperature and is completely frozen at a lower temperature. So it has a range of solidifying. In this range, steel is mushy like oatmeal or mud. Start adding more carbon, and the range of mushiness gets larger, until about 2% carbon. Add more than 2% carbon and the range of mushiness gets narrower, and then the range goes away at just over 4% carbon.

Solidification is not the only physical action where steel changes. Steel, when heated below the melting temperature to certain specific and critical temperatures, undergoes changes in the internal crystalline lattice structure. These structures are called phases. The magnificence of steel is based on these phase changes or transformations.

The word "eutectic" comes up often in steel discussion, but the word itself simply means "easily melted" in Greek. It may help to understand various applications of the word in the study of the science of materials to clarify the idea of eutectic principles in steel.

Back when I was very young and working in industry, we did a lot of welding. I worked as a maintenance electrician, mechanic, and instrumentation technician. This sounds like a lot, but mainly consisted of a small group of guys handling every single device, machine, driver, power feed, control, and regulation problem in a small-to-medium sized manufacturing plant. This meant wiring up new devices, troubleshooting failed systems, welding broken components, tuning and calibrating every machine and device the plants needed. I worked in half a dozen different plants like this: a plant that made concrete coated steel pipe, a fiberglass manufacturing plant, a secondary aluminum smelter, a pigment manufacturing plant, a circuit board plant, a radio crystal manufacturing plant, and even a large electrical generation station. Though I took vocational welding classes when I was still in high school as an advanced student, the classes were only a brief overview and introduction to real world welding issues we would encounter in the plants.

One of them, a seemingly simple one, was how to weld stainless steel to mild steel, or stainless steel to a high carbon, high alloy steel. A machine would fail, or corrode, and stainless parts were needed because, wisely, the plant maintenance manager wouldn't want to have to shut down the production line and fix the problem (due to corrosion) again. We came up with special welding rod made by Eutectic Castolin®. This is a great company that's been around quite a while, and makes some neat stuff, essential to maintenance and repair as well as welding advancements.

Now, each metal in the bond (lets say high alloy steel and stainless steel) each have a high melting point, and their melting points are different. But when you mix the two metals together in just the right percentages, you get an alloy that has a lower melting point t than both the parent metals! It's an amazing thing, and the welding rod from Eutectic Castolin® already had this perfect mixture in the rod. So when you welded with this stuff, it bonded to both parent metals at a lower temperature than they would ordinarily melt at, allowing a great penetration and bonding of the weld flow.

Simply put, eutectic transformation is a liquid cooling into a solid that has two phases.

Steels aren't the only metals to have eutectic properties. When I got into jewelry work, I quickly learned that eutectic bonding of dissimilar metals was necessary in all parts of the work. For instance, copper has a melting point of 1984°F/1085°C. And silver has melting point of 1763°F/961°C. If you try to fuse copper to silver, the silver will melt and dribble away long before the copper melts. However, if you mix up an alloy of 28% copper to 72% silver, the alloy has a melting point of 1431°F/777°C! This is great! This means that with that alloy between the pieces of silver and copper, you can solidly bond the two dissimilar metals at about 300° F below the lowest (silver) metal's melting point!

Metal alloys aren't the only materials that do this, and it's not only about melting, but also about the phase transformation of a solid. Understanding that in eutectic concentrations in steel means that several components in a specific combination create a whole that has a lower critical temperature than the individual components.

In the combination of steel, the elements iron and carbon, depending on their percentages in relation to each other and the temperature, give some steels strict eutectic points. To get gritty about this, the two atomic species form a joint super-lattice based upon their valence electrons.

What about eutectoid steel?

To throw another term into the mix, we have eutectoid steel. Eutectoid simply means "eutectic-like," but In this case, the word eutectoid describes a process of phase transformation where one solid forms into two different solids. Steel with 0.8% (actually 0.77%, but let's round the number) carbon can transform (with heat) to austenite, and in equilibrium cooling, austenite can then undergo complete phase transformation into pearlite (cementite and ferrite) without a transition zone and without any extra ferrite or extra cementite. This is considered eutectoid steel.

Hypoeutectoid and Hypereutectoid

Steels with less than 0.8% carbon are called hypoeutectoid steels, and hypoeutectoid simply means a mixture of components having less of the minor component (carbon) than the eutectoid composition. Hypoeutectoid steels can transform in the same equilibrium cooling phase transformation to pearlite and ferrite. Ferrite is a soft component abundant in mild steel. Railway track is a great example of hypoeutectoid steel. So are railway spikes. You might want to consider that when you see a knife made of a railroad spike. As knives, they are ornamental only, containing carbon in the range of 0.15% to 0.30%, creating soft, weak blades at their very hardest. But some folks like the look and they are easy enough to hand-forge.

Steels with more than 0.8% carbon have so much carbon that they transform into cementite before the eutectoid point and they are called hypereutectoid. Hypereutectoid means a mixture of components having more of the minor component (carbon) of a eutectoid composition. Hypereutectoid steels can transform in equilibrium phase transformation to pearlite and cementite, with many more abundant hard particles of cementite. Cementite is created because of the high carbon content.

These steels are therefore harder, more wear resistant and more durable in long-term use as knives and cutting tools. This is because the higher carbon can form abundant carbides. These carbides are not only iron carbide, but with high alloy hypereutectoid steels, are also molybdenum carbide, chromium carbide, and vanadium carbide, and combination carbides of multi-elements. Also, high carbon creates a more profuse and abundant martensite formation, which, after tempering, creates a stronger, tougher, more wear-resistant steel overall.

The important difference in these three types of steel in knife blades (hypoeutectoid, eutectoid, and hypereutectoid) mean more than just the carbon content, though that is the defining factor. The crystal morphology is different in all three of these steels. Hypoeutectoid steels form Windmanstätten patterns in their ferrite side plates projecting into the austenite grains. The microstructure of hypoeutectoid steels will then contain Windmanstätten ferrite and fine pearlite.

Hypereutectoid steels form a continuous network of hard, brittle cementite along the prior-austenite grain boundaries, and are typically only used where extreme hardness is required, such as in cutting tools.

In all types of steels, the ultimate tensile strength increases with increasing carbon content. However, yield strength varies little with increasing carbon content.

The only advantage of using a lower-carbon hypoeutectoid steel is increased ductility, which means a softer knife blade more likely to bend than break. This is not the reason to make a fine knife blade; cutting tools cut because of high hardness and wear resistance. Hard edges cut; ductile edges bend and dull.

Most chef's knives and kitchen knives made and sold today are made of hypoeutectoid stainless steels because they are inexpensive lower alloy, lower carbon steels. Being available in sheet stock, it's easy and fast to punch-press them into knife blades. They only require simple and fast heat treatment that is usually automated, and are easier to machine, grind, and finish. They also dull much faster, requiring constant steeling, honing, and sharpening with traditional methods.

A good use for a ductile blade is a machete, since it will likely encounter rocks and high impact. But making soft, ductile machete blades is really not the realm of the maker of fine knives, because soft, ductile steel blades are a dime a dozen at any hardware store, big box warehouse, or lawn and garden outlet.

 Hypoeutectoid steels have also been extensively used in structural steel applications, and were produced in extremely large tonnages. However, even the hypoeutectoid steels are being replaced in structures by High Strength, Low Alloy (HSLA) steels because of higher toughness.

Hypoeutectoid steels are limited in function, and simply make inferior knife blades. They do forge well, and that is their main attraction in this field. As I've detailed before, hand-forged blades are mediocre in every performance aspect when compared to blades that are machined, offhand, in stock removal method using hypereutectoid steels, particularly with lengthy, detailed, and high performance cryogenic heat treatments.

If you're familiar with this website, you know that all of the steels I use in knife blades are hypereutectoid. They have a lot of carbon, for good reason. You won't see this terminology used by knifemakers and knife manufacturers because they are often using hypoeutectoid (lower carbon) steels, and thus revealing this doesn't sound so great. Even in the stainless steels, hypoeutectoid types are the more common players in handmade knives. Steels like AEB-L, 13C26, and 14C28N that are commonly used for knives are low performing hypoeutectoid steels, having only 0.67% carbon. 

Hypereutectoid steels are harder to heat treat, with more complex and necessary deep cryogenics for maximum transformation into martensite, and more profound development of allotropes. They are not steels that can be fudged, estimated, or generalized in their treatment, they must be handled with high accuracy and control of the process.

Just because a knifemaker uses hypereutectoid steels doesn't mean he's treating them right. Most knifemakers absolutely do not treat them to their highest potential, missing the marks on temperature (deep cryogenics required) timing (low cooling slope of time vs. temperature), and multiple temper cycles that have strict control with deep cryogenic equilibrium stages in between. Hypereutectoid high alloy steels are complex steels requiring complex treatments to bring to their pinnacle of condition.

The reality is that higher carbon hypereutectoid steels are the better steels for knife blades. They are superior because they have high carbon content. Whether or not they are stainless steels, high alloy steels, or just plain carbon steels, the amount of carbon is critical to their performance and value.

Wordplay and Languages

The words hypo-eutectic and hypoeutectoid, and hyper-eutectic and hypereutectoid are different and have different meanings depending on the country and language, as the United Kingdom and European English version of the word is not the same as the American English version, so research papers and presentation vary somewhat.

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Understanding Martensite

Martensite is probably the most important crystalline structure knifemakers and toolmakers are concerned about, so I want to go into it a bit more. Martensite is formed from austenite (gamma-ferrite) detailed above. When heated to transformation temperature (the decalescence point described below), the ferrite (alpha-iron) and cementite (iron carbide) transforms to austenite (gamma-ferrite). The body-centered cubic lattice structure of ferrite transforms to a face-centered cubic lattice structure, containing up to 2% carbon in solution. The carbon has migrated from the orthorhombic cementite structure into solution, and also into the face-centered austenite. Since carbon atoms are 1/30th the size of iron, there are extra carbon atoms in the lattice structure floating around in the interstitial spaces (the spaces in between the iron atoms in the lattice).

In slow cooling, the carbon atoms will drift back into the alpha-ferrite arrangement (body-centered) alternating with layers of cementite and stabilize. In high carbon steels this slow cooling produces pearlite, layers of ferrite with cementite as the carbon diffuses into coalescing cementite layers (recalescence explained below). The cementite is in platelets.

This is an equilibrium phase, where physical changes happen at fixed temperatures with the material at thermodynamic equilibrium, without consideration for time. In simpler terms, equilibrium means staying at the same temperature for a long time, or extremely slow changes in temperature. These phase changes happen in equilibrium by diffusion, where atoms move due to thermodynamic and internal motion. The carbon moving within the face-centered and body-centered interstitial locations is a product of diffusion. These phases are represented on an equilibrium phase diagram, but martensite is not.

The reason martensite is not represented is because these particular phase diagrams are for materials at equilibrium. Martensite is not an equilibrium structure because it grows without diffusion, from it's parent austenite, inheriting its chemical composition. It does this by displacive transformation. It's also highly strained, a kinetic product brought about as a result of rapid thermodynamic temperature changes, again, not at equilibrium.

In fast cooling, the carbon simply has no time to move very far, so it's forced into the crystalline lattices making a new arrangement called martensite. Martensite is actually a distortion of the body-centered structure, making the lattice into a strained tetragon (rectangle). The actual movement of atoms within the crystalline lattice structure is no more than the distance between two atoms. The crystal structure has lots of dislocations, which can be envisioned by plenty of forced angles from many directions, strengthening the steel.

So the crystals are—in simple terms—interfering with others, strained, twinned, and bent. I know, you metallurgists are wincing, but I'd rather not go into the details of edge dislocations and screw dislocations formed by shear and wave front motion in evolving crystalline bodies. It's enough to know that many dislocations create an irregular lattice, which is much harder to break.

The microstructure of martensite has no simple analogy that is easy to understand in our physical world. The science of steel is rather complicated, so, without getting into morphological and crystallographic characteristics, the strength of martensite (over the same steel untransformed) is caused by: dislocations, fine twinning of the crystalline bodies, smaller grain size, and movement and segregation of the carbon atoms. Additional strength is caused by the formation of carbides. More on carbides later.

Martensite doesn't form all at once on cooling. It has a start temperature (M_s) and a finish temperature (M_f). In this cooling range, austenite transforms to martensite. The cooling has to occur quickly, otherwise, the crystalline structure will convert to bainite, or if cooled even slower, pearlite, the layered alpha-ferrite with cementite. Slower cooling also allows the austenite to stay austenite, or be retained as austenite. While there is always some austenite retained and some converted to martensite, the amount is very important. In the scientific sense, there is no absolute martensite finish temperature; this is determined at 95% transformation, since complete transformation is not known. So, M_f =95% martensite transformation. The important procedure is transforming as much austenite as possible into martensite.

But what about all that extra carbon? In martensitic transformation, it's forced into small, chunky particles rather than the layers and platelets of cementite as in pearlite. These tiny, extremely hard particles become carbides in further treatment steps, and are spread throughout the martensite, and increase the wear resistance tremendously.

Simply put, martensite is similar to the body-centered cube ferrite (bcc), but instead of a cube, it's body-centered tetragon (bct) with more carbon occupying the spaces between the iron atoms. The transformation reaction is very fast, at near the speed of sound. Since the bct structure is less densely packed than the fcc structure of austenite, there is a volumetric expansion, resulting in hardening of the steel.

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Decalescence and Recalescence
Transformation and Temperature

These are curious properties of steel allotropes, and can help in understanding how carbon migration, phase changes, coalescence, and the whole heat treating process works with steels. Decalescence and recalescence are physical properties based on temperature, and may be used to describe or indicate the critical temperatures where these phasic changes take place. However, they are not something manipulated or changed in any way by the knifemaker, simply something that occurs. Not understanding these reactions may lead to improper, ineffective, or less than optimum heat treating.

Decalescence

As the alloy is exposed to heat, the actual temperature of the steel continues to increase. At critical temperatures, phase change starts to take place. At these points, the steel temperature does not increase, even while additional thermal energy is being supplied by the furnace. Instead, the temperature of the steel decreases as the energy is used to transform one phase of the steel to another phase. Applying additional heat will not increase the temperature of the steel, only encourage further transformation. When transformation is complete, the additional heat supplied by the furnace will then cause the steel to increase in temperature in a more normal fashion. What's important here is that the energy is being used to change the crystalline lattice structure of the steel. This property of absorbing heat energy while decreasing in temperature during heating as phasic change is underway is called decalescence.

In decalescence, the important thing to remember is that plenty of ambient heat and ample time in the furnace is necessary for the phase change to take place, and temperatures must be closely held and applied. This is why the best heat treatment takes place in an electric furnace with accurate pyrometers, regularly checked and calibrated. Too low of a critical temperature, or too short of an exposure to that temperature, and complete phasic conversion will not take place. The blades need to be removed immediately after full phasic transformation, and not heated further. This is another reason that hardening is not accurate in a blacksmith/open forge environment. In that environment, judging the time and temperature for decalescence is only an estimation. This is why no industrial, machine shop, or heat treating facility uses open forges (or blacksmiths) for this critical step.

A critical point is that the decalescence point should not be held too short, nor too long, and the cross-sectional geometry of the blade being heat treated plays a large role in this. A thin blade must not be held too long above this temperature, a thicker blade must not be held too short. This varies from blade to blade, and the only way to control this is for the knifemaker to have an understanding and complete control of the process with accurate equipment and devices in use.

Decarburization

If the steel knife blade is heated for too long or for too high of a temperature after the decalescence point, the carbon will start to migrate to the surface of the steel and bond with any free oxygen, decarburizing the steel. This is a serious fault in heat treating, and it mainly comes from overheating the steel for too long or for too high of a temperature, or exposing the steel to an oxygen-rich environment during heat treating. In the machinist's guide and in other heat treating references, decarburization is listed as the number one failure in heat treating steel!

In my own studio, I've used protective environments for decades, including vacuum, inert gas (nitrogen and argon) purged furnaces, and stainless steel foil wraps that create an oxygen-depleted environment around the blades. In all of these types of decarb protection, certain other steps are necessary to ensure the proper and immediate transformations take place, such as adjusted timing, quench methods, post-purged environments, and manipulations of the environment furnace and quenching equipment. It's critical in all steels that will be used as tools to prevent decarburization during heat treating. Yet, in open forges by blacksmithing methods, decarburization is expected as if it's normal!

This is important, because If a blade is decarburized, two things happen. The first is that a black, hard carbon-rich crust will form on the blade, and will have to be removed. This "scale" is composed of a significant portion of the carbon that has migrated out of the steel to the surface and bonded with free oxides in the atmosphere. Grinding, sanding, and removal of this crusty surface of the steel means extra effort, changed geometry, and difficult finishing.

This indicates the second and most severe failure of process in decarburization. The carbon content of the steel is lowered, meaning less martensite, less carbides, and a softer, less wear-resistant, and weaker blade overall. It is a very bad thing; truly a failure of heat treating. You might read that blacksmiths and unprotected furnace heat treatment creates a scale that has to be ground away to "get to" the steel underneath that is not decarburized, but the depth of the decarburized steel can only be estimated, and is uncertain and tenuous. This is why no professional heat treating organization uses an open-air or unprotected method.

Think of this. There is one critical place on a knife blade where decreased carbon in the steel would do the most damage. That place is the cutting edge. Since it's the thinnest place on a knife blade, it has the greatest propensity to decarburize. There isn't much steel there, and the carbon can deplete from both sides, dramatically decreasing the carbon in the alloy... right at the cutting edge.

There is no way for decarburization to be seen in a finished knife blade, no way to correct it if it does happen. The result is a blade that performs less than expected, has less carbon available to create carbides, and has lower wear resistance overall. This is because loosing carbon converts the steel to a lower carbon alloy!  More about decarburization below.

The Hardening Temperature

This is the temperature slightly above the decalescence temperature that the steel is brought to in order to ensure that complete transformation has occurred. There are all sorts of interpretations of this temperature, some saying 50°F, some saying 100°F above the decalescence point, but it is truly only confirmed by experience guided by the manufacturer's white papers. Even the white papers may give several different hardening temperatures with the different results produced. Here, the experience of the knifemaker is key, along with accurate electric furnaces and calibrated pyrometers.

Recalescence

Recalescence is similar, only an opposite in physical reaction of decalescence. As the steel is cooled at critical points, phase change gets underway and even though the ambient temperature is dropping, the steel increases in temperature at these critical points. This is because a phasic change is happening. This is the steel trying to reach entropy, and it has to give up some latent energy while undergoing physical change to do this. So it heats up while cooling during phasic change as this change of state releases energy.

With recalescence, the same concerns exist as in decalescence, but with some more dramatic applications. Effective cooling will absorb latent heat during recalescence and phasic change, and enough coldness (absorptive environment) will allow the extra latent heat to be pulled from the steel, effectively aiding in phasic change. This is important in annealing.

However, in knife blades, during normal heat treatment, we do not want recalescence to occur! Recalescence is an equilibrium reaction, in other words, it happens in slow transformation, with austenite transforming to pearlite and ferrite. Unless we are fully annealing a blade, we don't want this to happen; we want martensite instead. In order for this transformation to occur, we need fast and immediate cooling so recalescence can not happen. This means an even more robust cooling environment with the ability to pull the heat from the steel at the most rapid rate possible without resulting in stress fractures or warping.

Because of these critical temperature reactions, it should become clearer that the most effective means to control this environment is an electric furnace and accurate freezers with calibrated and accurate pyrometers. To try to do this visually at the blacksmith's forge is simply a guess, which may be only a fair estimation in hand-forged hypoeutectoid blades. This method should never, ever be used in the treatment of hypereutectoid high alloy and stainless tool steels.

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Retained Austenite and Transformed Martensite

You'll undoubtedly read or hear about retained austenite (RA) in knife steel and tool steel discussions. No matter how a piece of hypereutectoid steel is treated, there will always be some austenite retained in the structure when it finally reaches room temperature. Everybody wants to limit or decrease the retained austenite because it's softer than the martensite or carbides in the structure, and decreases the steel's hardness after heat treatment. Retained austenite reduces the mechanical properties of the steel significantly, negatively influencing yield strength, machinability, fatigue strength, and even corrosion resistance. It even affects the size of the steel!

On the other side of the argument, some austenite is necessary for resilience of the structure and during tempering, the austenite is transformed to ferrite and cementite, toughening the structure overall. However, that is tempered austenite, not untempered austenite, and there is a huge difference.

Martensite expands when it forms, and any remaining austenite is surrounded and pressurized in small islands, with force exerted by the martensite expansion. In order for the austenite to transform to martensite, it needs to expand, but it's trapped. When the steel is brought into service, pressures and elastic deformation can allow retained austenite to transform to martensite, but the steel has already been tempered, so these tiny islands of untempered martensite are brittle and can cause fractures. Since it is on a microscopic scale, this would present in a knife blade as high wear at the cutting edge, exactly what you don't want in a knife blade.

Overall, knifemakers and tool makers want to reduce the amount of retained austenite, and increase the amount of martensite, giving a more durable structure that can then be tempered to the preferred balance of hardness and toughness that the blade and knifemaker, and ultimately the knife user requires.

Just what is that balance? It depends on the steel type, the intended use of the knife, the habits and application of the knife user, and the skills and understanding of process by the knifemaker. This is another reason a maker should, at the very least, heat treat his own knives. Sending a knife off to a heat treater who slaps a "58HRC" on the blade is a cheap and fast solution, but not participation of, understanding of, and result-based function of the knifemaker. It is not custom, it is not controlled by the maker, it does not demonstrate to the knife client that the maker has a grasp on the complexity of the process. If he doesn't have a grasp on the basic metallurgy of the blade, what does that say about the geometry of the blade, the fittings and fixtures, the handle? What does that say about his understanding of the knife use and application, durability, and longevity? What does that say about his practice creating the sheath, stand, or display? These are all parts of knifemaking, and they start with the blade.

Consider that chef's knives can vary in temper depending on the style, use, and preference. A boning knife may be favored by the chef to be from 55HRC (springy and flexible) to 62HRC (extremely hard, rigid, and wear-resistant). I've had clients requesting one of each! Knives used in combat and counterterrorism may range from shock resistant and tough to ultra hard, bordering on brittle, and every point in-between. This can depend not only on the user preference, but the blade and knife design and construction overall, including the features of serrations, armor-piercing thickened sections, or razor-thin recurve areas. It is a very delicate balance and it's only learned by experience and feedback, coming from years of custom knifemaking. So it's easy to see how a heat treating contractor farming dozens (or hundreds) of blades through his line at once cannot offer much in the way of variety or specific treatment.

How does one determine the percentage of retained austenite? X-ray diffraction, that's how (by ASTM and SAE standards). Since knifemakers are not typically in possession of this equipment, they must be educated on the standard practices and procedures to produce the desired levels of these structures in their shops and profession. The information is out there, available to everyone, and there is no reason anyone who makes knives and has a access to the internet should not have a grasp of this. Granted, most knife users are not interested in the amount of retained austenite in their knife blade, but they do want to trust their knifemaker or knife manufacturer to supply them with the best knife possible in their budget.

Simply put, we want to limit retained austenite to as small a percentage as reasonable, with as much of the austenite transforming into martensite as possible. It's pretty simple to perceive, but extremely interesting to understand how it all happens.

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The Quench

The word quench comes from Middle English quenchen, from Anglo-Saxon cwencan, causative to acwincan: to decrease or disappear. The definition currently is to destroy, extinguish, or in our field, to cool suddenly, as in heated steel, by immersion in water, oil, or air. Rather than look at quenching as an act of extinguishment or decrease of the heat, we must take a different view; one of transformation in the suddenness of time. In my view, it's not the heating that is the amazing thing, it's the quenching, and the results of the nature, timing, medium, rate, and depth of the quench.

Quenching, like most aspects of this profession, is a balance. It's a balance of cooling the blade steel as rapidly as possible, but not so rapid that the shock of cooling causes cracking and fracture. Want to know what I mean? Take a piece of 440C high chromium stainless steel and heat it to 1900 F. Pull it out of the furnace and quench it in cold water. It screams, it cries, it shudders, it vibrates as if the hounds of hell are tearing at its soul. And then it shatters into dozens of pieces that settle to the bottom of the bucket. Okay, just a visual for you to consider, but fun, anyway.

The balance a knifemaker walks is one of sudden and deep enough cooling to transform austenite into martensite, but not so sudden as to so severely stress the blade that it cracks. The exact quenching method is specified by the steel manufacturer or foundry, so it's not some mystical question that has to be resolved. Many steels (like O1 oil hardening tool steel) are designated by their quenching medium (as are A2 for air hardening and W2 for water hardening). The designation is not consistent and sometimes a particular steel type can be quenched in several different mediums. For example, ATS-34 may be quenched by air or heated oil, with slightly different results. This is another reason for knifemakers to heat treat their own blades, as entirely different properties are derived depending on these steps.

You may note that the medium designation is followed by the word hardening. This is what quenching does, it hardens the steel. It does this mainly by suddenly transforming austenite to martensite, taking a blade from a soft, plastic, glowing hot mass of steel to a cold, hard, stiff, and brittle one.

There are three considerations in quenching, and together, they play a pivotal role in the character and structure of the steel knife blade. They are:

1. The Austenitizing Temperature. This is the temperature at which austenite is formed, and it's typical to heat the steel just above the critical temperature so that complete austenetizing occurs. It is posted on all commercial steel's white papers and online.
2. The Quenching Medium (and its temperature). This is simple to determine; it is posted on all commercial steel's white papers and online.
3. The Quenching Rate. This is how fast to cool the steel, and often depends on the medium. A quench may be interrupted, staged, or controlled in other means. The rate may require first quenching in heated oil, then air, then water. Each steel alloy is different, and the manufacturer's recommendations on the white paper are the guide to this.

While the white paper specifies the recommended treatment and expected results, there are some issues with these documents and their presentation. More on that below. Generally, though, they are a reliable guide to processing individual steels.

Cold, Very Cold, Cryogenics

"Cold" treatment was well known, both by process and results since the industrial revolution. Old Swiss watchmakers would "age" parts in the snow, and Pierce-Arrow would age engine blocks by putting them outside in the winter. There are other stories of men and companies aging and treating steels to the cold for advantage of higher performance on the market, but they simply did not have access to the modern investigational tooling and devices we have to identify the results of the process. They did realize verifiable known and positive results.

When I started knifemaking, there were no cryogenically quenched knife blades. Guys were just starting on the right track, and we used dry ice and freezers, acetone and alcohol, and whatever we felt improved the rate and depth of cold to harden the steel. Granted, the steels performed extremely well even without this extra step. To this day, if steel knife blades are processed according to the manufacturers recommendations without  cryogenic treatments in conventional heat treatment methods, they will perform extremely well, because simply, the alloys are superior to plain, medium, and high carbon steels.

So why bother with cryogenic treatments at all? After all, it costs money, time, space, materials, electricity, and expendables to do cold treat. We do this because it produces a markedly better steel overall, and we can prove it, particularly in the high alloy and stainless steels.

When I started to expand, refine, and improve my quenching and heat treating process, I looked at it much like I look at all of the various facets of my work: improvement in steps. As I've explained in bio, at one point my blades were good, but my handles awful, so I tried to improve my handles. Then my sheaths were the worst, so I improved them. When I thought all of that came together, my embellishment was weak, so I worked to bring that up. Then, back to the blades, as the handles were now better than the blades. I look at this profession as one of continual improvement, constant upgrading and refining, coherently creating evolved works, and it was only natural to improve my steel processing as this went on. I started with no cold treatments, moved to long-term sub-zero freezing, then on to cryogenic treatments.

Let's define and identify these quenching treatments.

1. Conventional Heat Treatment (CHT): This is, simply quenching to room temperature, followed by tempering cycles. Understand that when properly performed, this most commonly performed process will result in long-wearing knife blades with good hardness and wear resistance. For high alloy hypereutectoid tool steels, CHT produces blades that out-perform all hypoeutectoid steel blades in every conceivable way! Conventional Heat Treatment is not bad, and knives with high alloy steel blades treated this way can be outstanding performers.
2. Cold. This term is invalid in the field of steel treatment, as it's vague and non-specific. In the old days, simply leaving an engine block or chainsaw bar in the outside cold of Buffalo, New York was truly cold, and yes, they did this for improvement of the machinings and castings. But cold is a relative world, so I'm just identifying it here as a historical reference. It's colder at room temperature than in a furnace or forge, so it's a relative concept. Yet, some sources talk about "cold treatments" in their advertising and literature today. Some classify cold treatments as -100°F! This is an antiquated term, so I'll try to stay away from the word as anything more than a generalized and relative condition as this field has refined enough to identify specific ranges of temperature of metal treatments. The funny thing is how many steel suppliers still use this on their white papers (technical bulletins). That's probably because there is a huge variation in equipment and processes, and they don't know what the buyers of their steels have in capabilities. They absolutely don't want to discourage anyone from buying their steels, so they tend to generalize by using the phrase "cold treatments." This is not the scientific description, but a sales advertising phrase. Please read further to understand the Standard Convention below, in this section.
3. Sub-Zero (SZ). Definitely a more specific word or concept, but one wonders: Fahrenheit or Centigrade? Typically, in our part of the world, we are talking about below zero degrees Fahrenheit. Substantially colder than freezing (of water), this is available with some common mechanical deep freezes, and some can be specially adapted to get down to -15°F to -20°F (-26°C to -28°C).
4. Shallow Cryogenics (SCT): This is defined as -121°F, or -85°C. Earlier conventions simply called this a cold treatment, and some still do today, but as the science has improved it's now known that this is a critical and distinctive cryogenic range where steel structures are greatly enhanced. This can be accomplished with mechanical refrigeration means. Some institutional researchers simply call this "cryogenic treatment." It's important to know that with some steels, the most significant improvements in steels are achieved in shallow cryogenic treatments, and somewhat less dramatic increase in deep cryogenic treatments. This usually refers to martensitic transformation and not eta carbide development (more on those later). Other steels respond better to deep cryogenic treatments, so it depends on the steel type.
5. Deep Cryogenics (DCT): This is defined as -185°C, or -300°F. This is accomplished with liquid nitrogen in various vessels, distribution means, or chambers. It is the lowest temperature range that high alloy hypereutectoid tool steels are typically treated to, and can affect the most dramatic change in the performance of the material. Some institutional researchers call this "ultra-cryogenic treatment."

I'm sure there will be some discussion and emails about this, but the term cryogenic simply means the study of materials at very low temperatures. Some define the temperature delineation of cryogenics at -238°F, but it is an interpretation and not specific. Cryo means cold; a cryogen is a material or solution used for freezing. The word cryo comes from Greek kryo, simply meaning icy cold.

I'm using standard convention detailed by some modern industries, who are well-versed in the technology: The Journal of Materials Processing Technology, The American Iron and Steel Institute, the International Journal of Emerging Technology and Advanced Engineering, and other various research sources (some detailed below).

This, then, is the target range of temperatures of quenching for our field:

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The Cryogenic Quenching Process and Factors

There are three critical factors in cryogenic quenching of steel:

1. The quenching rate
2. The quenching temperature
3. The holding time at that temperature (cryogenic aging)

Understanding these helps to grasp the whole process and what is happening to the steel.

1. Quenching Rate: How does one determine the rate of quenching? It's the fastest cooling possible without experiencing destructive results in the steel. These highly negative results are cracking, distortion, warping, twisting, and uneven cooling that produces cracking, distortion, warping, or twisting. To be specific, first we refer to the manufacturer's white paper and other online resources. Second, we use trial and error. Why is trial and error needed? Because most manufacturers supply information based on 1" thick sections of the steel. Most ASTM, AISI, and other institutional testing and evaluations are based on large, heavy, thick sections of the steels, and knives are none of these. So knives differ from rated and recommended processing because they are relatively thin. Knives in quenching must be handled quickly, with speedy movements and well laid-out and well-designed equipment helps a lot in this.

I remember having a relative in the studio heat treating his own knife. He was moving slowly, methodically, carefully, gently grasping the wrapped blade from the furnace, leisurely carrying it over to the table, cutting off the foil wrap, digging inside to extract the blade, all while I'm telling him, "Hurry, hurry!" He did not rush, and the steel quenched too slowly, and was barely hardened. We had to heat treat again. Speed is essential, and fluid, continual movement between the mediums is critical. Quenching has to be planned, thought out, even scheduled, particularly if multi-stage quenching to cryogenic temperatures is part of the process.

Specifically, in cryogenic quenching, the sudden and drastic exposure of the steel to shallow or deep cryogenic temperatures can impose such stresses and shock to knife blades that they can crack. Even if the crack is not visible to the human eye, too fast of a drop in temperature can have a detrimental effect on the actual crystalline structure of the steel with microscopic fractures, and that failure will present itself as high wear of the steel, and a markedly less-than-optimum condition. In order to quench at an effective and continuous rate, quench staging can be employed. Depending on the steel, an initial quench based on the medium (oil, air, water), followed by freezing to below zero, and then slow cooling to shallow cryogenic temperatures, and finally deep cryogenic temperatures, if required. The rate must be controlled carefully, and each type of quenching has specific means, specially designed devices, and equipment to control this rate so the cooling is continual, even, steady, and uniform for the specific cycle and range.

What is the specific rate of cooling below room temperature for most of these steels? 4-5 degrees Fahrenheit per minute. That means in order to reach -100°F, it should take about 40 minutes (from room temperature), and to reach -325°F should take about an hour and a half (from room temperature). This is why simply dipping blades into cryogenic baths of dry ice and alcohol or liquid nitrogen is a huge and destructive error, yet knifemakers who are uneducated in this process frequently do this, and tell others that it's the proper way to quench! Sad, truly sad for the knife client. The cryogenic process cooling rate is critical.

Like too fast of a quenching rate, too slow of a rate is not as effective. Remember that quenching quickly is the goal, and a continuous curve in the downward temperature scale is essential. This is because allotropes converting are metastable, and it's important to continue the cooling process without delay. While not as damaging or limiting as quickly dropping the blade into the deeply cold environment, the blade does need to keep cooling at a good rate. Being that knife blades are thin, relatively small pieces of metal, the cooling rate should be as fast as possible while adhering to that 4-5 degrees per minute Fahrenheit rate. Another consideration is uniformity in the cooling jacket, environment, buffer, and cycle. Once the rate is established and reliable, other factors can be tweaked, adjusted, and changed for a variety of distinct performance levels in the custom shop.

2. The Quenching Temperature: Again, a critical factor. This one is verified by pyrometer, and it's great to live in a time when we know, or can know, precisely what temperatures our blades are quenched to and when. In multi-stage quenching, each device or environment has a known temperature, and the instruments that measure this should be regularly verified and calibrated. This is another factor that doesn't typically  happen with "thermo-optical emission viewing" (looking at colors in hot steel), typical in hand-forging works. The actual temperatures reached may be recommended on the white papers, and most steels have extremely narrow ranges of required temperatures (as quenched to) necessary for the expected results. Some of these steels quench in stages, some may have to be interrupted or even held at intermediate temperatures during the quench cycle.

3. The Quenching Hold Time (Cryogenic Aging): This is an extremely important factor, as steel transformation does not happen all at once. Curiously, one textbook written in 2006 claims that the hold time at quenching temperatures does not matter, simply that the temperature is reached. This is totally in error. It has been proven time and again that cryogenic aging is critical, and some studies (Lal) have found that the length of cryogenic soaking is more important than the temperature of cryogenic medium! The quenching hold temp/time is critical to success of the process, and special means and devices must be employed to sustain this. Most early experiments had wide and inconsistent results, and it was traced to this critical "hold time soaking," which varied so much amongst the scientists and metallurgists that they weren't even sure cryogenics was worth the trouble!

Key studies have shown that it is not enough to cool the steel, but that it must hold a good long while at these low temperatures to realize the benefit of cryogenic treatment. Carbide development, nucleation, and precipitation is a sluggish reaction, and steels continue transformation for a substantial period after reaching the lowest temperature. Experimentation has shown the limits of this and what is also too long to be reasonably beneficial. What are the cryogenic aging times? 6 to 36 hours or more, depending on the steel chemistry, size, geometry, and expected results. This is another reason to keep the process in-house, to assure times are met and not shorted for the sake of economy.

People have asked me about what appears to be a conflict of process about the hold time at cryogenic processing. Here's a part of an email from a man trying to navigate his way through the processing:

I can see how this can be confusing, and I want to be clear. When I claim that the soak time is 6 to 36 hours, it's a generalized statement. I'll break this down for clarity:

First, this is a minimum recommended soak time, and actual soak times may be much longer. In my own work, if it's possible, I've maintained some blades at 60 hours or more. The time is the minimum, and what happens is that the improvements taper off, and eventually, it becomes more costly to hold the knives in this environment than realize additional benefits which would offset the continued cryogenic soak. Refrigeration, liquid nitrogen, the cost of continuing operation of the processor, and the practice of creating the super-cold environment outweighs what beneficial results happen. After a few days at this temperature, not enough gains are realized to justify the expense. So, as long a soak as possible, but don't put the steel away for weeks, there isn't enough improvement after a few days to justify the cost of the process. Percentage points of improvement are noticeable, but when fractions of a percent take days to achieve, it's not worth the cost.

The second part of this statement is the wide range (6-36 hrs.) I've included this range because of the variety of steels used for knife blades that may require cryogenic processing. Not all alloys are the same, and each one requires a different regime. I keep a detailed log of all my process operations and results, and that's the best way to hone in on specific, repeatable processing.

The third bit of confusion is understandable. In one section, you read that tempering cannot wait; it needs to happen immediately, and then you read that the blades are held in the cryogenic processor for days before tempering! The statement that tempering cannot wait is first based on studies of conventional heat treatment. Quench a steel like D2 to room temperature, and you'll see a hardness of 62HRC, but leave it at room temperature overnight and it will be 52HRC! So, in conventional heat treatment, you can see how tempering must not wait. The longer the time the steel is kept at a temperature between room temperature and 100°C (68°F to 212°F) after the complete transformation of martensite, the more likely the occurrence of quench cracking from the volumetric expansion caused by isothermal transformation of retained austenite into martensite.

What about in cryogenic processing? It's important to understand that room temperature is not cryogenic temperature. Leaving a blade at a static 68°F is not the same as holding it in an environment of -325°F. At room temperature, allotropes are metastable; they will change in time with (or without) mechanical pressure; this metastable nature is why it's critical to get tempering underway immediately. At cryogenic temperatures, steel is not metastable to any degree that would affect a change in the structure, apart from compression which forces precipitation of carbides at nucleation sites. In other words, conversion is still taking place. Once the steel blades are warmed to room temperature; all bets are off, and tempering must take place immediately, or the allotrope will convert. Again, the longer the time the steel is kept at a temperature between room temperature and 100°C (68°F to 212°F) after the complete transformation of martensite, the more likely the occurrence of quench cracking from the volumetric expansion caused by isothermal transformation of retained austenite into martensite. I hope that clearly explains the differences between holding at room temperature and compressing and converting at cryogenic temperatures.

Warming before tempering

The warm-up rate for the blades from deep cryogenic temperatures (-325°F) to room temperature before tempering is important to know. This is done by just letting the blades sit in still, room temperature air. On this page and several others of this website, you'll see photos and a video of my blades on racks with condensate (steamy-looking cold air) falling off of them or blades on a rack covered with dense ice. This is how they warm up. It's not a good idea to slowly warm them (leaving them in the cryogenic environment to slowly reach room temperature as the processor or environment warms); this may allow metastable austenite to convert. Conversely, hurrying the process along by sticking them in a pre-warmed oven is also not a good idea, since this is highly stressful. The balance between these is simple; they are pulled from the cryogenic environment, sit in still room temperature air until condensate on them turns liquid (above 32°F), and then moved into a cold oven to evenly ramp up to first tempering temperature.

How these factors contribute

To know just how each factor contributes to the steel improvement, I'll cite the studies from a scientific test of a low carbon martensitic stainless steel used in piston rings.

• The wear resistance was improved 43%
• The cryogenic soaking temperature was the most significant factor, contributing 72% to the increased wear resistance
• The cryogenic soaking time (cryogenic aging) was the second most significant factor, contributing 24% to the increased wear resistance
• The cooling rate was the third most significant factor, contributing 10% to the increased wear resistance
• The tempering temperature actually showed little significant change to the wear resistance, contributing only 2% to the increased wear resistance.

So what does this study show us? First, considering the steel used by the scientists, this is a fairly low carbon martensitic stainless steel, and I wish they would have done their test with a higher carbon alloy. What we do know is that the effects of this treatment are even more intense and amplified in the higher alloy steels, and this is one of the lowest possible improvement rates. Even so, it shows that the cryogenic temperature was the most critical factor, followed by the aging (soak) time, then the cooling rate, and finally variations in tempering temperature played the lowest role. Remember, the result we are looking at is increased wear resistance, and this is why temperature, time, rate, and overall processing is important to knife blades, as wear resistance is the characteristic we are enhancing. What are the improvements in higher alloy steels? They can be up to eight times the wear resistance of conventionally treated steels!

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Why are cryogenically treated steels better?
What, exactly, happens during this process?

The sections above have outlined the particular factors of cryogenic quenching in a general way. No specific results were described, and if you wish to dive into the technical side of what happens and what we know (and some of what we don't) this section will help clarify why knife blade steels that are properly cryogenically treated are better performers overall.

Cryogenic treatment results offer more than just a larger volume of martensite in the steel; the cryogenic treatment even increases toughness, which is counter-intuitive to most conventional ideas of what happens during this long, cold cycle. Modern study and the capability to examine the micro-structures of steel with improved microscopy and related testing equipment have given us new and continual insight into this amazing process. The process is so fascinating, an in such an evolving state that new research is needed and is currently underway. This means that modernly processed knives are not only the best they have ever been in history because of alloy content and manufacturing methods, but also the best because of treatments available. With equipment crossovers and secondary market of equipment sales, these processes are available even for small volume knifemakers. What an exciting time to be making knives!

First, let's look at high carbon steels (hypereutectoid steels) for the basis and reason for sub-zero (SZT), shallow cryogenic (SCT), and deep cryogenic (DCT) treatments. It's important to know that the most significant improvements in steels are achieved in shallow cryogenic treatments, and somewhat less dramatic initial improvement in deep cryogenic treatments. Further enhancements happen in long, extended cryogenic equilibrium soaking, and even more improvements happen with cryogenic sequences between temper cycles.  Since the exact results vary depending on the steel type, equipment arrangement, and protocols for each phase of treatment, this is another reason for the knifemaker to heat treat his own blades.

• Conversion of retained austenite: In high carbon steels, the main reason for cryogenic treatment is a larger proportion of austenite-to-martensite conversion. More austenite is converted to martensite, less retained austenite (RA) remains. This has been proven by x-ray diffraction, and there is no dispute that higher percentages of martensite create much more wear-resistant cutting tools, even after tempering. Martensite conversion occurs over a range of temperature, and on steel charts is designated by M_s (martensite start), and M_f (martensite finish). Steels with more than 0.3% carbon have a complete conversion of martensite temperature (M_f) that is below zero degrees Fahrenheit. So steels that are not quenched below zero that have more than 0.3% carbon will have significant retained austenite at room temperature (the temperature at which we use our finished knife blades). Even the low end of the knife blade family, eutectoid steel, with 0.8% carbon has a M_f of -50°C (-58°F)! The more carbon, the lower the M_f temperature is, thus the lower the temperature at which complete austenite conversion occurs. This is true then, for all hypereutectoid steels. What is the percentage? Depending on the steel and treatment, the retained austenite can vary between 50% and near 0%.
  
  Why is austenite a problem? We know that it forms at a critical temperature, and we know then, that (from our equilibrium chart) that it does not exist normally at room temperature, so it's metastable. Metastable means it is not stable, and though somewhat stable in our current state, it will eventually decay. Austenite (while tough) is soft, unstable, and its dimensional changes impart stresses in the structure. Heavy mechanical stress (pressure), and  temperature changes can induce additional transformation of austenite, creating dimensional changes and initiating cracks. In knives, this is not nearly the problem as, say, a ball bearing, but it's considerable, depending on the amount of RA. In the tool and die industry, RA is a negative, and a major cause for premature failure. While bearings and gears may work favorably with a 5% to 30% RA volume, a knife is not a ball bearing, and high hardness and wear resistance is critical, particularly at the thin cross section of the cutting edge. The lowest possible amount of RA is desired after quenching, if possible down to less than one percent of austenite retained. This is usually only produced by subzero, shallow, or deep cryogenic quenching.
• Precipitation of sub-microscopic carbides (η-carbides): When quenched to cryogenic temperatures and correctly aged, steels form eta-carbides (η-carbides). This is a very sluggish diffusion reaction, and another reason that cryogenic aging, or soak times, are critical to success. Eta-carbides are finely dispersed sub-microscopic carbides that tend to fill in areas of the structure giving it greater compressive strength, making it denser, harder, and tougher and more durable overall, improving wear resistance, strength and toughness of the martensite structure. These eta-carbides do not reduce in tempering, and can be physically measured by a particle counter. Technically, iron or substitutional atoms expand and contract, and carbon atoms shift slightly due to lattice deformation as a result of cryogenic treatment. Eta-carbides form in the martensite twinning structure boundaries, and have a considerable diffusing density. Some scientists suggest that these eta-carbides have a more profound effect on wear resistance than the reduction of retained austenite, in other words, they are the primary advantage of cryogenic treatment and the martensitic conversion is a secondary effect! It's is generally agreed that the eta-carbides offer a substantial measureable advantage as a result of cryogenic treatment.
• Long term cryogenic aging forces higher energy carbon atoms to move in the solution to areas of lower energy, coalescing in twinning boundaries, dislocations, and large-atom alloy location sites. This creates a more solid, denser crystalline structure, increasing toughness by interrupting dislocation energy, alignment, and position.
• Greater amount of carbides overall: In a recent study of A2 steel, in counting particles up to one micron, conventionally treated A2 has about 30,000 particles per square millimeter. Cryogenically treated A2 has about 83,000 particles in the same area. This closes the interstitial gaps making the structure more stable, stronger, and more able to resist loading, impact or force.
• Corrosion resistance improved: Because there are a greater number of carbides overall, cryogenic treatment improves the corrosion resistance of stainless steels. More chromium carbides are created, they are denser and more profuse within the material and this has been proven to produce a repassivization effect on the stainless steel surface. Additions of molybdenum (in 440C, ATS-34, CPM154CM and other high alloy steels) improves the repassivation effect
• Finer Asperity: Greater amount of complex, alloy and eta carbides creates finer asperity in the steel surface. This leads to a smoother surface, finer edges, and increased corrosion resistance of the steel.
• Material stabilization: The depth and range of heating and cooling (quenching followed by repeated tempering) increases the overall stability of the steel by thermo-mechanical compression. Like flexing a rubber band, this relieves micro-stresses in the metal, making it less likely to form what could become a crack or fracture. Though it's a simplistic comparison and while there are many highly technical reasons for this, I won't start listing them as it can get pretty deep into crystal morphology, transient states, enlargement of diffraction lines in crystalline lattices, and decomposition effects. Knowing that the average size of the mosaic crystalline lattice blocks increase with a decrease in cryogenic temperatures, and you'll get an idea of the relationship of matter states and temperatures. Add to that the stress fields being reduced in cryogenic treatment, and it's a clear advantage.
• Increased Hardness in Tempering: Cryogenic cycling between tempers completes the martensitic transformation that was started with retained austenite in the first temper. This hardens the steel more, creating additional hardening during tempers. This is not to be confused with secondary hardening that happens with some high alloy hypereutectoid stainless steels at a higher tempering temperature (Secondary Hardening). This is more easily understood as secondary effects of tempering transformation, and in practice, can be witnessed as an increase of hardness after each tempering cycle. This is only achieved through this multi-step cycling at cryogenic temperatures.
• Increased Wear Resistance: Carbon that was freed from the retained austenite crystalline structure in the first temper cycle conversion to martensite is available in cryogenic cycling stages to create further eta-carbides and critical transitional carbides that will coalesce in the second and third temper cycle. This will increase wear resistance as well has hardness, toughness, and corrosion resistance.
• Increased Toughness: The carbon that has moved from the retained austenite conversion in the first temper cycle relocates to lower energy areas, stabilizing the tempered martensitic structure, further increasing toughness in the third tempering cycle. It's been shown in some scientific studies that In some high chromium, high carbon alloys (D2) this third temper cycle increases toughness dramatically (up to 25%)!

With these factors, even medium carbon steels benefit immensely from cryogenic treatment, and the effects are more profound in the high alloy, high carbon steels.

Page Topics

Tempering

In order to better understand what happens in the entire cryogenic heat treating process, and to illustrate more specifics of the results, it's important to detail tempering. Tempering is part of the heat treating process that is performed after quenching and cryogenic aging. There is a lot of myth about tempering, and abundant misconception, confusion, and even error where the words "cryogenic" and "tempering" are used in the same phrase, for there is no such thing as cryogenic tempering.

It's noteworthy that some companies sell cryogenic processing equipment called names like "cryotemper" or "cryofurnace." These descriptors are their names for cryogenic processors married with tempering ovens that cool the metals to cryogenic temperatures and hold them at those temperatures for cryogenic quenching and aging, followed by draining of the coolant and slowly heating to tempering temperatures for automated tempering cycles. Understand that these are separate functions, all done in the same machine, thus the blending of the words "cryo" and "tempering." Still, there is no such singular operation as "cryogenic tempering."

Tempering is the process of re-heating the steel to force transformation of a percentage of the crystalline structure into another structure. The reason to temper is fairly straightforward. If steels (knives in particular) are left in an as quenched condition, they are far too brittle and unstable for working tools. All properly heat treated tool steels must be tempered to increase toughness and plasticity, and reduce brittleness and stresses. It isn't because the hardness is too high in untempered steels; it's the lack of toughness and the lack of thermal conditioning resulting in further crystalline microstructural changes in untempered or improperly tempered steels, and these changes can cause micro fractures and cracking, even on a microscopic scale. This presents as lower wear resistance, or a less than optimum cutting edge retention. Additionally, important carbides are formed in the tempering cycle, carbides critical to improved wear resistance.

Tempering is necessary to reduce stresses, balance the hardness and toughness of the steel, and increase wear resistance by the development of complex carbides. It works in several ways:

• Tempering transforms retained austenite (RA). This is necessary because retained austenite may not be stable. In cryogenic treatment, most of the retained austenite has been eliminated, but a small percentage will always remain. It is our goal as toolmakers to reduce the retained austenite as much as possible. After a blade is completed and is in use as a knife, retained austenite will convert to martensite if the temperature of the knife blade goes below the quench temperature. This is not likely in cryogenically-treated steels and another reason to use cryogenics. More significant is that retained austenite can convert to martensite under mechanical stress. Continually mechanically stressing the knife blade (typically at the cutting edge) may force the RA to convert to martensite. Not bad, you might think; martensite is a hard and durable structure. The problem is that martensite has a larger volume than the austenite it replaces, and this creates stress in the structure. What is the volume change? 4-5%! So room temperature stressing of these steels at the cutting edge, with retained austenite available, can create stresses that logically could lead to microscopic chipping or fracture of the micro-structure, or dulling of the cutting edge. The solution to this problem is to temper any fresh martensite formed, and this is why multiple tempers are necessary on all high alloy and hypereutectoid steels.
• Retained austenite contains a higher percentage of carbon (six times as much as martensite) locked into the crystalline lattice. This means that the carbon is unavailable for transitional, eta, and iron, and alloy complex carbide development. This results in lower wear resistance, lower corrosion resistance, and lower toughness overall.
• Austenite is converted to fresh martensite or bainite in the tempering cycle. Bainite is a plate-like structure of cementite and alpha-ferrite, and is harder than pearlite. It's also tougher than martensite so its presence makes the steel less brittle, without being soft.
• Untempered martensite is a metastable structure that decomposes when reheated. Martensite has a high dislocation density, and must be stabilized by tempering or the result is a stressed structure, prone to cracking.
• Unstable retained austenite leads to a shortened service life and fracture. This has been proven by many studies and is well-documented. Effective tempering reduces unstable retained austenite.
• Tempering reduces the stresses in the steel by thermal conditioning. It works by relieving quenching stresses by precipitating, coalescing, and spheroidization of iron carbide and other alloy carbides, giving the microstructure increased plasticity.
• Tempering must take place immediately! The longer the time the steel is kept at a temperature between room temperature and 100°C (212°F) after the complete transformation of martensite, the more likely the occurrence of quench cracking from the volumetric expansion caused by isothermal transformation of retained austenite into martensite.
• Cryo processing can not wait: if the steel is hardened, cryo must immediately follow and be part of the first quenching cycle. There are some companies claiming to improve already hardened and tempered tool steels with their treatment, but this only applies to thermo-mechanical cycling of the steel. While some RA may be converted, the amount is insignificant when compared to what happens on immediate quench.

Tempering Metallurgy Details

While the description above was fairly short and sweet, you might need to know what happens specifically in tempering, and why tempering metallurgy is so important in knives. It's the knifemaker's responsibility to control these critical steps in the process, because the design, geometry, and ultimate use and longevity of the knife exists in the tempering control. Miss the temper and no matter how well the blade steel was hardened, no matter how costly and time-consuming the cryogenic process, the knife blade will not be at its optimum condition. So many knives are shorted in this process, with missed steps, incomplete conversion, or inappropriate cycling steps, time, and temperature, that the cryogenic process of hardening is not recognized. Simply put, bad tempering practice will negate any well-executed hardening process. In this section, I'll describe exactly what happens during temper and you'll understand why this is so important.

What happens in tempering?

Stored energy of martensite—this is an important concept. The structure of martensite is carbon-rich. Remember that martensite is a body-centered tetragon, and thus has much less carbon locked into the crystalline structure than austenite which is face-centered. The sudden changes of quenching force carbon into the steel solution, which represents substantial energy. Carbon can migrate in steels even at ambient temperature because it moves so easily.

Once the steel has been fully hardened, with proper treatment, we can be assured that we have as little as possible of retained austenite. We have an extremely hard, brittle, penetration-resistant piece of martensite-rich steel, with abundant and profuse complex eta-carbides and alloy carbides.

How is temper set and what exactly happens during this process?

Temper happens in ranges and stages. These ranges are not specific points, but broad transition ranges that overlap considerably. The temperatures listed below are generalized for higher carbon steels, but it’s critical to remember that increasing alloy content will raise these ranges and retard the tempering process. The alloys that do this are chromium, silicon, manganese, molybdenum, and vanadium.

Range One: The first range is from 200-500°F (95-260°C), and specific ranges vary with steel alloy type. When the steel blade is heated to this range, more unusual carbon movement in the alloy occurs. The carbon atoms move toward lower-energy or “hungry” sites in the crystalline lattice, like dislocations. Some of this movement has already happened during quenching, particularly in lower alloy, lower carbon steels. In fact , in low alloy steels, 90% of the carbon has redistributed to these low-energy areas while martensite transformation was occurring. In high alloy, higher carbon steels, more substantial carbon migration takes place during tempering. Carbon moves to these low-energy dislocation areas and begins to coalesce during tempering. These atoms then group together to form transition iron carbides, mainly epsilon-carbide, which has a hexagonal structure and eta-carbide, which has an orthorhombic structure.

You might wonder why the carbon atoms wouldn’t simply bond with ferrite and form iron carbide like our cementite structure. Actually, this can happen, with the carbon directly converting to cementite, if the defect density is large. This is why it’s so important to eliminate defects with good steel hardening process, and using clean, high quality steels. You would be right, then, to question hand-forged and forge-welded (pattern-welded) damascus steels, due to the obvious high defect density created in making them.

Because the excess interstitial carbon has moved out of the martensite, the surface energy decreases; the martensite is more stable and less subject to fracture. In the higher alloys containing chromium, vanadium, niobium, and molybdenum, a small increase in hardening (supplemental hardening) can and should occur. This happens because the structure can be initially refined in tempering by filling of the dislocations with carbon clusters. At these cluster sites, alloy carbide precipitates of these heavier, larger atoms (Cr, Mo, Nb, V) grow. This is different from secondary hardening which happens in some alloys at a higher temperature range. At this stage, it's more of a refinement of the microstructure and initial building of the carbides.

Simply put, in the first tempering range, these things happen:

1. Movement, clustering, and enrichment of carbon atoms at defects and dislocations in the martensitic crystalline structure.
2. Martensite converts to a lower-carbon form as excess carbon atoms move out of the crystal, stabilizing the martensite.
3. Transition iron carbides begin to form at the clustered carbon groups
4. Hardness can actually increase slightly in some of the high alloy, hypereutectoid steels due to these carbon movements and complex alloy carbide development.

Range Two: The second temperature range is 400-600°F (205-315°C) and specific ranges vary with steel alloy type. In this range, some retained austenite can convert to ferrite and cementite, and this adds ductility to the steel. If the proper martensite finish temperature was reached, there will be little retained austenite, but there is always some, and it’s important to convert it to the more stable ferrite and cementite. Ferrite created in tempering is a tough, ductile component and the cementite is iron carbide and hard, but the important thing is that both are stable. For our particular use, in some high alloy hypereutectoid steels, some retained austenite can convert to martensite on cooling from tempering. In effect, this is like quenching, which demonstrates why it’s important to quench out of the first temper stage into cryogenic range, to make certain that complete martensitic conversion takes place as the martensite finish temperature is again reached. This also means a second temper stage must occur to temper the newly-formed martensite. Often, a third temper stage is required for complete conversion.

Simply put, in the second tempering range, these things happen:

1. Retained austenite is converted to stable ferrite and cementite, adding ductility.
2. In high alloy, high carbon steels, additional martensite is formed by quenching after the first temper stage.

Secondary hardening range: In high alloy hypereutectoid steels, often a secondary hardness effect occurs. This  happens mainly in steels containing molybdenum, chromium, and tungsten. Secondary hardening is a precipitation effect as cementite-rich carbides are replaced by fine alloy carbides. The temperature range at which this occurs depends on the alloy that is forming the carbides, but it's a relatively high temperature, often around 1000°F (538°C). This is a critical, beneficial range for alloys that must work at elevated temperatures and retain extremely high hardness, often around 70HRC. Some of the alloys are designed just for this critical range and application of extremely high hardness at elevated temperatures. In knife blades, this is detrimental. Steels treated for secondary range hardening suffer a tremendous loss of corrosion resistance which is usually why high chromium alloys are used in knife blades to begin with. They are also more brittle in this range.

Range Three: The third range begins at 500°F (260°C) and specific ranges vary with steel alloy type. Understand that in extremely high alloys, this temperature range is often above 1200°F (650°C). In this range, high complexity of movement and transformation occurs, and not all of it is good, particularly for steels to be used as tools! A transitional and metastable carbide, chi-carbide is formed. It has a composition that is between epsilon carbide and iron carbide (cementite). Increasing the temperature more, and the transition carbides start to convert and disappear, and are replaced by simple iron carbide (cementite). When this happens, martensite starts to lose its shape and changes from a tetragon to a body-centered cube. At dislocations, the cementite coalesces and nucleates, then starts to form spheroid shapes, which reduces the surface energy considerably. The ductility in the steel increases. The spheroidite grows, and smaller particles dissolve. The higher the temperature at this point, the softer the steel becomes. Eventually, as temperature increases, the martensite disappears as it’s converted to ferrite.

Simply put, in range three tempering, these things happen:

1. Transitional, metastable carbides form, and then begin to be replaced by cementite.
2. Martensite loses it’s shape and becomes ferrite.
3. At dislocations, cementite coalesces and forms growing spheroid shapes.
4. The steel hardness decreases, and ductility increases.

Ranges, Stages, and Cycles

The kinetics of tempering can be very confusing. Most knifemakers and heat treating companies simply heat the steel up for a while, and let it air cool, thus partially transforming some of the steel, and the result is adequate enough. This is not understanding or refinement of the process, and this is where the individual knifemaker who has knowledge of the process can excel.

• Ranges describe the temperature at which specific things happen in tempering. They vary depending on the steel type. Example: 440C's first tempering range is 300°F to 500°F (150°C to 260°C).
• Stages describe the condition of the steel as a result of tempering action. Example: After the first temper, the steel has tempered martensite, untempered martensite, and partially converted carbides.
• Cycles describe the physical process that the steel is in or has completed. Example: the blades are in the second temper cycle.

As each cycle of tempering is completed, the ranges determine the stages of transformation.

I hope that sentence makes sense, but if not, please read it again. The reason I am determined to get these concepts across is so that you who are reading this will understand that each cycle of tempering brings a certain and detectable refinement in the microstructure. After quenching, there is a certain condition, and after the first tempering cycle, there is a different and improved condition. The same thing happens after the second temper cycle, and these improvements can be detected. After the third cycle, an even further enhancement of the condition can be seen if the process is very good.

Few people look for or can notice these improvements, but they do occur when the entire process is refined, and the knife owner and user will definitely notice them in the condition of the knife blade itself as it is brought to its premium condition!

Alternate method: Or, you can just heat the blades up in a campfire of pennantia Baylisiana (the King's Kaikomako). This has to be done slowly under cover of darkness, because it's the rarest tree in the world, with only one living specimen. Then, certain chants must be performed from the Necronomicon Ex-Mortis (hats-off to Bruce Campbell). Quenching the blades in Westvleteren 12 ale, acquired from the gates of the brewery, will give you the perfect knife blade condition that we are all chasing.

And you thought tempering was so simple...

Snap Tempering

Performing a "snap temper" is tempering immediately after conventional quenching. The blades are quenched to room temperature, and then put in a tempering oven at an elevated temperature for the purpose of converting some of the newly-formed martensite to ferrite and cementite and bainite, thus softening the steel!

Why do this? It's done because in some cases, the stress of continued quenching to cryogenic temperatures can crack the metal. But this is only typical on large and irregularly-shaped metal pieces such as forming dies, which may have very thick and very thin areas. Knife blades are not forming dies, and it's been my experience that they cool uniformly enough that snap temper is not necessary, and is detrimental to the steel allotropes formed.

• The blade that is snap tempered does not have all of the austenite converted to martensite, and less effective and thorough conversion takes place, lessening the reason for cryogenic treatment overall.
• Some of the austenite is converted directly to ferrite and cementite (pearlitic structure) which is the softest, least wear-resistant structure of steel, the condition it was before heat treatment began!
• The carbon that would have been available to build complex carbides and alloy carbides will be, instead, bound in ineffective layers and clusters of cementite. Cementite (iron carbide) is markedly softer than complex alloy carbides of chromium, vanadium, tungsten, molybdenum, or niobium. Cementite is also a non-durable crystalline structure, this is why it flakes and pulls away and allows you to drill, file, mill, and grind pearlitic steel.
• The carbon will not be available to coalesce and aggregate in dislocations, and will not then be available to strengthen the structure.
• The blade will be more ductile, and softer overall, and be a lesser performer.

Several people have asked me if I ever had any cracking of blades in stress riser areas (around holes, filework, in radical geometrical changes of cross-sectional areas) in any of my blades. This is a great question, because heat treating contractors will tell you that this is the reason to perform a snap temper. I'll be very clear on this: I've never, ever, had one single crack of any kind in cryogenically processing any knife blade steel! Not a single incident. Take a look at some of the radically shaped blades I make, even with full engraving, and know that not a single incident of stress fracture has occurred. This is because metallurgical references state that snap temper is done for these reasons:

1. For complicated machined works with thin and thick areas, complex dies where heating and cooling rates vary greatly due to steel thickness on the same object.
2. To hold the steel at room temperature, for the convenience of heat treating process batches and schedules, since leaving it untempered in any way would leave it in a metastable state.
3. To negate any possibility of extreme hardness in the blade for fear of cracking someone else's knife blade, lessening the liability and lessening the performance overall.
4. To add ductility to the steel so that quick quenching to extreme temperatures (cryogenic) can proceed without slow cooling.

From this, it's clear that since knives are not wildly complex dies with thick and thin areas, the reason to snap temper is number two, three, and four: convenience and liability. Know that this is detrimental to the final steel allotropes and condition.

I believe that heat treaters who do this do it because it's easier and safer to take this step so that the likelihood of fracture is lessened, since the blades do not belong to them, and they don't really know the alloy makeup or properties (as they are simply told this by the person who sends them the blade to heat treat). Also, consider that heat treatment contractors do not typically know who the manufacturer or foundry of the steel is, and there are some variations of treatment between steel blades coming from different sources.

One more consideration. It may be possible that the heat treating contractor is not using proper processing protocol. He may be simply dipping the steel right into the super-cold environment (usually liquid nitrogen) without following the 4-5°F per minute rule. If he snap-tempers first, it's unlikely that he will get a fracture since the steel is no longer in complete quenching, because it's partially converted to ferrite. This is not a good practice and demonstrates a failure in processing strategy.

Additionally, the longer the time the steel is kept at a temperature between room temperature and 100°C (68°F to 212°F) after the complete transformation of martensite, the more likely the occurrence of quench cracking from the volumetric expansion caused by isothermal transformation of retained austenite into martensite. In treating large batches of steel, the snap temper allows the heat treater to take his time in the process, by removing stresses that could be caused by volumetric expansion. He can fit the cryo portion of the process into a more convenient time schedule. This may be particularly necessary if different types of steel are heat treated, since the furnace times and temperatures are different, yet the cryogenic treatments are the same. To do large batch processing, it's simply more economical and safe... for the heat treat contractor. This is detailed in the topic about batch processing and cost factors below.

For these reasons, I believe that performing a snap temper is a safer, cheaper way for the outside heat treating contractor to reduce the possibility of fracturing someone else's knife blade, and thus, incurring financial loss. It also may be a way to hurry the process, costing the contractor less to treat the blades. We can rely upon scientific testing that shows that snap temper, when necessary, permits less conversion to martensite, a lower density of martensite, and a lower density of carbides, curtailing performance, simply for the sake of economy. This is no way to treat a premium knife blade steel.

This makes me wonder, then, if this is another source of why so many are skeptical of the process overall. Just like the inadequacy of a proper cryogenic aging time, results can be less than optimum with a snap temper, which is usually only necessary for irregular shaped items with both thick and thin areas, like metal forming or plastic injection molding dies, or when the timing of immediate cryogenic processing is inconvenient for the person processing the knife blade.

Multiple Tempering

Tempering several times or multiple tempering is a critical process application, and should be carefully researched and performed according to manufacturer's white papers and based on the design criteria of the knife's actual expected use and geometry. Why is it important to have multiple tempering operations?

• In high alloy hypereutectoid steels, the first temper can create fresh martensite from the small amount of retained austenite. This martensite must be tempered in additional cycles.
• Bainite plays a role in the overall structure of some knife blade steels, as it is formed by decomposition of any retained austenite during the second temper. This increases the toughness of knife blade steels, so it's critical that the at the least a second temper is accomplished for increased toughness.
• Fine precipitates of carbides are formed in tempering, as martensite is converted and the carbon moves to nucleation sites of these carbides. Double tempering assures more complete carbide precipitation and softens the stiff martensite for a workable hardness with less brittleness as the high energy carbon atoms are moved out of the martensite and into dislocation areas.
• Cyclic conversions and Supplemental Hardening: There are studies indicating that the second tempering induces further relief of stresses induced not only by the hardening process, but by the conversions that happen in the first tempering cycle. The reason for this is because tempering at low temperatures only affects the martensite, and tempering at high temperatures also affects the austenite. After the first tempering cycle, the microstructure consists of newly formed and newly tempered martensite, carbides, with some retained austenite. During the second temper, newly precipitated carbides are formed and, along with the newly formed martensite, harden the steel overall. This is why in many high alloy hypereutectoid and stainless steels, there is an increasing of hardness during the course of several tempers, fully affecting these changes. The result of this can easily be seen; after each temper cycle, the hardness actually increases, which is contrary to what happens in low alloy and hypoeutectoid steels. It's a supplemental hardening effect seen in tempering cycles.
• The cooling process between the tempers is also critical! This is something a lot of knifemakers simply ignore, yet it's a critical process application. Since there is newly formed martensite, it makes sense that between heating cycles, the martensite conversion temperature must be reached, which means a low temperature hold, below the M_f temperature.
• Sometimes, three tempering cycles are required, depending on the steel type and geometry. High speed steels with high carbon require this, large or complex forms require this, and if high stability is desired, it's good practice. D2 can exhibit a 25% increase in toughness when triple-tempered with sub-zero holds between!
• One more very important point about tempering. Most tempering ovens have notoriously wide hysteresis bands. This means that they heat, and reach a temperature, and shut off power to the heating elements, and then cool until a lower temperature at which the elements are turned on again to start heating. Sometimes, this band (hysteresis band) can be 100°F wide! This leads to very bad process control, and inaccurate and widely variable results. This is the same reason you don't want to use a home food oven, toaster oven, or any other inaccurate oven for tempering. These ovens are not accurate, and the heat inside the chamber is not even, leading to poor and inconsistent results. This is why I my studio, I've switched all of these controllers to PID logic controllers, accurate to one degree Fahrenheit in laboratory grade ovens.

The properties of these high alloy steels are dependent on their individual microstructure and lattice components, which are created and refined during the entire heat treatment cycle. The final hardness alone is not the determinant factor of that microstructure, the entire process is, and from this section on tempering, you can see the role that accurate, meticulous processing plays in that structure and the ultimate durability, longevity, and value of the knife blade.

Page Topics

How Important is Carbide Development vs. Martensite Development?

The entire reason for having most of the refractory alloys in tool steels is carbide development. If a knife or cutting, machining, pressing, shearing tool is made of plain carbon steel (standard steel) or low alloy steel, no matter how high the carbon content, the focus of the steel's makeup and the heat treating is directed toward martensite development first.

Let's look at standard steels (plain carbon steels) to establish a starting point for this idea. In these steels, martensite is the desirable allotrope, and in order for the highest amount of martensite to be developed from austenite, all of the standard steels with carbon content over 0.3% must be quenched below zero degrees Fahrenheit. This means that standard steels with the designation 10XX, starting with the steel 1035 and up, must be quenched to well below freezing to reach the martensite finish temperature. No matter the quenching medium—oil or water—these steels must continue quenching to below freezing for complete conversion. Understand that most knifemakers don't know this, and don't do this, so they are settling for reduced performance and retained austenite, an undesirable allotrope.

In the standard steels, 1084 is the eutectoid steel, with 0.8% carbon, and I wouldn't even consider making any knife blade with a standard steel with less carbon content. This is because hypereutectoid standard steels (1086, 1090, and 1095) have excess carbon that encourages the development of iron carbides. Beyond the development of martensite is the creation of iron carbide from this excess carbon. In hypereutectoid standard steels, martensite plus iron carbides are developed, and in hypoeutectoid standard steels, martensite and ferrite are developed. Clearly, for knives, hypereutectoid standard steels offer vast performance improvements over hypoeutectoid steels.

If martensite and iron carbide offers good hardness and wear resistance in tool steels, why bother with the complicated and expensive process of cryogenics to create carbides of alloys? Why are high alloy tool steels created at all?

Alloys in steels are put there to improve many properties. What are they?

1. Elastic strength: tool steels must have increased and significant stiffness, controlled by the alloy content, geometry, and heat treating method. More about elasticity here.
2. Wear resistance: the longevity of any tool is borne in its resistance to wear abrasion. Longevity is rarely mentioned in lesser knives or manufactured knives.
3. Edge retention: this is technically different than wear resistance, and since you're on the advanced page now, I'll clarify this. The knife must be capable of being sharpened, and the edge must be durable and resist blunting. Wear resistance (above) is partially responsible for that, but edge retention is a combination of hardness and the machinability of the steel via treatment and alloy.
4. Toughness is the ability for the steel to absorb energy before breaking, and in a simple way, it can be considered the resistance to fracture. High alloy steels are nearly always tougher than standard steels, carbon steels, or low alloy steels when properly applied and heat treated.
5. Shock resistance is the resistance to rapid loading of the steel. Many people think of this as impact resistance, but it's actually the ability for the tool steel to absorb the energy of the force without breaking.
6. High temperature stability is important in all tools, and in knives you might think that since they are mainly used in ambient temperatures, this isn't as much of an issue. This is true, except for many of alloy steels that have low temperature tempering ranges. Some of them can't be exposed to boiling water without reaching this range!

Carbide development improves all of these properties. Carbides developed in dislocation areas and grain boundaries improve elastic strength (preventing deflection) and shock resistance by inhibiting crack propagation. Mainly, they improve wear resistance and edge retention. They do this because they are hard, much harder than iron carbides, much harder than martensite. Here's a simple chart that details this clearly:

From this, you can see that carbide development is critically important in tool steels, as all of the carbides are much harder than iron carbide. The Rockwell scale is not linear, so these hardnesses are substantial. Consider that there are multiple types of carbides with one elemental alloy, and these all differ in hardness. Also, some carbides are developed in multiple elements, such as chromium-vanadium carbides  or chromium-molybdenum carbides that have tremendous hardnesses.

You might wonder why some of the alloys are used in tool steels and not others, and some are limited. This is because they have other properties or limitations. Niobium, for instance, is very limited in solubility in steels, and tungsten offers high heat resistance. Molybdenum offers increased toughness and chromium vastly improves corrosion resistance. Titanium looks promising, but inhibits the growth of chromium carbides. There are numerous properties to consider in any high alloy steel.

The important thing is that all of these alloy carbides are much harder and more durable than simply iron carbide, and much harder than the martensite if a knife blade is tempered to 58HRC, 60HRC, or even 62HRC. Carbide development surpasses martensitic structure when considering all of the properties desirable in a good tool, and a good knife blade.

Why wouldn't a knifemaker offer these alloy steels to his clients? This, then becomes the question.

What are the drawbacks, the limitations, the restrictions in using these alloys and directing heat treating toward high carbide development?

1. Cost: high alloys cost more. They are more expensive to buy, and even more expensive to heat treat correctly. They are more expensive to grind, to machine, and to finish. They are definitely for higher end, higher performance knives.
2. Technical knowledge: from reading this page, you can clearly see that in order to get the most from high alloy steels, plenty of knowledge is necessary to understand the processing.
3. Experience: Beyond knowledge, years of working with high alloy steels will train the knifemaker. Notebooks full of process results and findings are derived from a tremendous investment of time actually heat treating knife blades. Beyond that, the feedback from clients that actually use the knives in the most demanding of environments and applications, like counterterrorism, marine rescue, combat, or at the hands of a professional chef, give feedback that is beyond measure. All of this is only available thorough experience, through heat treating many knife blades, making many knives, for many years. There are no shortcuts or educational venues that can replace experience.
4. Equipment: these high alloys take specialized equipment to correctly process them. This equipment is expensive, and takes considerable real estate, expendables, and energy in the professional shop or studio.
5. Sharpening: I thought it was important to include this factor. Many knife clients want to sharpen a knife the way it was done in 1950, with stones made of silicon carbide (standard sharpening stone) or aluminum oxide (India oilstone) or various rocks (Arkansas stones, Japanese Water Stones, etc.) If a knife user is set on this, high alloys are not going to be pleasurable to sharpen. Nowadays, we pros use diamond stones. They are easier, cleaner, faster, and better overall than rocks and other fused and sintered abrasives. But they are expensive, and if a client is going to spend the money on a high alloy tool steel blade, he is best served purchasing a diamond stone to sharpen them. High alloy steels hold an edge an incredibly long time, and with a proper diamond stone, take just a few strokes to sharpen.

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The Straw Man of Toughness

There are a lot of misconceptions in this field, and knifemakers themselves spread most of the myths about toughness. How is it that a misrepresented proposition applies to knife blades and toughness? After all, aren't steel properties well-known and proven by testing?

Here's how this goes: in hardening and tempering steel, a balance between hardness and toughness must be reached and applied for each type of steel, each particular geometry and design of knife blade, and the intended use (application) of the knife by the knife user and owner. Generally speaking, if steels are treated to be harder and more wear-resistant, they lose toughness. If steels are treated to be tougher and more fracture-resistant, they lose hardness (and thus wear resistance). You can get a basic idea of this physical trait of balance on my "Blades" page at the bookmark, "General Hardness Table."

The knifemakers and knife supply companies (mostly on forums) perpetuate a straw man argument. They do so because they believe or insist that there is a zero-sum game in knife properties. They claim that if a steel is made to be hard and wear resistant, it must be less tough. Conversely, if steels are made to be more tough, they must sacrifice wear resistance. Ergo, all hard steels are brittle, all softer steels are tougher than the harder steels. This is a gross generalization that is perpetuated to sell a particular steel, usually a lower alloy or heat treatment protocol overall.

1. The first misrepresentation is that they believe or present that all steels are the same. Yes, I know this sounds ridiculous; all steels do not have the same properties. If you look closely at who is making this claim, you'll find that typically, but not always, they are selling a particular type of steel and that steel is low alloy, or worse, low carbon. They are selling steel billets, raw stock, pattern-welded damascus, or finished knives made of lower alloy steels, and need to claim some advantage over higher alloy steels since they do not use them or sell them. It's advertising 101. Claim toughness is a problem in high alloys, and that is an easy enough argument to defeat, because knives need to be tough and resist breakage. Lacking toughness? Who wants that? This is the straw man: toughness is lacking in high alloy steels. This is absolutely untrue. Take a look a this very simple chart on my "Blades" page for some reality about fracture toughness.
2. The second misleading feint is that they believe or present that tough knife blades are more desirable than hard and wear-resistant knife blades. This is a flawed interpretation based on the assumption that a knife is going to be used as an impact tool, and not a cutting tool. If a knife is used as a hatchet or a hammer, it needs to be tough, not hard. If a knife is to be used as a cutting tool, it needs to be hard and wear-resistant, so it doesn't dull. In my 40+ years of knifemaking for the top fields that actually use knives (including top chefs, outfitters, military, and counterterrorism professionals) not one person has requested an impact-delivering tool first and a wear-resistant cutting edge second. They always request the properties of a tremendously durable and long-lasting cutting edge with extreme  wear-resistance first, and corrosion resistance second (related to the longevity of the cutting edge) and then toughness third. The people who claim high toughness is the most desirable property in a knife are almost always amateurs in their use of the knife. These are the guys who believe that a knife's main use is to chop wood, pry metal, dig and scrape and excavate and erect a log cabin in the woods with their knife. They would be surprised to discover professionals who use their knives to actually cut things. Knives to cut? What about two-by-fours and water bottles that need chopping in half? What about slicing a 2" diameter hanging rope? Where do you actually find a 2" rope in use, anyway? What about clearing land to build a bonfire in the dry forest? Hmmm? The straw man is that knives that lack toughness are less desirable, which is an absolute lie based on a premise that a knife is a chopping tool. If that were the case, knives would be made of softer, more ductile metals because they would never break, only bend, and bend easily. Sounds like a cheap machete, doesn't it?
3. The third bunch of straw is that they believe or present that a knife blade cannot be both extremely hard and extremely tough. They may not be aware of the fact that in high alloy hypereutectoid tool and die steels, hardness and toughness are available together in much greater amounts than in low alloy steels. Perhaps they are aware, but choose to ignore the fact that industrially, hard and tough steels used in the most demanding of machine applications are high alloy tool steels. While a knifemaker balances this, both properties are available to a higher degree in high alloy steels than in lower alloy steels This is why high alloy steels are made! Otherwise, why even bother if only one property (only hardness or only toughness) is allowed in any kind of steel? People who claim this apparently have little familiarity with high alloy hypereutectoid tool steels. They are, clearly, harder and tougher than low alloy or low carbon steels.
4. The fourth claim is an absolute lie. They believe or present that cryogenic processing reduces toughness, because it clearly causes steels to be harder. They may accept that cryogenic processing creates harder, more wear-resistant steels due to a more complete transformation of austenite to martensite and a more profuse and well-distributed formation of complex eta-carbides. They then may claim that because the steel is harder and more wear-resistant, it "by nature" must be less tough. This is the biggest error of all. Cryogenic processing dramatically and significantly increases toughness of all steels that are hardened and tempered, and it's provable, documented, and established with many years of testing by authorities, institutions, metallurgists, scientists, and engineers. Look up any study and you can see that toughness—no matter how hard the blade is set in final tempering—is dramatically increased by cryogenic processing.

Please think carefully about who is making this claim and why. On one forum, a person claimed to have contacted a metallurgist from a high alloy steel foundry. The metallurgist assured him that toughness was increased in cryogenic processing, and the guy selling the straw man argument insisted the metallurgist didn't know what he was talking about. The problem was not the steel; it was the metallurgist who actually worked for the foundry! Wow.

This happens a lot in knifemaking, and it's hard for technically-minded and educated people to take a knifemaker seriously who makes such unsubstantiated claims. In plainer English, this is one reason much of the public thinks of most knifemakers as stupid hicks.

Thanks for helping to stop misconceptions, wives' tales, and falsehoods in our tradecraft, science, and art through education. For more information, please read some of the references listed below.

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Austempering and Martempering

These are modifications of conventional heat treating involving interrupted quenching techniques, or more than one quenching medium. This is done to minimize distortion, prevent cracking, and decrease the potential for other stress and conversion problems. I will reveal that these processes have no place in hypereutectoid high alloy and stainless steel knife blades, but may have a place in lower alloy and lower carbon steel types.

Austempering is heating the steel to its critical transformation temperature, and then quenching in a hot medium, usually molten salts, that are high enough in temperature (above the M_s point) to form bainite instead of martensite. This is not typically desirous in knife blades, since martensite is the desirable allotrope. It is typically done industrially to increase shock resistance, not something necessary on most knife blades as wear resistance is diminished from conventional quenching and tempering.

Martempering starts the same, with quenching in a high temperature medium (usually molten salts), and then removing from the medium to allow the steel to cool in air, so that martensite can form.

Both of these processes are then followed by actual conventional tempering after quenching has completed, which brings me to a curious point. Why are these processes called aus-and mar- tempering? The are, in actuality, processes that happen in quenching, not actual tempering of the steel. So maybe they should be called ausquenching and marquenching, as this is more accurate to the step in which these modifications are performed! But this is the terminology, such as it is, and this may be another one of the reasons there is so much confusion in the metals trade about these terms!

In any case, both of these process modifications to quenching have a result. The elephant in the living room is that both of these processes result in high levels of retained austenite, most undesirable in knives! Retained austenite reduces wear resistance, reduces strength, and leads to deformation as the steel is placed in service due to the problem of mechanical transformation, dimensional variations, and distortion at room temperature. All of these results are unwanted, and totally unnecessary for the knife blade heat treating process.

Austempering and martempering do have their place, in ductile and white cast irons, in high silicon shock-resistant steels, low alloy carbon steels, and some specialty metals, but I don't see any advantage of either of these processes in the treatment of a durable, wear resistant knife blade. The only reason I can see performing this (based on extensive testing and scientific results published by researchers) is to make an inferior steel (1095, 52100) more shock resistant than conventional heat treating process.

When you read the advantage details of austempered and martempered steels from companies who sell this service, you'll see why it's done. Most of these use entirely automated processers, and that is why they are economically preferred. From the austempering and martempering industry, the advantages are:

1. More resistant to shock (the only really valid reason to do these processes, but when is a knife an axe or machete?)
2. Less distortion, distortion control (valid, but not a regular concern as a knife is not a metal forming die or a gauge block)
3. Clean surface for electroplating (not a knife blade concern or issue unless you're a factory producing tacky, chrome-plated blades)
4. Resistant to hydrogen embrittlement (not a knife blade concern)
5. Uniform and consistent hardness (this happens with any properly treated blade steel)
6. Tougher and more wear resistant (than conventionally treated low alloy steels, but research does not always support this claim, and if you notice, the comparison is to conventionally treated steels, not cryo-treated steels)
7. Hardness target: 38HRC to 52HRC (Crap! That's soft! No thanks!)
8. Greater ductility (I'll stop there, this is NOT something you want in a knife blade!)

The steel types typically austempered and martempered are SAE 1045 to 1095, 4130, 4140, 5060, 5160, 52100, and 6150, distinctly low alloy steel types. Since these steels are inferior in many ways to high chromium, high alloy martensitic stainless steels, I don't use typically use them. Now, when you see the process identified, you'll know more about it and its applications in the world of hand knives.

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Normalizing, Annealing, and Spheroidizing

These are, technically, softening processes for steels, but their application depends immensely upon the alloy type!

Normalizing

Read enough about knives and knife blades on the internet, and you'll come across the term "normalizing" sooner or later. Just what is normalizing and how does it work? More important, does it play any role at all in working and processing modern, high alloy and stainless knife blade steels?

Hey, just what is normal? With steel, everything is variable and changeable, so there really is no normal, so let's just get to the definition of the process of normalizing.

Normalizing is heating the steel to a temperature above the transformation range (where alpha-ferrite and pearlite convert into austenite) and then cooling it in still air. That's it.

What? Why? I will firmly declare that in working with high alloy hypereutectoid stainless and tool steels, we call this "hardening!" Yep, if you try this with any of these steels, they won't become normal, they will become quenched, and extremely hard.

So what is the purpose of normalizing? The purpose is to soften the steel and reduce stresses to make it more workable! What? I'll soundly declare that if you try this with these upper-tier high alloy, hypereutectoid and stainless steels, you won't be doing any work with them at all, as they will be in the lower 60s in Rockwell hardness, and a file and drill bit will just glance off the hardened surface. So in working with high alloy steels, normalizing has no place or purpose at all.

Normalizing is then for lower alloy steels and carbon steels.

 Normalizing is a process that's cheaper and a bit faster than annealing, but based on the same idea. You take the steel to its austenitizing temperature, and then cool it slowly. But "cool slowly" is a general term, and needs to be suited to the individual steel alloy. For instance, in one reference, normalizing is done at 100°F temperature drop per hour, in another just sitting in room temperature air. The purpose of normalizing is the same as annealing, to reduce stresses or hardened areas before machining or working the steel. But the normalized blade is not annealed, and the properties of the steel are not uniform (as in annealing). Because normalized steel is not uniform, stress are created, and then the blade may then need stress relieved. So normalizing is not a final condition, but a part of the working process of a typically lower alloy steel.

Normalizing is done with lower alloy carbon steels as a cheaper and faster alternative to annealing, since it doesn't take as long and is not as expensive as having a dedicated oven slowly lowering the temperature of the blade in many, many hours. Because normalizing is not really effective in extremely high alloy tool steels, and annealing is, the word normalizing is an indicator that the knife blade is a lower alloy type. In all my decades of making fine, high alloy tool and stainless steel knife blades, I've never had to normalize a single one of them. Oh, I've annealed a few, but I can count them on one hand. The important thing to note is that high alloy and stainless tool steels are not normalized, they are annealed, and diligent efforts should be made so that annealing is never needed.

Annealing

Annealing is full softening of the steel. In annealing, the steel is taken to its austenitizing temperature or a recommended temperature just below it, and then cooled very slowly, extremely slowly, to allow the equilibrium transformations to take place. Every process temperature, time, and step of annealing is different depending on the steel alloy content, and the white papers (technical data sheets supplied by the steel supplier) are a guide to this fully softening process. Annealing is done to create the most ductile, most malleable steel possible, for several reasons.

• One of the reasons to anneal is to reduce stresses created in machining steels. If you have complex machining or forming operations, stresses can be created and areas can be work-hardened with localized hardening making it difficult to achieve further machining operations. For instance, say you are drilling a hole in steel and overheat the area because of a dull drill bit. The area of contact can instantly harden, since so many of these steels can quench harden in room temperature air. Then, when you try to continue, the steel is too hard to drill, and it must be softened. With the steels I use, the only option is to fully anneal the blade, or use a drill that can drill through the hardened area, usually a tungsten carbide drill.
• Another reason to anneal is a full-on disaster, like a blade warping out of heat treat. It can't be straightened, it is ruined, unless you can fully soften the steel to straighten it, and start the blade treatment over with.

The important thing for me, as a professional. is to never have to anneal a blade in the first place! Both of these scenarios happen because of other failed steps or mistakes (bad process control or dull cutting tools) and I never, ever purposely want to have to anneal a knife blade. I'll also clarify that in some of these steels, full annealing is almost impossible; they stubbornly refuse to return to the state they arrive from the foundry (fully spheroidized and annealed). This is another reason so many knifemakers do not like working with high alloy tool steels and stainless steels; they are unforgiving of error or casual attention. They need to be made right, the first time, and processed once, for correcting an error may not even be possible.

Spheroidizing

Spheroidized metals are in their fullest, dead soft condition, and this is typically how they arrive from the foundry. The term spheroid refers to the spheroidization of the plates of cementite contained in the pearlite structure, making them big and round and granular and thus, ductile and easy to machine. Spheroidizing is a step beyond annealing, and is expensive to achieve, as it takes many hours in the furnace with extremely slow cooling so the equilibrium phase transition can take place. In spheroidizing process, the steel may need to be held for an extended time at the austenitizing temperature, and cycled in the higher ranges before extremely slow cooling results in a fully spheroidized structure.

 Usually, spheroidization is not necessary, in the decades I've been making blades, I've never had to attempt this on a single one. Since most steel billets arrive at the studio in this condition, they are already at their easiest working condition, dead soft, and as soft as they are ever going to be. This might be surprising, though, to those who work with low carbon or low alloy steels, as even in their dead soft condition, these high alloy and stainless tool steels are comparatively tough and difficult to machine.

Now that you know these three important conditions of metal: spheroidized, annealed, and normalized, you'll understand why steels are shipped from the foundry or supplier in "fully annealed and spheroidized" condition, the reasons for annealing, and why no high alloy steels and stainless steels are ever normalized.

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Cryogenic Processing and High Alloy and Stainless Steels

Stainless high alloy steels are the fastest growing steel types made. This illustrates how important these steels are to the world. Martensitic stainless steels constitute the majority of high alloy hypereutectoid steels I and other makers of fine handmade knives use, simply because they are the very best. Even without cryogenic treatment, their performance, strength, wear resistance, corrosion resistance, and durability overall surpass all lower alloy commonly hand-forged steels by many times and in all characteristics. High alloy modern tool steels, martensitic stainless steels, and powder metal technology tool steels benefit greatly from cryogenic processing.

While this information is still being studied, and not all of the effects are well-understood, it's clear that the performance of these steels is terrifically enhanced by cryogenic treatment. It's best to break these properties and results down into individual aspects that have been proven by studies and scientific experimentation:

• Because these extremely high alloy steels are heavy in carbon, their M_f transformation temperatures are sub-zero. This means that at the very least, they should be quenched below zero Fahrenheit to assure as complete as possible the transformation from austenite to martensite. Cryogenic treatments are the most effective for this transformation.
• Martensite is a critical component of cryogenically treated steels, and has a hardness of up to Knoop 800 HK. This is four times harder than annealed or non-treated steel, so it's important as the basis for high wear resistance to improve the amount of martensite overall through cryogenic treatment.
• Martensite plate size is something seldom discussed, but it's understood that a reduction in size of the martensite plates leads to a finer grain, more interlocking boundaries, and a harder steel. What kind of martensite plate reduction are we talking about with cryogenically treated steels vs. conventional heat treating? How about a ten-time reduction of martensite plate size? This is an order of magnitude and astoundingly demonstrative of the cryogenic effect. The smaller size means a harder, tougher, and more wear-resistant blade.
• Because of the transformation sluggishness of carbide precipitation detailed in the previous section, the hold time (cryogenic aging) at extremely low temperatures should be significant. In my past works, this hold time proved to be a beneficial result, and even though cryogenic temperatures were not reached, holding the blades well below zero for many hours (10, 20, or more) resulted in a superior blade performance. While this undoubtedly aided in the carbide precipitation, cryogenic treatments are much more effective at producing these results.
• With all carbides, their effectiveness depends on how fine they are, how well-dispersed, how high the volume overall that is precipitated. A critical point is that the three elements chromium, molybdenum, and vanadium have the highest solubility in austenite, therefore they precipitate the highest volume of carbides. This is why these three are big players in high alloy steels.
• Since there are so many elemental alloys included in these steels, dispersion of these elements within the material becomes a concern. In cryogenic treatment and aging, the element solubility decreases, so molecules move within the structure. The vacancies migrate, and concentrations of single elements disperse, leading to a more even distribution overall.
• More carbon moves around, bonding with chromium, creating a larger volume of chromium carbides (Cr₂₃C₆). Since all stainless and high chromium tools steels contain a large amount of chromium (440C, D2, ATS-34, 154CM, N360, CTS-XHP, CPMS30V, CPMS90V, CPMS35VN) and a large amount of carbon, significant amounts of chromium carbides are formed during cryogenic aging. Iron carbide (cementite) has a Knoop hardness of 1025 HK, but chromium carbides have a Knoop hardness of 1735 HK, 1.7 times the hardness, resulting in higher wear resistance. This is another reason that stainless steels are flat-out better performers than carbon steels, which have little or no chromium to form chromium carbides.
• Contraction is the physical process that takes place in deep cryogenic processing, in the aging cycle, and does not typically occur in shallow cryogenics. The austenite and martensite are so cold that they contract, which physically forces carbon to diffuse, resulting in a greater density of carbides and a more homogenized distribution of carbides.
• Many of these steels (like ATS-34 and 154CM) are high in molybdenum. Cryogenic treatment helps produce a higher volume of molybdenum carbides, and they are 1.8 times harder than iron carbides, leading to higher wear resistance. Molybdenum has been specifically proven to disperse and move within the crystalline structure at cryogenic temperatures, resulting in a higher volume of molybdenum carbides in high carbon alloy steels, and another reason that long cryogenic aging is critical in these steels.
• Some of these steels (O1, CPMS35VN) contain significant amounts of tungsten. Cryogenic processing increases the amount of tungsten carbides, which are 1.85 times harder than iron carbides, leading to higher wear resistance.
• Several of these steels (O1, CPMS30V, CPMS90V, and CPMS35VN) contain significant amounts of vanadium, and cryogenic processing increases the amount of vanadium carbides. Vanadium carbides are 2.6 times harder than iron carbides. This leads to a tremendous increase in wear resistance.
• Toughness: Since significant cryogenic aging allows more homogeneous distribution of the micro-carbides, and since the stainless and high alloy steels have a very large proportion of these carbides, low temperature thermal conditioning produces microstructural and crystallographic changes resulting in an increase in toughness.
• Hardness: Of course, a dramatic increase in hardness occurs in these high alloy tool steels when cryogenically treated. Consider that the blade will be tempered back, made less hard overall, during the tempering process, and that hardness doesn't contribute as much as in the initial quenched hardness. This is misleading for several reasons: first, because the blades are tougher and more resistant to fracture overall, they can be tempered to a higher hardness without being brittle. This means a much more wear-resistant blade. Secondly, the improvement of wear is non-linear; a ten percent increase in wear resistance offers a much greater increase in durability and longevity overall. When you consider that the cryogenic processes of these tool steels simply produce a higher hardness, that hardness translates to many times the durability and longevity of a tool used to cut.
• Resistance to cracking or fracture: Conventional considerations about steel suggest that harder steels are more brittle—see the "Straw Man of Toughness" section above. There is a mistaken, persistent idea that cryogenically treated steels are, even after tempering, more brittle and subject to cracking, but this is not the truth. Scientific metallurgical studies have proven that the abundance of microcarbides created in these high alloy steels assist in enhancing micro-stress distribution, improving (by reducing) fracture growth in the material overall. Simply put, cryo-treated high alloy tool steels are more fracture-resistant than conventionally treated or sub-zero treated steels.
• Fatigue life: Since the crystalline structure is improved overall in cryogenic processing, it is well-known and established that cryogenically processed steels (and many other metals) benefit from a long-term fatigue life improvement. In considering fatigue life, an important factor is the repetitive forces of stress over a long time, which is much different than a singular, initial force. Studies have shown that springs—particularly valve springs in high performance racing cars under high, continuous, forceful movement—have benefitted from cryogenic treatment with many times (up to 7 times) the life of conventionally treated springs. This translates to a longer fatigue life for the knife blade as well, particularly at the cutting edge, where tremendous forces and deflection are in play.
• Nickel is limited in these high alloy martensitic stainless steels. I mention this because of the critical effect of nickel on the austenite structure. Nickel improves ductility and machinability (not something you desire in a hard, wear-resistant knife blade). Worse, it's an austenite stabilizer. So it's likely that less martensite conversion will take place at sub-zero and cryogenic temperatures as nickel content is increased. Nickel is not typically alloyed in these steels, but in a few it has a very low volume and is used because it depresses the transformational temperatures (A1C and A3C) and shifts the proeutectoid and pearlite formation times to longer time periods. This may be useful in larger, thicker sections such as dies and metal forming tools, but those are not cutting blades.
• Wear reduction: This is improved in cryogenically treated tool steels by a proven reduction in asperity ( Dr. Sudarshan of Materials Modification Inc. and Dr. Levine of Applied Cryogenics). Asperity is the roughness of the surface, and when steels are cryogenically treated, the wear of the surfaces is typically reduced by half, even though the same polishing methods are applied! This is because the cryo-treated steels have less microscopic peaks and valleys, contributing to an improved polish, improved finish, and lower wear.
• Appearance: Here's a characteristic that you won't find discussed on any scientific paper, because these studies are concentrated on and funded with the intention of examining and improving the physical and material performance, not the appearance of finished steel. However, you will see it discussed in the realm of wear resistance in polished surfaces, focusing on asperity, and it's been proven that after cryogenic treatment, the surface can be highly polished, better polished, with less peaks and valleys leading to a smoother surface overall. Since only artists and fine craftsmen are typically interested in a tool steel's finished appearance, it is up to us to reveal what the steel looks like after cryogenic treatment as opposed to conventional heat treatment. It stands to reason that changes in the crystalline lattices of cryogenically treated steel would change the outward appearance particularly when finished to a high degree of smoothness, as in mirror polished knife blades. Here is what I know:
  
  High chromium martensitic stainless steels like 440C or ATS-34 are processed with conventional heat treat (CHT) or with sub-zero heat treating (SZT), and the steels are then tempered and finished by grinding and then polishing. These steels are beautiful in their own right, with mirror polishes showing some grain texture. These textures appear like a much diminished and less noticeable version of D2 steel's "orange peel" granularity pattern, seen when held in just the right angle of incident light. While D2 has a much bolder and profound pattern, this same type of effect is seen in ATS-34 and 440C, with curves in the pattern following grind terminations, trailing points, and other geometric features of the blade. If the blades are cryogenically treated, these patterns are tremendously reduced or disappear completely. The cryogenic treatment makes the finish of these two steels much more like the finish of powder metal technology tool steel, for example CPM154CM and CTSXHP. The surface is extremely clean and uniform, and little to no grain can be seen. This makes sense, considering the greater conversion of austenite, but perhaps more so the precipitation of fine carbides throughout the structure. Simply put, cryogenic treatment produces a more even, uniform, smooth, and beautiful finish than conventional heat treating.
• Corrosion resistance: This is a complex interaction response to an environment, so I've broken it up into subtopics:
  • Finer finish: The key to this first consideration is in the previous topic, appearance. Because cryogenic treatment produces the possibility of a finer finish (depending on the skills of the metal finishing knifemaker), it stands to reason that the surface is more corrosion resistant simply because the surface is smoother, with less irregularities, and fewer boundaries of different allotropes where corrosion or oxidation could start. This is believed to be due to a larger amount of microscopic carbides in spherical shapes, and a smaller, more refined structure overall.
  • Martensite: There are several differing opinions on the physical corrosion resistance of CHT vs Cryo-treated steels. One consideration is that since more martensite is formed, and martensite is less corrosion resistant than austenite, that the retained austenite would help to increase corrosion resistance. This seems logical, but it is not the case, even in conventionally treated steels. It is well known that conventional heat treat alone increases the corrosion resistance of all steels, particularly high alloy stainless steels. It's also known that the harder the stainless steel is after temper, the more corrosion resistant these steels become. In this comparison, it then seems counterintuitive that corrosion would increase if more martensite is left in the steel, since harder steels with more martensite are clearly more corrosion resistant.
  • Carbides: it's clear that the uniform distribution of micro-fine eta-carbides helped to decrease the corrosion potential of these steels, simply due to the increase in percentage of these carbides. The increased amount of chromium carbides in these cryogenically treated steels further enhance corrosion resistance throughout. Studies have also shown an interesting repassivation effect due to higher levels of chromium carbides occurs, further enhancing corrosion resistance.
  • Water contact angle testing: Here's some interesting stuff.Pure water (deionized) is used to conduct a test of CHT and CryoHT steels. In this test, the contact angle of a water droplet is used as an indicator of the steel's hydrophobic or hydrophilic response. Hydrophobic means having little or now affinity for water, and hydrophilic means having an affinity for water, or to be easily wetted. Of course, since water is the critical factor in most corrosion response, we would want a stainless steel to be more hydrophobic, or resistant to water. This resistance (or acceptance) of water is measured by the angle of contact of a water droplet resting on the surface. It has been proven that cryogenically treated steels exhibit a greater contact angle, and are more hydrophobic than CHT steels. This is believed to be due to large amounts of carbide precipitations in spherical shapes, which allows a smoother surface.
  • Lattice Size and Electrochemical Response: studies have shown that cryogenic treatment can reduce the overall lattice sizes, enabling better corrosion-resistance performance. This is because cryogenic treatment produces a material that is more dense and homogeneous, increasing the electro-potential resistance, enhancing corrosion resistance, and preventing corrosive media from directly penetrating deeper downward into the steel.
  • Residual Carbon: since cryogenic treatment produces more carbides and less residual carbon, corrosion resistance is further enhanced. This is due to the a greater amount of small carbides and crystal chromium homogenizing with the crystalline boundaries, with better corrosion resistance overall.
• Working and sharpening: Is the cryogenically treated knife blade a better performer? Why of course it is, and from the data presented on this page, it's easy to see why improvements of 100 to 800 percent in wear resistance lead to a blade that has much greater longevity, durability, use, and value.
  • But what about sharpening? What about the only service that the knife owner must perform himself during his life, and the life of the knife? Since steels are more wear resistant when cryogenically treated, it stands to reason that they would be resistant to the stone and harder to sharpen. They are. There simply is no way for a knife to be extremely wear resistant and yet be easy to sharpen; that's a myth. But along with advances in steel alloys, we have advanced in our sharpening methods and materials as well, and it's not as complicated as one might think. For instance, you don't need a power driven sharpener, a rack with sticks, a clamping guide, or any other gizmo to effectively sharpen the most modern, super-hard, super-tough high performance alloy tool steel blade. You need diamonds. Now, if you wince at the thought of diamonds and dollar signs go scrolling vertically through your eyes, please know that diamond hone sharpening is actually the most reasonable breakthrough for knives that has happened since these alloys have appeared. Nothing is as hard as diamond abrasives, and though you might think your cryo-blade is so hard it can't be sharpened, you underestimate the hardness of diamond. Diamond hones will last indefinitely if you take simple care of them: they don't change, don't wear, don't get curved, concave, or clog. The very best hone will cost about $75.00 at the time of this writing, and it will be the last hone you'll ever buy. Realistically, you don't need a whole range of grits, but it's nice if now and then you can get additional grits to upgrade your sharpening tools. $75.00 is a night on the town, a big tank of gas, or the cost of a very cheap suit. But unlike Arkansas stones, silicon carbide, aluminum oxide, India oilstones, ceramics, and other conventional types of stone, diamond cuts through all of these stones themselves; diamond can actually dress, grind, and lap the other stones. Diamond abrades through all steels, and will keep on cutting until the knife blade wears away, no matter what it's made of. You don't need oil, or water, or a rack, stand, frame, or electricity.
  • Working with cryogenically treated steels is a dream. This seems counterintuitive, since they are so hard and wear resistant. Please bear with me while I get a bit technically descriptive in a language that most metalworkers will relate to. This section is for those who will sharpen, and also for those makers who will grind, shape and finish a cryogenically treated blade.
    
    First, in sharpening, here is the noticeable difference: When using a diamond hone to sharpen, a conventionally treated blade will smear a bit. What this means is that the blade deposit left by sharpening (the swarf) seems a bit gooey, with larger strings of steel depositing on the diamond stone. They seem a bit clingy and sticky, requiring a vigilant cleaning of the stone so the blade has access to the sharpest of the diamond grits. Effective edge improvement stops at about 400-600 grit, with higher grits just polishing the edge and no marked improvement in edge quality. This is fine for most purposes, particularly when you consider that a medical scalpel is only finished to about 400 grit. But with cryogenically treated steels, there is a noticeable difference. The residue left by sharpening (the swarf) is very powdery and smooth. Instead of a clingy metallic deposit, it seems more like a fine hard powder. This is probably due to the higher martensite content and even more so by the high micro-carbide content of the cryogenically treated blade. Because this powder is very fine and easy to blow off of the stone, sharpening is a bit faster. More importantly, the powder effect extends into much, much higher grits (if desired) and this allows a sharpening up to 1200, 3000, or even 8000 grit. This leads to a super-smooth sharp blade, the smoothest and sharpest I've ever seen, and I've seen more knife blade cutting edges than just about anyone alive.
    
    Second, in working: this refers to working with the cryogenically treated steel blade, and is then more for other knifemakers than the public. After cryogenically treating blades, as a knifemaker, I have to finish them, usually to a bright mirror polish. Some tactical knives have an abraded, media-blasted finish, but in either case, they have to be ground. As with sharpening, conventionally treated blades leave a sticky, clingy residue to the abrasive belt, particularly fine grit belts in the 40 micron to 5 micron range. The cryogenically treated blade grinds easier, is stiffer to deflection (something not studied in cryogenic testing), and leaves a smooth, powdery residue. The only challenge is in mirror polishing, which is more difficult in the cryo-treated blade. But it's worth the extra work; these blades are beautiful.
• Unusual and remarkable effects: There are some strange effects that happen when a high alloy tool steel is processed cryogenically. Though these characteristics do not affect the typical doubling or tripling (or more) of the service life of the cutting tool, they are of interest to those who make knives particularly, but metalworkers in general, and are interesting and substantial.
  • Dimensional stability: It's proven that the cryogenically treated steel is more dimensionally stable, in that gauge blocks accurate to .0001" can be made and expected to stay that way for many decades, whereas non-cryo-treated steels will change. But there is another very interesting aspect of this treatment. When the knifemaker drills holes in the blade before heat treatment, after conventional and sub-zero treatment, the holes are ever so slightly larger in diameter (perhaps .0005" on a 0.125" diameter hole), and pin placement is easy with plenty of play. This is due to the volumetric change during heat treat. Properly cryogenically treated blades do not show "growth" and the holes are exactly the same diameter as drilled, which means they are tighter, more accurate, and less forgiving of error. So, in this case, it produces a tighter, more accurate metalwork, and the maker had better be paying close attention, as these dimensions can not easily be changed! In that way, metalwork with cryogenically treated blades can be more difficult than CHT or SZT blades.
  • Harmonic frequencies: The cryo-treated blade is a different allotrope, so it has a different frequency range of harmonics, or ringing. This is noticed when pins are driven and peened in bolsters, when the blade experiences the ringing effect of hammering or impact. Only knifemakers would probably notice this, but these blades will ring at a higher, more harmonious frequency than CHT or SZT blades. This hasn't gone unnoticed in the musical instrument field, and many instruments and bells are now cryogenically treated. The musicians who have this done report a more responsive, brighter tone with higher overtones. This is a specific process called resonance enhancement. It won't make your knife sing; it's just an interesting effect.
• Using cryogenically treated knife blades: Of course these blades are superior in use, just as sub-zero and aged blades are superior to conventionally treated knife blades. Sub-zero quenched and aged blades have been my mainstay for decades and their performance is well-established and appreciated by the thousands of knife owners who have used and sworn by them for decades.
  
  When I started cryogenically treating knife blades, I wanted to see something, something profound that I could relate to without all the scientific testing apparatus employed by all of the scholars, metallurgists, scientists, and institutions referenced on this page. I wanted to see some result I could relate to for anyone who has read this far and has an interest in this process. What would that be?
  
  In the shop, I heat treated a 440C blade with cryogenic processing. I finished the blade; it was a blade that was being donated (thanks to an anonymous client and my own personal contribution) to a United States Air Force Pararescueman for his use in combat rescue service and duty. I finished the blade and sharpened it to 8000 grit on diamond hones. I started cutting. I cut paper, cardboard, and wire, I cut hemp rope until I simply grew bored, and realized I needed a much more aggressive encounter. I wanted to make the blade fail. I was after a failure, a bending, breaking, or chipping of the edge. Dulling would be nice, too. So I started cutting other metals. I cut aluminum until, after realizing nothing was happening to the edge, I started chopping on the aluminum. Now, this is something I strictly state that hand knives are NOT designed for, in any way. But I wasn't getting anywhere in the cutting stuff, and I was intent on some demonstration of failure. Please note that this was an extremely thin cutting edge, something you won't see on just about any other maker's handmade knives and something never seen on any factory knife in this steel type and hardness. When nothing happened with the aluminum, I started chopping... brass. This was a 1" diameter hard brass,80B Rockwell, free cutting brass used for screws, bolts and fasteners, nearly as hard as mild steel, and some tough stuff. I whaled on it with the knife, chopping chunks out of it like a little axe! When I examined the thin edge, there were just a few tiny dings, invisible to the eye, but imperfections i could feel with my fingernail none the less. So I stopped, satisfied, and resharpened it. Then, I pushed it further. I clamped a block of mild steel in the vise, and started carving the corner of the steel block away with this very thin cryogenically treated blade. I wasn't hammering wasn't landing blows, but was applying heavy pressure to literally carve away the steel block. It cut the steel with ease, and there was no dulling, not one little bit, of the edge. Cutting steel. With a hand knife.
  
  I know this is an anecdotal account, simply a easy visualization, but sobering nonetheless. I don't expect any knife to be used this way, but it's encouraging to experience the result of cryogenic treatment, chopping a hard brass bar and carving steel. There is no doubt if I continued, I'd be able to chop or cut them both it in half. By the way, the blade was 440C, treated to shallow cryogenic temperatures, double tempered, with a final hardness of 60HRC.
• Cost: From someone who has created more cutting edges in his lifetime than most people have ever seen, I can soundly state that cryogenically treated blades are worth the effort to create a markedly superior knife blade in every conceivable way but one: economy. While inexpensive knife blades have their place in current mass-marketed and primitive hand-forged works, this is not the kind of knife I make, nor do I want to. Since the cost of creating a cryogenically treated blade is born mainly in the equipment and process, once that is established, the cost is fairly low. It takes more knowledge, equipment, more electricity, expendables, and time to create the superior cryo blade, but it's something that my clients deserve, so that is what I offer. And, as with all knifemaking, it's a fascinating advanced process that the very finest steels deserve. Below are how some of these steels individually benefit from this process:

O1: This high alloy oil-hardening tool steel is a standard in the industry for a reason. It's a great hyper-eutectoid tool steel with about 0.9% carbon and the version I use has high tungsten and vanadium with a bit of chromium, though not enough to be stainless steel. I use it when clients want a great performing black colored blade, because the finish and bluing is excellent on this steel. On one website about steels used in woodworker's tools, the writer claims that because O1 has a higher martensitic conversion than other steels, cryogenic treatment is not effective. This is flat-out wrong. While O1 does perform well with conventional heat treating, cryogenic treatment vastly improves this performance. How does it benefit from cryogenic treatment? O-1 can have up to 8.5% retained austenite when quenched to room temperature (20°C). While this does not seem to be a lot, it is significant, and proves that at the very least, O1 should be quenched to sub-zero temps and held there to reduce the amount of retained austenite. So much for the woodworker's assessment of O1. Now here's the really important result and proven by highly specific and controlled technical scientific studies: in treating O1 to shallow cryogenic treatment (SCryo), the wear resistance was improved 221%. In treating O1 to deep cryogenic treatments (DCryo), the wear resistance was improved 418%. Simply put, either of these treatments dramatically and substantially improves the wear resistance while making the blade tougher, and the finish better overall! Why not do this?

440C: This high alloy martensitic stainless steel is a great performer. In SCT, its hardness is increased by 4%, and in DCT, by 7%. and by bringing this steel to a shallow cryogenic treatment, it has 128% improvement of wear resistance. There is some conflicting information and data about 440C in the cryo field, with claims that deep cryogenic treatment does not significantly improve this steel performance. These claims would lead you to believe that 440C doesn't need the -320°F liquid nitrogen quenching of DCRYO. One study claims in deep cryo, it only has a 121% improvement of wear resistance, so it's assumed that 440C has better results in wear resistance improvement in shallow cryo. However, there's a very important thing that is seldom mentioned by researchers who evaluate these things, and something I've learned from working with 440C in both shallow and deep cryogenic processing. When 440C is processed in deep cryogenic processing, with multiple tempering and DCRYO immersion between tempers, it's markedly more dimensionally stable in DCRYO than in SCRYO. It's also tougher, stiffer, and much more wear resistant. I know that much of this research is done without extended long-term cycles and soaks at deep cryogenic temperatures, and one of the reasons it's so important to document and continue testing. In the future, look for longer-term cyclic testing on this steel; the performance using my T3 process is over-the-top outstanding, and I'll be documenting this as time permits. It's interesting to note that the standard by the United States Air Force for all parts made of 440C in any aircraft are that they are cryogenically processed, and that has been the standard since 1995!

D2: This steel is an incredible steel and benefits undeniably and astoundingly from cryogenic processing. Since D2 has so much carbon 1.7% and so much chromium (12%), it creates abundant chromium carbides even with conventional heat treating. The martensite is also profuse in the structure, due to the extremely high carbon. In shallow cryogenic treatment, D2's wear resistance is increased 316%, which is wonderful. In deep cryogenic treatment, D2 becomes another animal. DCT increases the wear resistance of D2 up to 820%! Over eight times the wear resistance of conventional or even sub-zero heat treatment is an astounding result, and it has been proven over and over again in numerous scientific studies how profoundly D2 responds to this procedure. If you have a conventionally treated D2 blade, you already know that this is a very wear resistant alloy, one that takes and holds an edge for an incredibly long time. Now imagine the same species with eight times more! When properly done, D2 also benefits from the precipitation of finer carbides which lead to increased toughness as well. D2 benefits from multiple tempering cycles (at least three), because they promote the precipitation of secondary carbides, and with triple tempering an increase in toughness of 25% is experienced when compared with double tempering.

ATS-34 (and 154CM) is a great performer all around. When given conventional heat treatment, it results in a high performance blade with high toughness and very good corrosion resistance (though not as good as 440C). When cryogenically treated, some very interesting results take place. The steel as in all high alloy martensitic stainless steels, develops extremely fine microscopic carbide particles. The finish is smooth and excellent, and because of the high molybdenum, two additional results occur. The first is that the creation of molybdenum carbides is abundant, particularly when given a good, long cryogenic aging. This brings up the hardness significantly and the wear resistance is abundant. The second benefit is that because of the high toughness in this alloy due to the high molybdenum, the blade can be tempered to a higher hardness overall, without fear of brittleness. The result is an extreme improvement of high wear resistance and high toughness, along with improved corrosion resistance. This same result can be experienced with 154CM, since technically, they are the same alloy. Since steels like D2 experience up to an 800 percent improvement of wear resistance with cryogenic treatment, In other studies of high molybdenum, high chromium, high alloy tool steels it can be suggested that a 200 to 300 percent improvement of wear resistance may be experienced, even though no certified testing data is available for these steels in the current research literature. My own research has demonstrated tremendous improvements in these steels when cryogenically treated, so why not do this?

CPMS30V, CPMS90V, CPMS35VN, N360: In all of the other steels I use in making knife blades, the cryogenic treatment certified testing data has not been done. This is probably due to several reasons; mainly the proportional rarity of these steels to common machine tooling steels, and the expense of the studies, along with requests from the steel suppliers for study details. Considering the typical results using high speed, cold work, and high alloy martensitic stainless steels, it is generally expected to achieve a minimum 200 percent increase in overall wear resistance, with up to a 800 percent increase in wear resistance possible with deep cryogenic treatment of these steels. Why not offer this treatment for clients who want the very best condition possible of their blades?

It should now be clear why cryogenic processing of these particular steels is one of the most important improvements that can be implemented on a high performance, high alloy steel knife blade.

Page Topics

More about Carbide Precipitation

Here's a query about a particular steel (D2) and the cryogenic holding time:

This is a great query, because it's an opportunity to clarify some effects of the process, to the best of my understanding and experience, from what I have learned in study and practice. The first thing to note is that the M_f of D2, the temperature at which martensite is finished forming upon quenching, is about -150°F. Each steel is different, so it's best to make sure that one knows the makeup and recommendations of the individual foundry or steel supplier first. Even to finish martensite transformation, the temperature of -150°F must be reached, and this cannot be done with dry ice or even with shallow cryogenics. D2 is definitely a steel that needs deep cryogenic processing at -325°F for proper heat treating. Think about this when you hear or read that D2 is worked in an open forge and quenched and tempered in an archaic forge setting! D2 that is hand-forged is ruined steel.

I believe that Mr. N. suspects that conversion takes place slowly, sluggishly because of the temperature, and this is not quite the concept. The beneficial reaction of carbide precipitation is caused by the temperature, so I'll go into it a little bit deeper here.

Martensite formation is what happens when the steel quickly reaches its martensite finish temperature. This may not be a true cryogenic temperature; in the case of the D2 described above (-150°F), it's colder than the shallow cryo (-125°F) but not as cold as deep cryo (-325°F). At the martensite finish point, the steel has as much martensite as it's going to have, and that part is pretty much done. But this is only part of the story; the formation of eta carbides is critically important, and some studies suggest that they contribute to the wear resistance of steels more than the martensite.

Much of what happens at this temperature is due to compression. If you are studying this page, you probably know I've mentioned compression before. While the studies are complex and detailed, I'll try to put them in plain English. After the martensite is formed, expansion and compression play a role in precipitating carbide development. The martensite formed is, in simple terms, cramped, expanded, forced, bent, and strained, but the cooling continues. Due to laws of the states of physical matter, the steel is compressed by the cold. The metal shrinks in the extreme cold, pressures increase within the structure, and this physical state forces lattice deformation, shifting carbon atoms, stacking, arranging, and shuffling the crystal form. Carbides then form by the spinodal decomposition of some of the martensite.

While this can get pretty complicated very quickly, and there are great minds and works on the process at the crystalline level, the concept is this:

1. Quenching initiates the forming of martensite from austenite, and the martensite finish temperature is reached. Retained austenite is at a minimum if this is done correctly. Martensite contributes greatly to the strength and wear resistance of the steel.
2. Continued quenching and holding at cryogenic temperatures forces compression of the crystalline structure, and fine eta carbides are formed from the distortion of the martensite structure. The carbides contribute more to the wear resistance of the steel than only martensite. They also enhance the strength and toughness of the martensite matrix, more so than the simple removal or conversion of the retained austenite.

The point here is that the extreme cold does not slow the process in a detrimental way; the extreme cold forces the process to continue past martensite conversion on to eta carbide precipitation. Now consider this; at least one study confirms that deep cryogenic treatment for extended times (20 hours was used in the experimental process) actually increases the inherent driving force of carbide conversion that takes place in tempering. This means that there are structural changes that happen in Dcryo that enable more carbides to be formed when the blades are tempered! And this study was done on D2.

Very neat and exciting, isn't it? Okay, maybe it's a cheap thrill; it's not like the day you bought your new car or your boat, but for guys like me, it's a thrill to be alive at a time when we are learning, improving, using, and sharing all this data.

Page Topics

Grain

What is grain, and what role does it play in steel knife blades?

The word "grain" in steel refers to the particles of the crystalline lattice, and how the word is interpreted and in what context it is used changes the definition of grain. For instance, a grain may mean the singular crystalline lattice of a microscopic particle of carbide, or it may mean the group of bonded lattices that are surrounded by another material. Grain may mean the visual appearance of finished steel or freshly broken steel, or it may mean the finest particles visible under an electron microscope.

Literally, the word grain is defined as the discrete particle or crystal determinable in the matrix. So you can see that the type of particle, the size of the particle, and even the viewing apparatus used to see the particle identifies the type of grain being described. Grain study and structure is common in steels, it can determine the material, size, shape, and bonding structure of the crystalline particles, and thus their percentage in determining the effects of various thermal treatments. There are studies about grain size, grain shape, and grain boundaries. There are studies and procedures for lapping the surface, etching the metal, and examining and counting grains under a microscope. From this, you can see that grain complexity is a science into itself, and belongs, in our case, in the realm of the metallurgist and materials scientist.

When knifemakers talk about grain, you should probably take what you read with a grain of salt. Sorry for the bad pun, but in all seriousness, grain manipulation, grain bonding, grain sizing, shape, and structure is beyond the realm of the knifemaker, no matter what forum or venue he is posting on.

This strange fascination with grain probably hearkens back to the blacksmithing or hand-forging days, when you could heat treat a piece of metal and then break it in half, and visually examine the grain. A large crystalline grain would mean it wasn't at it's best hardness, a small, fine grain meant you were close to the mark. But this is far and away from scientific grain testing and study, something I will flatly claim is out of the realm of knifemaking.

The reason this is not the knifemaker's realm is that knifemakers only control the shape of the steel blade, its geometry, and the process of heat treating and finishing of the steel, and do not control grain structure. Mistreatment and bad practices on the part of the knifemaker will result in an inferior blade performance, and some of these defects may be visible in the grain structure. Understand that no knifemaker is working under an electron microscope, and no knifemaker imparts some special magic in his process to manipulate grain changes in the steel that are improvements on standard process and cryogenic processes.

The reason I'm detailing this is that I've seen these "grain discussions" for years, on forums, in postings, on websites, and in bulletin boards, and nearly all of them are bunk. Guys are claiming that chromium carbide grains are too large to bond at a cutting edge, that grains are soft, grains are hard, grains are improperly placed. All of these discussions are meant to try to explain why their knives, their idea and interpretation, their choice of alloy is somehow superior than other choices.

No where is this more apparent than in the most persistent misrepresentation (lie) spewed in knifemaking, that somehow carbon steel blades are superior to high alloy stainless steel blades. Guys use this grain argument over and over, and in creative yet unsubstantiated ways to claim that these inferior steels are somehow superior, and they use grain discussion to bolster their argument. After all, who is really examining grain, and to what degree? Do these guys have Ph.Ds.; are they published scholars; do they have any evidence by such to prove their claims?

The next time you read some claim about grain, consider the source and challenge the source if you must, but it will be a fruitless endeavor. When a knifemaker creates a knife, he only knows what to consider to prevent disasters like grain growth during extended soaking at austenitic temperatures, and non-conversion of allotropes by not reaching or holding the steel until its martensite finish temperature. Correct processing in all steps is critical, but a knifemaker will not improve on the steel apart from the best processing procedure possible. Some things we do know about grain:

• Grain boundaries hinder the movement of dislocations. The more grain boundaries that exist, the more difficult it is for dislocations to move, and as a result, the steel is harder, stronger, and stiffer.
• Coarse-grained steel contains larger and fewer grains and grain boundaries than fine-grained steel with many more grains and boundaries
• Proper heat treating creates fine grain, making the steel harder, stronger, and stiffer.
• Alloying elements, particularly chromium, block slip planes in grain boundaries, adding to mechanical strength.
• Proper cryogenic treatment and aging produces even a finer grain, with more boundaries that hinder fracture propagation, making the steel tougher.
• Some elements are added to these steels (like vanadium) that help to create initialization nuclei for grains to grow, creating a finer-grained steel. There is continuous, ongoing research in the microscopic grain precipitation field, with new alloys, new combinations, and new reactions over every horizon. What an exciting time to be alive!
• Grain sizes in steel are incredibly small. Knifemakers may claim that the grain at the cutting edge has some bearing on edge holding due to grain size, grain bonding, and grain interaction, but this is just total nonsense. To understand the magnitude of sizes and structure in the cutting edge and some humorous notions, please take a few moments and read:
  "Which steel has the greatest "tooth" for the cutting edge?
  --and other carbide particle nonsense"
  on by "Blades" page. There, you'll get an idea of actual particle grain size, and how this has no bearing on a cutting edge sharpness. What does matter is the steel type, the steel processing, the geometry of the blade, the sharpening angle and the sharpening media grit size, and little else. Steels do not maintain or lose cutting edge sharpness because of grain bonding, grain sizing, or grain manipulation, and it's time this fairy tale was put to bed.

Bad Boy Chromium Carbides

It's interesting to note that nearly always, when knife enthusiasts and knifemakers are faulting grain in knife blades for bad or inferior performance, they are writing, talking, or posting about chromium carbide. "Chromium carbides are the culprit," they claim, with "large grains that make the steel impossible to sharpen," or "chromium carbides have bad or inferior bonding to other grain particles," or "chromium carbides pull out of the steel," or "chromium carbides lead to an impossible to sharpen knife."

Knifemakers on forums claiming to be machinists even state that these carbides are soft! This is clear and obvious ignorance, as any engineering and metallurgy source will quickly and clearly prove that these carbides are extremely hard, and add tremendously to the wear resistance and durability of the alloy in a myriad of ways. This is one of the main reasons chromium is used as an alloy, after all! I'm not talking about simply adding chromium in a lower carbon steel for increased corrosion resistance; I'm talking about chromium in high carbon martensitic steels, where the desired result is chromium carbide because it is so beneficial!

A lot of makers like to cherry pick bits and pieces of data they think bolsters their argument of why chromium is bad in steel. One study suggested that chromium carbide particles were pulled out of the steel in high wear testing. Of course, this does not mean that chromium carbide was in any way less beneficial, yet makers will glom onto any bit of data they think means that the entire machine tool industry is wrong, and they are right for choosing a lower alloy to hammer into a blade. Note that in this one study, the claim was that these were sub-microscopic effects only occurring on high speed, high pressure, high temperature machine cutting tools, at sub-microscopic levels in tribological testing (the science and engineering of interacting surfaces in relative motion).

This is not in hand knives! This is because hand knives in use experience tremendously lower pressures, feed rates, temperatures and abrasive motion stresses than high speed tool steels, hard-surfaced tool steels, or machine-driven specialty steels, where tribological studies are necessary. No one is using a knife blade cutting at 200 surface feet per minute to cut a carbon steel bearing block at high feed rates and elevated temperatures. A hand knife is not a milling cutter, in other words, so these discussions bear little on knives.

Again, to understand how minute these particles are and they do not determine the ability of a knife to be sharpened and to cut, please read about carbide particle nonsense on my "Blades" page at this bookmark. It will open your eyes to these ridiculous claims and illustrate just how small grain is, and how comparatively wide the sharpest cutting edge is.

Another common ploy among the ignorant is to claim that high alloy steels in general, and stainless or high chromium steels in particular, are inferior to carbon steels made into knife blades because these high alloys are not used in particular machine tools. They cherry pick (again) to bolster their argument, by citing specific machine tools that may use a lower alloy cutter, or axle, or guide, or runner, or former, or some other component. Then they'll claim, "See? I told you that high alloys and stainless steels are bad, otherwise these machine tools would be equipped with them!"

What they don't go on to clarify, as I will here, is that the economy of manufacture prohibits the use of higher alloy steels; they are just too expensive to use in machines that are made cheaply! Most of the time, these machines are made in foreign countries, (India, China, Singapore, Taiwan, Pakistan) and other locations by firms that are not known for their use of high quality production, high quality parts, or high quality anything! They are budget-driven and volume-driven firms, not quality-driven firms. So these manufacturers opt for cheaper products overall. Even in machine tools that are not cheaply made, the use of an extremely high alloy is not often justified when a less expensive steel will do.

Another limiting factor is that the extremely high performance value may not be necessary or applicable in the range of wear or exposure. For instance why have a 440C stainless steel drive shaft on a planer that has a plain steel chain driving it? The application may not require it, and the use is not appropriate. Just because there are high alloy (and expensive) steels available, this doesn't mean that they are the best choice for all equipment or machinery that does not meet that high quality standard, so they use the most economical steels. Is this the reason why a knifemaker would hobble his clients with a lower alloy, lower performing steel?

Now think about this for a moment: would you want these cheaper, lower alloy, less wear resistant, less tough, less durable steels used to make the turbine blades for that aircraft you or your family is flying on? How about the ball bearings for the landing gear, with improved corrosion resistance? That would be (detailed by SAE and AISI, AMS 5880 standards) as the "premium aircraft quality product: 440C." It's got at least 16 percent chromium for a reason. This is not a casual hobby designation, this is the Aerospace Materials Specification (AMS) standard.

Note that these guys typically have something against chromium carbides. Why is that? Could it be that they are fans of carbon steel blades, steels that have very little chromium, and are fans of non-stainless steels in general? Are they looking for excuses to use, promote, and work with non-stainless steel blades because of their own interest and skill level, and can't admit that their blades are inferior? .

Of course this is the reason, because if they knew anything about metallurgy at all, they would know that chromium, the hardest metal on the periodic table, is a wonderfully positive addition to steels, used even in low alloy steels like 52100 (the princess of many hand-forging knives) as an addition to improve hardness and wear resistance! Here's an interesting detailed example of this foolishness on my "Blades" page:
Does Chromium hurt or help the blade?

The only thing a knifemaker can do is choose a steel that is the best he can afford for the project, suited to the knife project and expected exposure and use, and heat treat it with the best possible method. The high alloy and stainless high alloy martensitic steels are the best performers made for the applications of fine handmade knives, unless the knife is designed for decorative and primitive appearance with pattern-welded damascus or temper lines and rough finishes, or the knife is made with extreme economy (cheap) in mind. There is a reason high alloy hypereutectoid steels outperform all others in these applications in professional, industrial, and military use, and there is a reason that machine shops are not using blacksmith-made products in any type or circumstance. When someone is claiming grain boundary, grain interaction, grain shape, grain inadequacy in any way, he is talking nonsense.

Page Topics

Why Cryogenic Processing?

After reading and studying this page, the question in your head about fine knife blades should be "Why not?"

This may be the most complicated topic on the page. The reason is because people don't want to change and grow, they don't want to admit there might be a better option, they may be ignorant of science, scientific process, or metallurgy, they may want to justify their own way of doing things, or their concepts and understanding of the idea of process applications was limited. it simply may be out of financial and investment reach in the studio, as these process and equipment add to the expense of fine knife creation.

These same attitudes nurture the tired, outdated myth that carbon steels make better knife blades than high alloy stainless steels, that hand-forging is somehow better than high alloy machining and laboratory-grade heat treating with cryogenic treatment, and that a primitively-made knife has some durability value rather than decorative only. These are all past myths, and it's surprising how they are defended, propagated, and reinforced by knifemakers who should know better, and should be better educated on their metallurgy. We've got guys selling carbon steel damascus chef's knives for tens of thousands of dollars, simply because of popular television exposure, claiming the experience of every century of blacksmithing as their own, merely to justify their outdated process! Would it help to know that there is no modern machine that endures any measurable force and stress that has one single part that is hand-forged? Would it help to know that there are no blacksmiths at NASA, at any AISI member's business, at no machine shop, at no research facility? Yet these stubborn myths remain, mainly because of money and ignorance.

One scientific study in 2010 relates that while heat treating overall was well understood and employed, cryogenic treatment was still in its infancy. This is a critical point, and I'm very excited about the prospects of what this relatively new understanding of science offers for our field. Even companies who have this performed on knife blades may know little of the process, simply because new and revealing details are being discovered and uncovered every year. Though it was known back in the 1930s and 1940s that cold treatments were beneficial, it has taken decades for the details to be wrung out via studies and advanced microscopy, equipment, communication across fields, and testing, and may take many more years for it to become the standard, though I absolutely believe it will.

• Financial interests of accounting and the cutting tool industry: It's clear that one of the main areas of drastic improvement of cryogenically treated high alloy tool steels is in the cutting tool industry. These are the makers and suppliers of milling machine cutters, drills, reamers, taps and dies, broaches and every other related cutting tool made of high speed or high alloy tool steels. For example, one might wonder why a manufacturer of milling cutters would want to improve his product. If he improves his milling cutters in a way that doubles, triples, or increases the life eight times on the machine tooling floor, he would, by logic, sell only half, a third, or (yikes!) one-eighth of his cutters. To justify the volume loss, he would have to charge two or three to eight times as much for them. In this highly competitive market based on continual monthly sales flow, this is not reasonable. If he can't sell that idea to his accounting department, and justify the volume loss and equipment cost in the competitive field, it won't happen. Sure, the end user or machine shop would benefit from longer tool wear and less tool changes, less ordering efforts, lower shipping costs, and higher production, but the supplier is not the user. Since the supplier is the one who would have to pay the expense for the cryo treatment while losing volume sales overall, they will fight the change. I'm not the only one to notice this, and one of the leading researchers into cryogenic treatment of steels, professor Randall Barron, was asked why razor blade companies wouldn't cryogenically treat their blades, since studies had proven a minimal doubling of the useful life of the blades after treatment. They said no because "then they wouldn't make as much money." See how this all works? Planned obsolescence is the same in ALL mass-marketed volume sales industries, because sales have to keep going to earn the shareholders profit. Do you then wonder why computer programs and operating systems are constantly and continually upgraded, forcing the computer user to buy a newer computer to run the new programs while support for older programs is discontinued? This is particularly troublesome because usually the existing computer is fine and functional; it's forced obsolescence. All the while, these same large corporations are pushing ads of how gentle they are on the environment, while being horridly wasteful and hoping you won't notice. For the high alloy steel industries, they will only be dragged into this technology kicking and screaming, since it's an investment, a cost, and gamble. The hard economic facts of the manufacturing-use-supply chain are not usually considered by scientists and metallurgists conducting these studies, since they don't actually work in the manufacturing or machining field. So, unfortunately, the industrial application of cryogenics is painfully slow in becoming universal.
• The steel foundries: It can be considered that the same results of cryogenic treatment in the cutting tool industry could be applied to the steel foundries themselves. If the foundry suggests that all of their steels used in dies, cutting tools, and other high wear applications should be cryogenically treated, what would the result be? It could be that 2, 4, or even 8 times the wear resistance and longevity could occur! This means that there could be a huge demand drop, as all the devices and tools made from these steels would be replaced less often. In a world where volume is king, why would a foundry shoot themselves in the foot by offering a simple way to make the tools made with their steels last longer? They wouldn't; that doesn't make economic sense. So data, testing, verification, and process standards are slow to adapt in these trades, just like the cutting tool trades above, so more mass units of steel can be sold. Consequently, the data on these steels is somewhat limited, even though cryogenic processing has been well-documented and supported in science.
• Changing and growing: When I was talking to metallurgists about this, they offered that these are new developments (cryogenics), and many industries do not want to employ new processes when what they have been producing was simply "good enough." One even asked me why I would be pursuing this new technology when I had been doing fine for decades. This is a valid argument, and one that I could apply to this very page on my website. I've certainly been doing very well for decades, and have plenty of orders, but still, I took the time and effort to do intensive research and critical time to build this page, word by word, article by researched article, for no obvious benefit apart from building a better understanding, a better knife, a better framework of my tradecraft. This is the reason I love making knives, because the learning never stops. You can take this field as shallow or deep as you prefer, and in this world it's hard to find a field so wide and boundless.
• Better Options: Everyone who wants a knife wants the best knife possible. Though conventionally treated and sub-zero treated tool steels for hand knife blades perform well, why not offer a better option to the knife user? Better options are offered in fittings, handle materials, geometries, sheaths, stands, and embellishment, so it's only fitting that cryogenic treatment and aging of the blades takes its place in the list of custom options of fine handmade knives.
• Scientific process, study, and publishing: There are numerous scientific studies that have taken place in the last 7-10 years about cryogenic processing; I encourage you to access them for free on the internet. This is a great time where this knowledge, never before available to the public, can be quickly and freely accessed and assessed by the individual or professional. Because one development begets another, these discoveries and confirmations will continue to accelerate. Another powerful facet of these developments is the ability to apply what has been learned in one profession and science to be applied to another. For instance, studies in cryogenics in microbiology has allowed us the very equipment, supplies, information, and tools for reasonable research in a small metals laboratory environment. This simply was not possible ten years ago, and as the technology grows, so does the result.
• Varying test results: This is a tough one. Not all scientists are adhering to strict method, and the results then show wide variations. While I'm not a scientist by profession, it's easy to see from the standpoint of a technician that some steps, some variants, some choices to make comparative studies must be backed by strict protocol of process and testing method. For instance, in one study, a comparison of twist drills was made. The control was the same material as the cryogenically processed high speed steel, but the two were tempered entirely differently! The control was austenitized, quenched to room temperature, and then tempered three times without any deep thermal cycling, but then the test piece was cryogenically quenched, and tempered only once. This is a missed test, there are too many variables in the temper cycle to establish a reasonable control. No wonder the test conclusions were negative toward the cryogenic process, when the fault was the tempering! I've read several sources that claim that some industry standards need to be established here, and since the science is relatively new, it's up to people who do cryogenic processing to keep strict and accurate records and protocols, carefully evaluating their results. This is truly leading edge science.
• Justification of current process: Many metals workers are clinging desperately to their outdated processes. I know this feeling well; I was a photographer who almost resented the digital advances in the craft that allowed inexpensive processing and printing without the necessity of a costly darkroom and lab. I was proud of all I had learned in that field, and had invested tens of thousands of dollars in chemical photography process. It took real skill to make chemical glossy prints of knives, understand all the qualities of light and chemistry, and serious investment to make it work in a business setting. But technology was not stopping for me or any other photographer, and I had to take what I learned in the older process and apply it toward the new one, digital photography.
  Knifemakers can be the same way. It's comfortable to stick with what works and has worked, and what has been tradition for so many years (sometimes decades, or in the case of blacksmithing, centuries). But the development of exciting new alloys has simply made these antiquated processes (like hand-forging) an art of the past. Though the ancient, time-honored technique of hand-forging and working in hypoeutectoid or low alloy steels does and will continue, as the knife-using public becomes educated, these lower-performance steels will not be the preferred performers, but only visual creations of an old art, just like chemically-processed black and white art photography is today. This is because these blade steels can be easily proven to be low-performance, and study and education will march on and build a foundation of truth that cannot be ignored by the knife aficionado. Ask any real knife user (military, counterterrorism, professional chef, butcher, or any other hard-use knife owner) whether he prefers a markedly better knife blade, and I doubt he'll choose a hand-forged carbon steel blade, no matter how pretty the hamon line is.
• Process applications technology: This is a bit more complicated. Cryogenic processing is a process application. It's something that goes into a knifemaker's shop and studio, just like the grinder nest, the buffing room, or the machine tool setup. A typical example is the grinder, something in every knifemaker's shop. While the tendency is to focus on the machine itself, there is always so much more. Taking up more real estate is the belt rack, the dust collection, the wheel rack, the tool cart, and everything else it takes to maintain, equip, run, service, and operate the grinder. Knifemakers quickly learn that like any process, the environment of the shop itself allows the grinding process to take place, from the power panel that allows electricity from the service drop, to the discharge of the grinder swarf from the dust collector. Good grief, a professional knifemaker must have a spotless and grit-free padded bench to set the blades on between finish grinds so they don't get scratched! All of this requires real estate and equipment.
  
  The cryogenic process application technology is the same animal. You must have the equipment, specially adapted to the process of knife blade treatment. Knives are not heavy plastic injection molding dies, so the process must be adjusted accordingly. It takes real estate, specialized racks, frames, and cryogenic liquid transport and manipulation equipment. It takes dedicated electrical power feeds. It takes specialized insulation, temperature monitoring equipment, and a smooth workflow direction for expedient momentum. It may take a variety of chambers, pots, buffers, and piping. This represents a sizeable investment in not only equipment, but time, making and arranging the working guts of the process. Taken as a related process and extension of heat treating, the extreme cold environment is positioned right by the extreme hot environment of the furnaces and ovens, and they must not interfere with each other! This is a lot to bite off, and much of this has to be designed and created by the maker, since the exact necessary equipment for the process is not on the market.
  
  In my studio, while cryogenics is a certain challenge, it's considered as another process. It's logical to consider the cryogenic method applied along with machining, heat treating, leather work, lapidary, anodizing, passivating, photography, textile work, engraving and many other meticulous and specialized fields.
• All processes must be immediate and concurrent: This is a critical factor in heat treating, one that bears closer examination. It has been proven that timing is critical in all of these processes, in order for successful, reliable, repeatable, and highly effective results to be gained, the entire process must be performed in quick, immediate, expedient order, without delays, in the same facility, oftentimes in the same room. The processing of the steel must not delay in reaching cryogenic temperatures after heat treat; steels change moment by moment in these circumstances. Tempering must not wait, nor the deep cold aging between tempers, every part of the process must integrate with the other. Though you may hear of companies offering "ship and cryo" services, these are unproven and of dubious effectiveness, particularly compared with expedient and continual timing and flow of the profession heat treating regime.
• Competitive advantage: The cryogenically treated knife blade offers a serious competitive advantage to blades that are not treated this way. The studies and use of these steels proves it, beyond doubt; they are much better performers in wear resistance, toughness, corrosion resistance, durability, and longevity than non-cryogenically treated steels. I want to make the very best knives possible for my clients. My clients don't need to read and understand this page; many of them won't even bother, but it's important to me to supply the very best knife possible within my capabilities. Because the availability of cryogenic processing equipment and supplies has become available in the last few years, it is now possible for me to do this in my own studio. When I began making knives in earnest back in the early 1980s, this would be only a dream, but thanks to technological developments in both steel process and related equipment and supply fields, it's something I can now offer to my clients.

Unlike the company that sells milling cutters by the thousands, I strive to make the very best, and do not function by volume but by quality. This is part of the division between knifemakers as individual artists and craftsmen, and knifemakers as companies that want to make and sell a lot of knives. Realistically, I don't want to sell a knife to every person who comes to the website or hears of my name. I only want the right knife to go to the right person. That is a unique and sobering conversation that I'm lucky to have. The right knife is built in extremely high quality throughout. The performance must also be a robust as I can possibly supply, and that is the reason why cryogenic treatments have a place in my studio.

Page Topics

What About Dry Ice Baths for Quenching?

How does it work? Dry ice (solid carbon dioxide, CO₂) is purchased and crushed, then put in a liquid that will remain liquid at the cold temperature of the dry ice, and a bath is constructed. The knife blades have been brought to their hardening temperature, and then quenched to room temperature, and then are dropped in the very cold bath to complete the quenching.

It's pretty important how the bath is made, chilled, and stored, as are the components of the bath. By the way, these baths are fairly common in use for benchtop laboratory study and process, when a very small amount of cooling is needed, and the lab does not have the funds or resources to purchase a dedicated mechanical chiller with a circulatory bath.

Sounds simple enough, and it has its place, but also has some pretty significant limitations that the knifemaker needs to know and the knife client needs to be made aware of, particularly since there are significant factors that may result in a less than optimum knife blade. While dry ice bath quenching is an improvement over conventional heat treat, it is not the pinnacle of quenching methods and in this section I'll detail the process, advantages, and limitations.

Dry ice is very cold; it sublimates (turns into gas) at −109.3 °F (−78.5 °C). If you can get it in a saturated solution of liquid, that solution will be at a very cold temperature, close to the shallow cryogenic process temperature. Put the knife blades in the solution after quenching to room temp and let them get really cold. Sounds great, but there are some limitations.

In the old and early days, we used acetone for the bath medium, but this is not a good choice due to its instability, flammability, and reaction potential with nearly all plastics. Acetone is a solvent that loves to dissolve things, so other solvents should be used. One of them is ethylene glycol, but this is poisonous, and it's probably not a good idea to keep a volume of it around. Ethanol is another bath solvent, but it's highly drinkable and might disappear when you have visitors in the shop that appreciate fine liquors or Everclear® (just kidding). Denatured alcohol is a common enough agent, though it evaporates quickly at room temp and is flammable so storage has to be carefully considered. Most shops don't have a flammable safety cabinet, and this is the proper way to store this type of agent.

You might read that dry ice sublimates at −109.3 °F (−78.5 °C), so it might be considered that this is the temperature of the bath, but this varies greatly. The dry ice can actually be much colder, depending on how it is made, where it comes from, how it is stored, and how it is transferred. The liquid in the bath may be warmer, unless continuous agitation is employed. So the temperature can vary quite a bit, so to keep accurate records of process, a thermocouple should be employed to determine the temperature of the bath.

There are several substantial issues with using dry ice baths for quenching. While they are a step in the right direction, they are not optimum for quenching steel knife blades and have substantial limitations for these reasons:

1. They use solvents, which are flammable an/or unstable and require special storage. Since they are used in a quenching process after heat treating, this raises safety concerns, as like-process does not mean like-storage and use. Using a flammable solvent near orange-hot blades can be dangerous and serious precautions must be taken in using flammable solvents near furnaces, ovens, kilns, and other heat-producing equipment. No sparks, no ignition source, no pilots, no open flame of any kind should be in the same room as the flammable solvent bath, and the area should be well-ventilated to prevent any vapor buildup. If non-flammable bath solvents are used, these must be stored according to their reaction potential, toxicity, and with appropriate safety practice. Of course, this is not a major concern for the knife owner, client, or customer, but for the maker and his process.
2. They require dry ice. This may or may not be readily accessible, and it's a consumable in the heat treating shop that can not be normally stored for any significant length of time without specialized equipment.
3. They are not accurate. I know of no maker that is regulating the temperature of the bath with control systems, and I don't even know of one who measures the temperature of the bath with a thermal sensor. This is a significant concern for the knife owner, user, or client, as most clients require the absolute best heat treating process possible, and that only comes from accurate control, measurement, and regulation of all parts of the process by the maker. While temperature variation may not matter much for most low alloy steels, in order to understand and define intricacies and specific regimes for reliable results with different steel types, monitoring temperatures is important to fine tune every step of the process. This fine tuning exists in the record, that is, the recorded process of each heat treat, kept in a log, compared to previous results, where tuning and adjustment of process particulars can assure repeatable, reliable results in every heat treating regime.
4. The bath must be made prior to every heat treat, and made in an insulated container that will contain the bath for a long period of time. The solvent must be stored after the bath is rewarmed to room temperature, in a method where most localities require flammable storage and ventilated cabinets. Remember, we are not talking about a quart of paint remover; baths may end up being several gallons. No sparks, no ignition source, no pilots, no open flame of any kind should be in the same room as the flammable solvent bath, and the area should be well-ventilated to prevent any vapor buildup. If non-flammable bath solvents are used, these must be stored according to their reaction potential, toxicity, and with appropriate safety practice.
5. The bath must be mechanically agitated, and crushed dry ice added to maintain uniform and stable thermal environment. Where the blades are within the bath is very important; they must be suspended away from the sides of the bath container and not in contact with the container walls. If you stir the bath and let it sit, the area surrounding the blade will be warmer than the rest of the bath. Only continual mechanical stirring will maintain uniform bath temperature during quenching and aging.
6. Submersion of the blade into the bath can cause thermal shock that is highly detrimental to the blade. This is one that few people discuss, and it's extremely important to the final knife customer, client, or owner. While a knife blade may not openly crack or visibly fracture, this thermal shock of dropping a blade into a dry ice bath may cause sub-microscopic fractures (sub-microscopic means smaller than 0.2 micrometers), that are unseen but cause premature failure of steel demonstrated by high wear at the cutting edge. The knifemaker or knife user won't even know about this apart from a cutting edge that doesn't seem to last as long as a properly cooled blade. In order to alleviate this problem, thermal staging should be employed so that the blades are cooled relatively slowly but consistently before reaching the final bath temperature. I don't know of any knifemaker who even discusses thermal staging and slow cooling in these baths and processes, but the scientific literature absolutely has limits and specific rates of cooling that are required by testing and steel authorities.
7. They can't reach deep cryogenic temperatures, and they rarely reach shallow cryogenic temperatures. This means that they can only be used for steels that require mild shallow cryogenic process, which leaves out many of the hypereutectoid high alloy stainless steels completely. Most of these steels require -320°F and that means only liquid nitrogen.
8. The most important thing is this: Cryogenic Aging. At -78°C, this is just above the temperature of shallow cryogenic processing. By the way, it's not a "cold" treatment, at least according to The Journal of Materials Processing Technology, The American Iron and Steel Institute, and the International Journal of Emerging Technology and Advanced Engineering. We know that in shallow cryogenic processing which is colder than dry ice baths can reach, this is the range where the greatest results and improvements in steel quenching happen. One might then ask, "Why is this a problem, since this seems like it's close to the right temperature for the steel?" It's because that although it is in the right range of temperature, the reaction of carbide precipitation is extremely sluggish, taking many, many hours. I know of no knifemaker who is holding the blade in the dry ice bath for 10, 20, or 30 hours, and this is what is necessary for the most beneficial carbide precipitation! While this dry ice bath temperature will result in less retained austenite, and a greater conversion to martensite, holding the blade in the bath for 10 minutes, 30 minutes, or an hour or so is not long enough! And the baths are not quite cold enough, needing about another -10°F to reach merely shallow cryogenics, even at their optimum temperature. Dry ice can't even come close to deep cryogenic temperature, the temperature at which a multitude of beneficial results takes place. These results are a more complete transformation of austenite into martensite, and profuse eta-carbide formation, leading to a higher development of carbides overall after tempering, a stronger, tougher, more wear-resistant and more corrosion-resistant knife blade. This is not possible with dry ice!
   
   This is one of the main issues with cryogenics in the metal trades industry: this misconception that simply reaching temperature is enough, and then it's done. It's been proven again and again that cryogenic aging is a hugely beneficial procedure that everyone is trying not to do. They don't want to wait, it costs money, it costs time, it cost in expendables and equipment to keep something very cold for extended periods. Even the manufacturers of cryogenic treatment equipment have told me that this is a big problem for them, as the conception of "reach temp and you're done" has permeated the heat treating field. They've told me that many of the industries that use this equipment are set on the idea that a process must match the worker's shift time, so their production lines aren't delayed.
   
   As I've written before, and as is revealed by many studies, even the metalworking trades are confused about cryogenic process and benefit because of the misunderstanding and misperceptions about cryogenic aging and the distinctive benefits of eta carbide formation. By the way, many scientists, researchers and metallurgists believe that these submicroscopic (smaller than 0.2 micrometer) carbides are more important to wear-resistance than the martensite conversion! Many of these carbides form at shallow cryo temperatures, and are not just limited to deep cryo temperatures. Dry ice reaches neither temperature.
9. Deep thermal cycling between temperatures should take place in this temperature range. Yes, the same cold environment of deep cryogenics should be the same environment that the blades experience between the multiple tempers. This can be realized by supplemental hardening, described in detail in this section of the "Tempering" topic of this page, the increase of hardness with each additional tempering cycle. Without this cryogenic soak and aging between tempers, optimum conversion of allotropes will not happen. Dry ice is not in the cryogenic range stated by The Journal of Materials Processing Technology, The American Iron and Steel Institute (ANSI), and the International Journal of Emerging Technology and Advanced Engineering

What to do? While dry ice baths are a step in the right direction, and they are better than conventional heat treating (CHT), there are better methods, and this is why I don't use or recommend dry ice baths for the ultimate treatment of hypereutectoid and high alloy stainless tool steels.

However, if  a maker is determined to use a dry ice bath for quenching, he can attempt to overcome some of these limitations listed above, and then he should be able to explain these to his clients:

1. He can use a non-flammable solvent, or have a dedicated safe area to work with flammable solvents, including flammable storage safety cabinets. When using these solvents, let's hope he doesn't forget his PPE (personal protective equipment), or his knifemaking days will be shortened!
2. He can find a cheap and plentiful source for dry ice, as he'll need plenty of it regularly. Perhaps he can buy and install a dry ice generator, but then he'll need a CO₂ tank and storage.
3. He can find a way to regulate the temperature of the dry ice bath. He'll probably need a heater or some method to control the temperature of the dry ice before it's put in the bath, and to control the ramp-down rate. He'll need accurate thermocouple and temperature indication, and a control system to make it all work.
4. He can make or find a hefty insulated container to store and use the bath in. A lunchbox cooler won't do, and I don't know of any horizontal rectangular Dewars, but one never knows.
5. He can make and install a circulating or agitation system, plus a cage or suspension method to keep the blades in the core of the bath. This might be linked to the temperature control system detailed in step 3. above.
6. He can find, make, or create a way to slowly cool the blade down to the temperature of the dry ice bath. I'm not sure how this one would work, but he would need to eliminate the thermal shock of immersion. Most steels require a 4-5°F drop per minute maximum, so from room temp to -78°F should take 28-35 minutes. Can't just drop it in! Maybe he could start with room temperature solvent with the knife blade in it, and then take 28-35 minutes to cool the bath down by slowly adding chunks of dry ice. That might work!
7. True shallow and deep cryogenics required? Sorry, this one's insurmountable. Only liquid nitrogen will work, unless the knifemaker has discovered some unknown dynamic of physics and room temperature superconducting. Let's hope he shares it with the rest of us...
8. He should maintain the bath at temperature for 30 hours. Yep, he'll need to stay up late to do this one. Or he can create an automated system to keep adding dry ice so he can go beddiebye while the cryogenic aging continues. Or, he could create a really big, big bath with a lot of insulation that lasts for 30 hours...with automated circulation...
9. For thermal cycling, he should make sure the bath continues to be available while the tempering is taking place, so he can move the blades back to the dry ice bath between tempers. Actually, this one is fairly simple since tempering typically takes only a couple hours per cycle.

If you, as a knife client, customer, or user, now think that dry ice baths are not the most efficient way to quench steel knife blades, congratulations. This is something I learned years ago before I moved to other methods and equipment. Remember, dry ice baths are a step in the right direction, and they are better than conventional heat treating (CHT), but there are better and more reliable methods. Because my clients deserve the very best steel treatment possible for their blades, this is why I don't use or recommend dry ice baths for the ultimate treatment of hypereutectoid and high alloy stainless tool steels.

Page Topics

Cryogenic Treatment After Hardening and Tempering

I've described the role that cryogenics plays in the quenching process of knife blade steels, and how the deep dive and hold into the realms of cold has a positive result and improvement of steels, particularly high alloy steels used in the finest knife blades.

There are companies that sell a service of post-heat treatment cryogenic exposure or immersion. They call it "Cryogenic Processing," "Cryogenic Treatment," and even "Cryogenic Tempering," which are all very confusing terms, as the industry has no standard process definition for what they are doing. If you do some reading into the service they are selling, you'll see that they are offering a post-hardening, post-tempering immersion into either liquid nitrogen, nitrogen gas, or nitrogen spray and a cold cryogenic soak. Are these viable services, and do they help the steel, particularly knife blade steels? It's important to understand where and when this process takes place in the tool creation, and ask some serious questions about why this is considered.

The general sales pitch is this: send us your completed tools and steels, already hardened and tempered, and we'll simply run them through the liquid nitrogen and this will improve the wear resistance, toughness, hardness, etc. That's it. The steel is already hardened and tempered, and at some indeterminate point in the future, you simply get it very cold and hold it there for a while, and the wear resistance is somehow improved.

If this appears a bit questionable to you, I understand and concur. Considering where in the process cryogenics plays a role (in quenching and before tempering), this seems to me also to be an empty promise, particularly when some of these companies cite the statistics of increased wear resistance that occur during cryogenic quenching as being available in their process! I'll flatly claim that this is misleading at best, and a lie at worst. Perhaps they are just ignorant of what happens during actual cryogenic quenching process, but I don't know why, since the results are extremely well documented. Conversely, I can find no studies bearing any important or substantial result of cryogenic processing after hardening and tempering that improves the steel in any way. If there were proof, this would be standard process for all steels, in all tools, and it is not. We would be digging out all our old cutters, chisels, drill bits, metal forming dies, shear blades, and every tool steel item we could find and dragging it through liquid nitrogen to improve it, and we don't. We don't because this is not a proven improvement to the steel, and therefor it is not the standard. The standard and premium process is cryogenic treatment during and as a part of quench, not post tempering! Where is the actual tribological testing to prove the claims of these post-tempering process improvements?

Perhaps the reason for this is that once complete tempering cycles are done, the steel is about as stable as it's going to get. The conversions have taken place, the thermal cycling is done, the steel allotropes have stabilized. While it's possible that some retained austenite may be converted in steels that have inadequate martensite conversion, it's unlikely that the amount is significant to realize any noticeable improvement in wear resistance after tempering.

I've done my own studies and experiments and concluded that once a knife blade is hardened and tempered, at any indeterminate point after that, simply getting it very cold does not markedly change the hardness and performance of the blade. I haven't tried this with all steels, just the most predominant types I use for knife blades, and I've seen no reason whatever to do this.

Of course, there are many types of steels and applications, and a large amount of variables that have to be tested to confirm a viable advantage to post-tempering cryogenics. When considering the heavy sales pitches and promises, I would like to see some viable scientific studies done on the actual steel items treated. While cryogenic processing during quenching is extremely well documented, post-tempering cryogenic exposure is not. Could it be that some companies are simply selling a service that is related to quench process in terminology only, not backed by proof? Where is the actual tribological testing to prove the claims of these post-tempering process improvements? I'll leave that up to you to consider. In my studio, cryogenic processing is part of quenching, not some afterthought that is somehow supposed to change an already stabilized and tempered steel.

I'm only writing about high alloy hypereutectoid and stainless steels, not brass, aluminum, castings, or nylons. I'm not writing about plastics, tissue or food products either, so if you are one of those companies performing this "treatment," please be kind enough to cite studies that prove your claims. Unfortunately, there are few references included in the advertisements for these processes, even though documentation is fairly accessible and common these days.

Now for the few companies that request that you send them hardened and quenched steel items that have not been tempered for them to cryogenically treat and then temper them, it's critical to remember that timing is key in this process. The longer the steel exists in a quenched and untempered condition, the much greater the risk of dimensional instability and stress, leading to micro fractures that may reveal themselves as high wear, even after tempering! Martensite can not sit at room temperature; it is metastable until fully tempered! Add to that the steel isn't at full stability until several thermal cycling operations. Full quenching and tempering must take place immediately for these high alloy steels. There is no time to wait, to ship, to get in line for some distant company to run them through a cryogenic process and then temper them at their convenience.

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Important Note on White Papers (Data Sheets):

Most manufacturers and foundries have white papers with clear illustrations, charts, and comparisons available, for free, available online. They are also called data sheets and other descriptive terms. Beware, though, that these are advertising documents, and some of the claims can be outright misleading, so some interpretive logic has to be applied by the knifemaker or knife enthusiast seeking a particular steel.

For instance, one supplier claims that their steel has the same or better corrosion resistance than 440C, but that is a gross generalization, as it doesn't even indicate the condition of the 440C. The steel this supplier is selling can not be mirror polished, so that is a huge factor in a steel's corrosion resistance, detailed here. Another factor is that the hardness of the 440C is not specified, and the corrosion resistance of 440C directly corresponds to the hardness of the finished steel. Still another consideration not identified is that steel corrosion resistance can vary depending on how they are quenched and treated!

These documents often feature comparison graphs (more about comparagraphs here), but the graphs may not specify ranges or details of the comparison. So you might see a bar graph with bars of the advertised steel towering far above the bars of a comparative steel (typically 440C, since it is the standard others are compared to for a reason).Be aware of these types of graphs circulating on the internet, without specifics of how steels are compared. Note that in every single case, the steel being peddled is the one excelling in all relative characteristics above others (shocking, no?), and the testing method and treatments are not even specified. More on knife blade testing here.

These advertising documents also may not specify what happens or what is expected in cryogenic treatment. They may mention cold treatment, or freezing, or refrigeration, but typically with little information about the duration, temperature, and rates necessary for an expected result. I've contacted engineers and metallurgists with some of these companies before, and after long discussions, it's clear that extreme testing with scientific method and in substantial lots has not been done on many new steels. This is because, according to them, most steel manufacturing is done overseas, and our colleges and universities (who used to be supplied grants to do extensive experimental testing) are suffering from the same funding limitations as businesses are. The manufacturers relied upon research institutions for the testing and development, and that's not being funded, so many questions are simply unanswered. It then becomes the province of the business experimenter to do all the research and testing, and as a dedicated knifemaker and businessman, I do my own bit of testing in the cryogenic field, with my results being field tested in some of the most demanding occupations and uses in the world. It's an honor to do this, and I'm committed, as all makers should be, to the furthering of their trade, craft, skills, and art.

If properly followed, the results a knifemaker or knife manufacturer experiences will align with those posted on the white paper or data sheet, with refinements and enhancements of the process recorded in the knifemaker's own data set. These guides are proofs and resources on their own, and worth their weight in gold to the researcher and materials developer as well as the knifemaker. They also define the knifemaker's own proof of concepts and skill, with pitfalls and errors in processing avoided and successful results and process expected over the range of steels he uses.

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Hardness Testing

It's critical to understand the importance of hardness testing, not just after the heat treatment process, but all along the way. This is the only practical, frequently referenced, reliable, and logical instrument designed and used to determine correct process of proper and effective heat treating of knife blade steels.

Hardness testing may take place at many times, particularly in establishing a baseline of process if cryogenic treatments, multiple tempers, and various aging and timing steps are used. These baselines, carefully recorded and evaluated, give the knifemaker a greater and specific understanding of each steel he uses. The hardness tester plays a large role in these process adjustments, as does the heat treater's log and record. Though the data sheets and white papers for each steel give an idea of hardness and results, these are only a rough guide, and I've found throughout the years that results can vary tremendously from these supplier-side references. The engineers and metallurgists at the steel company will assure you that results vary greatly from their data sheets, and that the only way to be certain of each item is extensive testing, comparisons, and evaluations of each item processed. The only way to be sure of the individual knife treatment regimes is to accurately and frequently test each knife along the way. This also assures the knives fit the maximum performance criteria that the high-tech alloys are designed for.

Testing can and, depending on the method, should occur before heat treating, after the initial quench, after a shallow or deep or cryogenic processing, and before and after each temper, if necessary. This is a lot of results and data, and can give the maker tremendous insight into how, when, and thus, why changes are occurring in the allotropes and crystalline structure. For example, it's critical to know that 440C, which is not known for supplemental hardening after the first temper in any reference, can actually experience this if the maker is pushing his steel process into a refined and extensive custom procedure. Testing can also indicate the important tempering times and temperatures for desired results. From my own experience, I can assure you that the data sheets are not accurate, and typically, no manufacturer or foundry is experimenting with high process control to achieve improved response, due to the reasons listed in the topic "Why Cryogenic Processing?" on this page.

In my studio, I typically use the hardness tester pictured below. How the device works: The blade is placed on the anvil, and tension is adjusted to relieve play. The device uses a specially ground diamond-tipped penetrator for high hardness testing, and this diamond must be regularly checked and examined under a microscope to assure its integrity and geometry. Other penetrators (tungsten carbide) are used for softer metals and lower scales of hardness. A 10kg (22 lbs.) load is first applied, which causes initial penetration. The major load is then smoothly and accurately applied. In the case of a diamond penetrator, at this high hardness range, I use a 150 kg (330 lb. load), and penetration occurs. That's quite a bit of pressure on the tiny diamond, so the integrity of the diamond, the mounts, the anvil, threads, bearings, linkages, and dashpot that regulates the rate of pressure application must be smooth, clean, and frequently checked, serviced and tested for accuracy. The penetrator only penetrates so far, and the penetration stops. The major load (150 kg.) is removed, with the minor load (10 kg.) still applied, and the penetration is measured. This gives an accurate reading of the exact hardness at the point of penetration, and thus the relative hardness and temper of the whole blade. The testing mark is usually placed where it won't be seen, since the mark is permanent in the blade.

Though the hardness tester is a fairly common sight in complete machine shops, it is sometimes neglected. This is a fine instrument, capable of very accurate readings when properly maintained and used. The delineation of this machine is close to a millionth of an inch, so any error is significant! For example, a slight deviation of the penetrator by the compression of a dust in the bearing surfaces of the anvil that leads to one millionth of an inch displacement can cause significant error of the final reading. This is a delicate instrument and has to be regularly cleaned, calibrated, and maintained.

One more important hardness testing consideration: any malformation, tiny microscopic irregularity, minuscule flaw of, around, or on the diamond penetrator will give inaccurate results. I've seen makers reveal that they have impossibly high hardnesses, and I suspect this is the error. If the penetrator has a chip in it from careless or sudden load application, angled application, or general wear, it will give a reading much higher than the hardness actually is. This is because the carefully ground geometry of the diamond is critical to accuracy. A chipped or worn diamond will tell you a knife blade is 67HRC, when it's actually much lower. The diamond can't penetrate as much, so the instrument gives an inaccurate reading. You'll need a microscope to examine the diamond for flaws, at least 30 power is usually sufficient. More reasons to have the machine regularly calibrated and tested against standards, and the measured hardness of the blades tested on an alternate instrument, by another lab, firm, or entity. There are other issues with hardness testing and these instruments that I may go into in my book.

Condition of the blade for testing

The condition of the steel blade or item to be tested is very important. Steels with a rough finish do not test accurately. Imagine trying to establish a standard measurement of penetration in a surface with a bunch of lines (grind abrasive marks), irregularities, or contaminants on the surface. The steel surface to be tested must be very clean, smooth, and clear of debris, inclusions, or contaminants.

I've read where hardness testing is done on a fire-scale surface, and this is an amateur and glaring mistake. To get an accurate reading, the maker must be reading the steel surface that is the final finished surface that the knife will have, essentially the core of the steel. This can only be reached after heat treating by removing perhaps .0003" to .0005" (half a thousandth of an inch) or more of the surface. This is significant grinding, and I've read where outside heat treating companies who are not keen on grinding on someone else's knife blade surface simply take a Cratex® (rubber impregnated with silicon carbide) wheel and clean off a tiny spot to do the testing. This is a minimum of consideration, and perhaps more cosmetic than accurate. Spot abrading does not produce a highly accurate and valid test, and the reasons are several.

• Simply abrading away the surface scale does not assure that the finished surface is reached.
• A rubberized abrasive wheel does not remove enough material to reach the actual core.
• It also does not assure regularity and flatness of the surface, since these wheels create their own uneven peaks and valleys caused by the abrasive and variable pressure.
• Since they work on high speed mandrels, they create localized heat which may substantially effect the point contact of the surface at the abraded spot. Pressure creates localized heat and heat affects the crystalline structure of the steel. You might not think this is important, but remember that the tester is capable of delineating measurements of close to one millionth of an inch, so localized heating can effect the surface considerably.
• Silicon carbide, the abrasive used in most of these rubberized abrasives, can be deposited into the actual surface of the steel. While you might not think this happens, please understand the microscopic nature of what is being measured. Adding some incredibly small contaminants might not effect the visible appearance of the steel, but remember the magnitude of the measurement.

Another factor in establishing an accurate reading is the degree to which the surface is finished. Some ASTM standards require "lapped and polished" specimens! None of this "grind the spot to 400 grit and you're done." Remember, this is in the final test that is done, the number that will accompany the knife. While in my experience the degree of finish can vary the final reading at up to one point depending on the finish, in this world one point is significant variation. Standards also recommend that the underside of the tested piece is also finished, as any microscopic movement or deviation against the anvil (think sliding or slight displacement or compression) will affect the measurement.

The surfaces to be measured must be parallel, testing surface to anvil contact surface. This seems obvious, but an angled surface will cause sideways pressure as the geometry becomes a wedge which will effect the reading as pressure is applied.

Thickness must be considered; a blade that is 1mm (.040") is too thin to test in the standard Rockwell tester. This is not often seen but on thin chef's or fillet knives, but is also a reason that the blade is not tested in a thin region (near the cutting edge or on thin tangs).

Testing must occur away from any edge (or hole or any irregularity). It stands to reason that since the metal is deformed, if the test is near the edge of the material, it will be deformed at an angle, and the reading will not be accurate.

Time is seldom considered, but did you know there are standards as to how long the pressure is maintained at the contact point of the penetrator? This is because steel deforms plastically, even very hard steel, and it may take minutes for this to occur.

All of these reasons add to the logic of a maker testing his own knife blades. The tests should be standard, an average of multiple tests, and the maker can control all of those tiny yet significant factors that contribute to the viability of the tests. Farming out knife blades for heat treating will not establish these practices, an outside heat treater does not get paid to experiment or vary process, or test along the way, or establish standards in his own testing regime. This may be well and good for the hobbyist knifemaker, but for the professional, a higher standard of measure is desired by both the maker and knife owner.

I'll go into this in greater detail in my book, but I've included it here to help the reader understand just what is required in establishing an accurate guideline and framework of testing for knife blade hardness to aid in processing the steel to its highest performance, and establishing accurate benchmarks for the client and knife owner and user.

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Outside Heat Treating Contractors

In this section, I use the word outside because this refers to people who heat treat outside of the premises of the knifemaker's or knife manufacturer's own studio or shop.

There is quite a bit of money to be made by contracting heat treating, with very little relative expense. Once the equipment is paid for, it's only electricity and expendables (mainly liquid nitrogen and gases) to pay for, and the business can be quite lucrative in large batches. This is because there are many people and companies who make knives (and other cutting tools) and few who understand or who can perform the process of proper heat treating and processing. More about this in the Equipment topic below.

There is nothing wrong with outside contractors, particularly if the maker can not reliably heat treat his own work. But us older guys who see this in the trade know that if the maker doesn't heat treat the blades himself, he's handicapped and limited in the knowledge and scope of his work, and is simply not in charge of the entire process of making a knife. I'm sure I'll get some hate mail over this, as guys frequently write to me to justify what they are doing and how it's the best, most proper, most reasonable way to do things, and it may well be, at least for them.

If he's in the business of knifemaking and calls himself a professional, his clients simply hold him to a higher standard, one that assures he knows all of the particulars of the field, and steel work is the entire scope of the blade. If he doesn't perform all of the necessary operations of the blade making, how will his client know that his blades are worth their salt? Will they then question his ability to interface a permanent handle with the blade, create and install the fittings, the handle material, and the sheath? Every part of this operation that is contracted or farmed out means that other hands are involved, other ideas, other skill levels, other process and procedures, and other levels of quality are in play.

Cheaper and Easier

This concept of contracted outside services help can then be logically extended. Since it's easier to send a blade out for someone else to heat treat, it's clear to the client that the maker is interested in the easier way. Most guys won't get this, but understand: IF A KNIFEMAKER DOES THIS, his potential clients will know that he is taking an easier step, and will wonder what other easier steps he may be taking. This will affect his reputation and standing as a knifemaker, and it's been demonstrated over and again in this field.

A manufacturer or maker contracts out his heat treating, it's cheaper and easier. He may then contract out his blade construction, logically to foreign shops (in India, Pakistan, Taiwan, or China) because it's cheaper and easier. He contracts out the handle supplier, and the sheaths, all the while making things easier and cheaper. Reasonably, he's then competing with all the other makers and manufacturers who do this, and the clients know he's making easier and cheaper, so they will only pay easier and cheaper prices for his work. This continues a downward spiral called "lowballing," where the only option is to make a knife easier and cheaper. That way, you can compete with easier and cheaper knife sources, but that eventually means moving all of your operations to a foreign company because, here in the United States of America, we have extremely high median annual earnings, and people don't want to work for cheap, yet most people buy cheaper and easier. This is particularly true of the majority of knife interests (and represented in our national trade deficit!) and that is why most of the knife information, companies, exchange, forums, websites, postings, and conversation is focused on the low-end market of cheaper and easier mass manufactured knives, who inevitably, contract their work to offshore labor.

Cheaper and easier is fine, if that's the direction one wishes to go, but know this; it becomes a business based on cheaper and easier volume, and this is not how fine works are created. Worse, this is not how fine works are purchased by those who seek them out.

For the sake of argument, let's say that a maker is not interested in fine works, owning and understanding the whole process, or meaningfully contributing to the art, craft, and science of knifemaking. That's fine, but don't expect him to create a knife better than any other mass-marketed product, and don't expect to be successful at it, unless he's building a company about cheaper and easier. Also, don't compare cheaper and easier knives to those that are completely made, in house, by a professional, because the two are in different leagues altogether. And don't expect a cheaper and easier knife to ever be of value to a client, as it will end up a poor performer both in the physical sense, and in the investment sense.

Simply put, if a knifemaker starts contracting out for the sake of expense, there is no logical stopping point, and I've seen it over and again. A knifemaker makes knives, and that means he makes the blades.

For Knifemakers:

Every maker who uses an outside heat treating contractor swears they are the best ever! If you don't believe this, I challenge you to find one maker who complains about the result of their chosen heat treating contractor. After all, this would be detrimental to the sales of their knives, wouldn't it? It's always been curious to me that every single knifemaker crows about the results of their chosen heat treating contractor's results, no matter who they are. Are there any realistic comparisons of each contractor's process or results? Does the knifemaker even know or understand the process and timing of the procedure, since different procedures (like snap tempering) produces lesser results? Or, as knifemakers, are they simply taking what they get and then using the heat treating contractor's name to bolster their own knives value and performance expectations? Perhaps, just perhaps, they can justify why they don't do the heat treating themselves, because, after all, they could never be as good as old (insert name here)'s heat treating process...

It was shocking to read in a forum posting about a knife blade that was treated by an outside contractor, and the maker then had to deal with rounded moons of different appearance of the steel on the cutting edge and spine of the blade. These "blemishes" were there after grinding and sanding, and the maker wondered if there was something wrong with the steel. Then, another maker who used the same heat treater chimed in, claiming that the marks were there from the "torch process" that the heat treater used to straighten the blade! What? This is an atrocious screw up on the heat treater's part; no torch should ever be used to straighten a blade; hell, no torch should even be in the same room with a knife that's been heat treated! This is a ruined blade, a blade where localized heating has completely changed the integrity, crystalline structure, and the entire allotrope of the steel in spotty locations, and that is why it looks different. It's botched, it's ruined, it's garbage and unlikely to be saved, and the guys discussing it shrug it off as it it's something that's normal and routinely done! How terribly sad, not for the maker (who is plainly ignorant of the process) or the guys who think it's normal (who are plainly ignorant of the process), but mostly for the guy who buys the knife and finds that it has soft spots, or high wear areas, or reduced toughness in spotty locations, or finds that the blade eventually cracks from dimensional instability. Yet the heat treating contractor goes on to treat blades this way as if he is doing the right thing, and the guys who use his services claim he's the best... truly sad.

What if the heat treater screws up your blade? What will they do? You'll have to get a clear contract from each individual business to find out. Most of them will give you a value (coupon or money or purchase potential) for the size of the blade in raw stock. Yep, you get another bar of steel, raw, so you can start over. This may be fine for some makers, but it's not how I would want to do business. After all, the value in the blade is in the labor that has (hopefully) gone into the blade before heat treat. That may mean profiling, drilling, grinding, filework and in some cases, even engraving! Yikes; that could be many, many hours and that's a hellofa hit! As a maker improves his work, this can be a substantial amount of the value in the knife, and I'll flatly claim that it's usually many times more than the value of the raw steel. But if a heat treater screws up, a bar of raw steel will be all you'll get for your trouble. That is, if he admits he made a mistake at all! I disclose this because typically the heat treater will blame the maker: you've ground it too thin, you've ground it unevenly, you've got the wrong type of steel, or other reasons that a blade may be "less than optimum" after heat treat. By the way, less than optimum means warped, cracked, bent, curved, wavy, or damaged.

Heat treating overall is a fairly simple and straightforward process, clearly outlined in the manufacturer's white pages and through endless online sources. This is another reason to do these operations in the knifemaker's own shop or studio, so the maker himself is responsible for the very best treatment of knife blade throughout the entire process, without delay, with complete control of the process and thus the results. This is why many of us old-timers claim that if you don't heat treat your own blades, you're not really a knifemaker.

Heat Treating Purist Knifemakers

In the decades I've been doing this (full time professionally since 1988), I've heard a continuous, unsolvable argument about heat treating by the knifemaker, and farming out this process step to others. It's sad to see that it has caused such grief and embarrassment, complaint and conflict among makers, and here's how it goes:

A guy wants to get into knives. He makes some with simple hand tools and then quickly realizes he needs more equipment. He gets what equipment he can as time goes on, and the improvement and enjoyment of his craft builds. This is how it should be. I don't know of any maker who was gifted an entire functioning shop or studio, all at once. So the modern knifemaker is then building his skill set, his tool set, his process understanding, and the result of this should be obvious and apparent in his works, his completed knives, sheaths, stands, cases, and accessories.

I'm different than most guys, I suppose. I didn't get into making knives because I wanted to make and have knives; I got into this because I was fascinated by the process of heat treating. That you could create such wide and variable ranges of steel hardness, toughness, flexibility, corrosion resistance, and appearance of steel is what captured my interest. As I stated in my bio, an old welder told me if I wanted to understand heat treating, then make a knife. The reason he knew this is because a knife blade is truly a special case. It's not structural steel that must support a piece of equipment or building, and it's not tooling steel that must be made to its absolute highest hardness for extended wear resistance at high feed rates. Knives are special, they have to be hard and tough, wear resistant and tenacious, a bit flexible, and (in most cases) corrosion resistant.

I started with a torch; a #12 rosebud (heating tip) of an oxyacetylene rig. I quickly learned that the process with this rather crude tool was tenuous at best, and highly uneven, but it was all I could afford. It didn't take long to come up with the resources to buy my first burnout oven, because at the time I started, there were no dedicated knifemaker heat treat furnaces. I modified the oven by doubling the element size and modifying the controller, to make it into a rapid ramp furnace. I then built a furnace in a discarded refrigerator body, and it had an internal furnace and internal quenching chamber, was evacuated by vacuum pump and infused with dry nitrogen! This was a real beast, it could gain 500 degrees F a minute when empty! I progressed beyond that, and am still progressing.

My point is that I've always believed that the heat treatment of steel was the basis of knife making. The very basis. It's what differentiates the knife blade from the raw stock and separates the cold chisel from a scalpel; it is the very foundation of why a knife is a knife and not just a piece of metal with a cutting edge on it. Most makers (and clients) agree.

There are those who, for whatever reason, do not wish to do their own heat treating. I know why: it's too expensive, it takes to much room, too much equipment, too much effort. They may not be confident in their capabilities, they don't understand the process, they simply may not be able to afford it or justify the expense of the equipment. I understand; I've stated I can barely afford my own shop, and this is how most creative artists and fine craftsmen work! It's okay to have knives treated by someone else, as long as it's disclosed, and as long as the knifemaker can explain what heat treating is and how it works for his clients. Most clients don't want to know the intricacies of carbide nucleation and propagation in evolving crystalline bodies; that's the knifemaker's realm.

The Purist vs. The Absolute Purist
-and the comparative argument

Frequently, I've seen makers become highly defensive about this topic. Old guys like me might claim that you're not really a knifemaker if you don't do your own heat treat, and the sparks fly! You'll hear or read the argument of the absolute purist, which seems to be a go-to sarcastic and defensive posture based on this principle: "If you don't dig your own ore out of the ground, smelt it yourself, then you aren't a knifemaker either. Take that, you heat treating knifemakers!"

What an embarrassing comment, trying to compare mining with knifemaking. That's like saying to the fine artist painter that unless he shovels the titanium bearing ore out of his own mine (remember, to the absolute purist everything must originate by the hands of the maker), then he cannot call himself a fine artist. Perhaps Michelangelo wasn't a real sculptor, because he didn't hoist his own marble from the quarry by himself. A jeweler could not call himself a jeweler because he didn't mine his own gold bearing quartz. See how this goes? So the absolute purist shovels this cynical argument back on the maker who does his own heat treat, as if heat treating your own knives is somehow wrong!

This is an argument of building or creating raw stock vs. building or creating a knife. In insulting comparison, the absolute purist claims that unless you build your own raw stock, you cannot call yourself a knifemaker, thus equating building raw stock to building a knife. Of course, no one believes this, so the guy making this argument has just excused himself from heat treating because, in his mind, the two are equivalent!

But the comparison and technique is based in total falsehood. Let's get this clear. This is not a discussion about heat treatment; this has somehow become a comparative evaluation of raw stock production vs. treatment of a machined, forged, and/or hand-worked piece of raw stock and the two are of totally different operations, concepts, and even different fields! Yes, mining is a different field than knifemaking, foundry creation of steel is not knifemaking, but heat treating is knifemaking! If you don't believe so, then just quit heat treating your blades altogether, after all, it's not part of knifemaking, right?

Guys will go on to claim that cutting out a blade is the same thing. Have someone else cut out your blades, after all, it doesn't affect the "quality" of the knife. Why stop there? Why not have someone else grind them, shape them, sand them, polish them? Why stop there? Why not have someone else attach a handle, sand and finish the handle, polish and embellish the knife? Why stop there? You are still calling yourself a knifemaker, right? Why not have someone else make the sheath, make a stand, and sharpen the knife? Why not just have every single facet farmed out to different companies, people, organizations, and groups, in many different countries, and have the knives delivered to your door, and then call yourself a knifemaker? Why not?

There is no clear definition here of what constitutes a "knifemaker," specifically, exactly, and repeatedly. There is no designation of "knifemaker" on the United States Government's IRS Principle Business or Professional Activity Code which is how this business is classified by our government. So, really, who defines what is a knifemaker? You could bake apples at a travelling renaissance fair and call yourself a knifemaker! You could glean old newspaper from the county dump and call yourself a knifemaker! Why not? Where is the line? There is none, really, unless some authority decides to make one. So, go ahead, call yourself a knifemaker, no matter what you do... right?

This, then, becomes a discussion of integrity. That's a powerful word, right? It opens up all sorts of concepts like morality, virtue, reason, and truth. Man, those are those tough words that are borne out through years of service, concepts that involve others. I'm not saying that if you don't heat treat your own knives, you don't have integrity. I will say that if you make the base comparison that raw stock manufacture is the same as heat treating a knife blade for the purposes of justifying why you don't heat treat, you don't have integrity.

Then there is the work scope argument: I've seen these guys take it a step further, claiming that unless you're a professional heat treater, you can't treat your own knives adequately! That's the ultimate in farming-out justification.

Okay, let's accept that premise just for fun. This means that you can't drill an adequate hole in a knife blade, because you're not a machinist. A typical machinist has completed a certified, often regulated level of training or apprenticeship recognized by an official governmental organization. The machinist fixes the work to a trammed table on a dedicated machine, centers his hole using a centering indicator, or digital readout, from blueprints, and starts with a spotting drill progressing to a combination drill/countersink and then follows with screw machine drills, larger and larger drills, followed by a drill just slightly smaller than the final required hole size, and then he reams the hole with a chucking reamer, and then measures it with a calibrated pin gauge. A knifemaker therefor can't drill his own hole; he doesn't drill professionally; he's not a machinist! By the way, ask a machinist try to grind a knife offhand, and he'll tell you to go to hell, because he's not a knifemaker!

More fun: a knifemaker can't possibly fit a handle to his blade, because he's not a carpenter. The carpenter would carefully store the wood, making sure that the moisture content is correct, using dedicated tools to plane the raw stock and determine the specific grain arrangement desired. He then profiles out the stock, kiln-dries or kiln-ages the stock to the specific required condition, and then may hand-plane it and hand-scrape it in just the right direction and orientation for the application. He may use special chemical stains that react with the tannin in the wood to create the appearance he desires, then after this chemical treatment, uses stabilizers to stop any further reaction. Then, he starts sanding... a knifemaker could not possibly do this; it's not in his scope.

On the other hand, is a knifemaker who claims you're not really a knifemaker unless you heat treat your own knives a purist? Perhaps we can compare this to other trades, say, a jeweler. A jeweler may not mine his own ore, but he does his own cutting, fitting, soldering, and casting. A woodworker or carpenter may not grow his own trees, but does all of the cutting, shaping, fitting, bending, and treatment for surface and appearance. The potter or ceramics artist may not dig his own clay, but he does all his own shaping, forming, and firing.

Reverting back to the absolute purist, he may argue that some jewelers do not do their own casting, that some ceramics artists do not do their own firing, and that some parts can be farmed out! Do you see how this argument festers in unsolvable circles?

Here is the really important thing: most artists, craftsmen, and creative people would prefer to have as much control of the process as is possible, particularly involving the core of what it is they are making or creating. For a knifemaker, the very core of his work is the performance of the blade (after all, he's not a letter opener maker or a spoon maker). The performance of the blade is established by steel choice, steel geometry, and steel heat treating, and all of these are the responsibility of the maker of the knife. If you don't think so, ask a client this: "Who is responsible if one of these factors fail in a knife they purchase?" For instance, if the heat treating is missed and the blade is soft or has soft spots, or cracks from dimensional instability, you won't get away with telling the client that it's the heat treater's fault. More importantly, If the quality of a sharpened piece of steel even matters, why wouldn't the maker want to control this essential step? Why is it then that people take the stand of the absolute purist (if you don't dig your own ore, you're not a knifemaker) as a moral equivalent to justify why they don't heat treat their own blades?

It's because they want to be called knifemakers, that's why. If the name "knifemaker" did not matter, why not just assemble kits and call yourself a knifemaker? Why not buy the entire knife assembled and created overseas in a boxed form (these are available) and glue on a piece of wood and sand it, and then call yourself a knifemaker? Why not just buy the knife already completely assembled and then just etch your name on it and call yourself a knifemaker? Don't laugh, I've had people ask for this! You can easily see the progression of logic, and it's all based on the fact that the word, "knifemaker" means something, and means a lot!

To argue these concepts are a baseless, useless endeavor. A guy who doesn't want to heat treat his own blades will not change; the guy who believes that knifemakers should heat treat their own blades will not change. It's all about integrity of the individual. If you do it, explain why; if you don't do it, explain why: that's all the client wants to know. The client will make and understand the distinctions between the two. And if you feel some discomfort in telling a client that you don't heat treat your own blades, you probably need to examine that a bit more and make some adjustments in your process, abilities, and knives. That way, your client will be confident in your abilities and understanding of what it takes to make a knife.

Look, it's okay to have blades heat treated by someone else, particularly if if the maker can't afford the equipment, space, time, and effort of doing this critical operation himself. But in doing so, let's hope the knifemaker doesn't simply rely solely upon the heat treater's name in the trade. Let's hope the maker wants to educate himself on the process, and start heat treating because that is the core of working with knife blades and the foundation of building a good knife.

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Example of Bad Outside Heat Treating

On one of the forums, an inexperienced knifemaker was shocked to find that his steel blade had a huge, rounded fracture in the spine, only discovered when he was grinding it. It became clear when he removed the outer "crust," which was, I suppose, carbon that had migrated to the surface in decarburization, which should never be there in the first place. However, the decarburization failure was not the main issue. It was clear that he was a beginner or novice, because he asked if the process of grinding would have caused the large crack. He explained that he had the knife blade treated by an outside heat treater, so he suspected his grinding technique, or perhaps the steel type as causing the fault, since "everybody" was using this heat treating contractor.

Let me be crystal clear here, so clients, knife enthusiasts, knife owners and users, knifemakers, and readers can understand this, as this is a great educational moment. No grinding technique or operation with abrasive belts can cause fractures.

Abrasive belts simply sand and grind (abrade) the surface, and unless you somehow pinch the blade between the belt, contact wheel, and frame of the machine or tool rest, no impact, no force occurs that can damage a properly-treated blade. Even if intense localized heat was created in the steel, this would never cause a fracture, it would just overheat the blade leading to other issues, like loss of temper.

In the discussion, the steel type the guy was using was immediately suspect by the other forum posters, and this was also the wrong path for discovery. No matter what knife blade steel is used, grinding would never cause a fracture. Add to this, the steel he was using was Nitro V, a distinctly low alloy, low carbon stainless steel that has the main advantage of being "easy to grind" and forgiving (and ductile), and a fracture seems absolutely incongruous. The posters kept blaming the steel, over and over, not even considering that the heat treating process could be at fault. By the way, if a steel is truly horrible and can't be used or sold, it will not be manufactured, as knives are only a small part of steel use and applications.

The forum posters all chimed in, with mostly incorrect ideas, but more than one mentioned "the straightening process used" by the particular heat treater, a large outside contractor that is mentioned a lot by novice knifemakers. The actual crack was a great indicator of the culprit, as it is big and round, like a dent a hammer would make in wood if you slipped off the nail, after being heated with a torch.

Hammer? Torch? Steel? Heat treat? Straighten? NOOOO...

If your head is spinning at this idea, good for you. A hammer has no place in a heat treating facility; there should be armed guards at the door handcuffing and perp-walking anyone that comes near a heat treating area with a hammer! What about a torch? No torch of any type has any place in a heat treating facility, under any circumstances! What about straightening? Another horrible word that has no place near any heat treater, and no consideration in the heat treating conversation! This is clearly why knifemakers should heat treat their own blades.

I've read about this before; this particular heat treating contractor taking a hammer and/or an oxyacetylene torch to knife blades after and during heat treatment (usually during or after quenching) to "straighten" a blade. This is a horrid error, a sign of total incompetence, and hobbyist, part-time, or unskilled full time knifemakers seem to think this is okay! They mention "localized heating" and this is a despicable, terrible, and destructive technique to attempt to fix a screw up!

Let's get clear and gritty, and I'll speculate about what I think is going on. Understand that this is my opinion based on 40 years of heat treating and knifemaking. You can take it or leave it, control your own heat treating (or make sure your knifemaker knows what he is doing) or accept that someone is going to take your knife blade, when it is most vulnerable and during critical phase transformation, to a dark room, and abuse it with a 5000 degree flame and a ball peen hammerhead on a stick.

The first suspicion is the word "straightening" NO knife blade should ever need straightening of any kind, unless it's hand-forged and only then before stress-relieving and while it's still red and plastic in the forge. If a blade comes out of the heat treating oven or quenching medium with a curve, warp, wave, or distortion, something was done wrong: obviously, terribly wrong. In the thousands of blades I've made in four decades, only a few have been distorted in any way, and I can immediately identify the source and cause. It was always an error, and never the fault of the steel type, the quench medium, or the ovens and furnaces.

For instance, if a blade is not held vertically in the heat treating oven, it can relax, curve, and bend, and when quenched, the curve stays. Another error would be quenching in liquid (water, brine, or oil depending on the steel type) and improper entrance into the liquid or improper agitation. A severe and rare occurrence would be waviness due to an extremely thin grind, an overly thick spine, and the difference in quenching rates. All of these errors are caused by heat treating or geometry faults, and no "straightening" is ever done! The way these errors are corrected is this:

1. Completely anneal, stress relieve and soften the steel, straighten the blade and correct any distortions, and heat treat entirely over again with full hardening and tempering.
2. Quietly put the blade in the trash, be a man, and start over.

Notice that the word "hammer" and "torch" does not appear in any of the two correction techniques. Even the blade that is fully annealed can be easily straightened and flattened by the pressure of the bare hands, since it's fully annealed and soft. It can be tested against a precision granite machinist's block for confirmation of flatness, re-ground to make it absolutely true, and the heat treating can be repeated.

If a knife blade is in transformation (in ANY part of the heat treating process), or has been hit by a hammer, heated by a torch, or deformed any way during or after heat treating, here's what will happen:

1. There will be substantial and detrimental crystalline changes in the steel in the area where localized heat and force are applied.
2. The blade will crack and fracture.
3. The blade may not immediately crack, but may crack later in the knife owner's hand.
4. The blade will absolutely have uneven or horrible wear characteristics, poor performance, and will be prone to failure.
5. The knife blade will be ruined.

I've seen this before with this same heat treating contractor. What was surprising was to read that the same heat treating contractor had had the same "results" with other knifemakers, yet these guys keep sending them work! Obviously, the heat treating company either doesn't know what their employees are doing, or is producing bad results on purpose! More than likely, individual custom knifemakers are way down on their list of priority process, since they are a large industrial firm with much more substantial clients. Why would any knifemaker keep sending this company blades to heat treat? Why?

Now, here's the real shocker. This particular heat treating contractor is a big name in industrial heat treating. The company has an annual revenue of about $5 million and employs about 45 people. They have been in business since 1979, yet claim over 300 years of experience (this is a fairly commonly bloated claim by companies with more than one person). On their bulleted list on their website, one of their "Why Choose Us" points is that they have "Straightening Capabilities."  I assume each employee has their own personalized company hammer...

This is another reason that knifemakers should absolutely heat treat their own knives. Even if the maker can only afford the heat treating furnace and none of the cryogenic equipment, and can only afford to do a conventional heat treat, his results will be known, understood, and completely in his own hands. Even a conventionally treated knife blade is better than one that has been hammered, abused, heated in spots with a torch, and will fracture or prematurely wear, bend, or fail.

One more thing: what if, just what if the knife blade fails due to this "localized heating and straightening?" What if it fractures during use, and the blade injures someone or pops into their eye? Who will be liable for this poorly made and treated blade failure? Will it be the knifemaker or the large heat treating contractor? In these litigious times, this is an extremely important consideration. In all tools, knowledge of correct manufacture and construction operations and techniques bears heavily in a court of law.

As a knifemaker, are you prepared to explain why you trust an outside heat treating contractor after reading this section? As a knife owner, are you ready to accept that your knife was in the hands of an outside heat treating contractor?

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More Bad Outside Heat Treaters Information

I really wish I didn't have to do this; I wish that people representing companies who call themselves professional were just that, professionals. I consider a professional a person who has integrity, who doesn't make flippant, offhand, false, or inflated claims. Unfortunately, in much of knifemaking, there is a distinct lack of professional integrity, and an abundance of people who really don't understand the basis for their profession, and this includes heat treating contractors.

As I’ve stated before, if you are a knife buyer, a person who takes your hard-earned money and expects to purchase a premium knife, you deserve to know every bit about how your knife blade was treated, what to expect in performance, the reasons for the treatment, and why you would spend considerably more than you would for a factory knife. Since this is the case, if you are a knifemaker using an outside heat treating contractor, you also deserve to know every bit about how your knife blade was treated, what to expect in performance, the reasons for the treatment, and why you would spend considerably more for custom heat treating.

You would think that heat treating contractors would be specialists in their tradecraft, that they would know their process inside and out, and that a knifemaker who uses them could rely upon the integrity, knowledge, and deep understanding of the process to produce a premium knife blade treatment. You would think that, but in most cases, you would be wrong.

I watched a group of videos produced by a small knife company featuring one of the most predominant heat treating contractors that knifemakers use. The person who was presenting themselves as a professional began to explain heat treating. While I will acknowledge that most of the basic information he gave was correct, he made a lot of statements that were simply untrue, or were based on misunderstood, misapplied, or faulty concepts or information.

Please remember that I’m not writing about a hobbyist, a casual knifemaker, a beginner, or a novice. I’m writing about the statements made by a professional heat treating contractor, advertising in knife media, online, in knifemaking supply company catalogs, on websites, and praised by knifemakers. This would be someone who you would think you could depend on for knowledge and integrity.

If you think that all professional heat treating contractors know their stuff, allow me to enlighten you with some actual statements presented by one of them on a YouTube video, compared to the truth. Remember, there are knifemakers sending steel to this guy's firm to have it "professionally" heat treated. Sad, truly sad.

• Claim: "Oil quenched steels have less alloy; they're using just basically carbon to get their hardness."
  Truth: Steels that are oil quenched have a range of alloys, not particularly more or less than other steels. The quenching media suggested for some of the highest alloy steels is often air or oil, depending on the properties desired. Just look at any manufacturer’s data on the quenching media and you’ll find oil suggested as an option. Alloys with high chromium, high vanadium, high tungsten, high niobium, and high molybdenum all have oil-quenching process parameters. Some steels are actually classified as “oil-hardening,” and they are all tool steels, all high in alloy content. The common steel O1 is a high alloy oil-quenching tool steel, containing significant amounts of chromium, vanadium, and tungsten. These alloy elements are in the steel to increase the hardening potential, strength, and wear resistance of the steel.
• Claim: “The only thing (alloy) in an oil-hardening steel is carbon.”
  Truth: Let’s first assume that he’s not talking about all of the high carbon, high alloy stainless and tool steels that can be hardened by quenching in oil (440C, ATS-34, CPM154CM, etc.) and assume he’s just talking about the Tool Steel Classification of Oil-Hardening steels. These steels all contain significant amounts of complex alloying elements: manganese, chromium, vanadium, tungsten, molybdenum, and in some cases, cobalt! Clearly, the representative knows very little about oil-hardening steels and the statement is wrong.
• Claim: "High alloys enable the steel to get harder quicker without severe quench."
  Truth: the alloy content does not enable steels to get harder quicker. In fact, the alloys retard or slow the hardening process; that is why many of them can be quenched in air as well as oil. The severity of the quench is a tenuous term. Is it more severe to quench steel in water to room temperature in 30 seconds, or by air to 325 degrees below zero in an hour? "Severity" is not a metallurgical term for a reason. Therefore, this comment absolutely makes no logical sense.
• Claim: "With O1 and 1095, we're looking to get that down to where you can hold it in your hand in about 15 seconds."
  Truth: this would not be possible using the correct heat treating protocol—if this is being done, the steel is improperly heat treated. The proper way to quench both O1 and 1095 is to bring them to their critical austenitizing temperature (1650°F for 1095 and 1500°F for O1) and then quench them in oil that is pre-heated to 125°F (don't put your hand on that!). For a very small knife blade, this takes about 30-45 seconds. Larger blades would take 45 seconds to over a minute. Most steel foundries suggest that these steels are never allowed to reach ambient temperature before tempering begins. The only way to get a knife blade to cool that quickly (15 seconds) is by quenching it in water or brine, which would ruin both of these steels. Let’s hope he’s not actually doing this.
• Claim: “The quick (oil) quench traps that carbon and makes a carbide and that gives you your hardness."
  Truth: What the—? The purpose of all quenching (oil or otherwise) is to create martensite from austenite. Martensite is not carbide, carbides come much later in aging and tempering. Carbides are not formed in quenching, as quenching does not allow time for diffusion of carbon into carbides. He was probably confused by the fact that carbon atoms do not move very far in quenching, but that is not the claim. The allotrope that creates hardness in quenching is martensite.
• Claim: “Air hardening steels have less of a chance of warpage and fracture because they cool slower."
  Truth: Any steel knife blade can warp, and it is not a factor of the quenching media. Warpage happens because of a mistake in heat treating. One reason would be that the steel was heated too fast for differential temperatures to be eliminated, such as in objects with large, thicker areas and small, thinner areas. Another reason for warpage is lack of proper support during heating as hot blades are soft and will simply bend in the furnace. Another reason would be improper quenching, such as lack of proper agitation in the quenching media, slow or intermittent dipping into the media (called interrupted quenching), and lack of stirring of the quenchant (in liquids) leading to hot and cold areas and uneven cooling. Warpage is always a mistake in heat treating, not the type of steel and quenching time. You might consider this carefully if a heat treating contractor blames the knifemaker or the steel type for the warpage. This happens a lot, by the way.
• Claim: "Cryogenics only applied to industry during the last 10 to 20 years."
  Truth: Cryogenics has been used in industry since World War II, which ended in 1945. Germany used cryogenics in the 1930s industrially. Knife manufacturers have used cryogenics in knives with stainless steels since the 1950s. The YouTube video was made in 2012, so the guy commenting was off by at least 50 years...sigh.
• Claim: "—you quench the blade quickly (from austenitic) and give it a couple lower temperature tempers to take a little of the hardness, the brittleness off the top and add a little bit of strength."
  Truth: This is a flippant comment referring to “snap tempering” before cryogenic cooling. Snap tempering is done to reduce the amount of martensite (the desirable allotrope) in order to hinder complete transformation. This is done (as described on this page) to allow a convenient time period for the heat treating contractor to get to the rest of the quenching, or to soften the steel because he may not be following the 4-5°F per minute rule in cooling, and nothing else. It does not add any strength whatsoever, and in reality, reduces the strength of the steel blade since it converts some of the martensite to ferrite and cementite.
• Claim: “Cryo treatment tightens up the grain, adding strength, corrosion resistance, and wearability; a tight grain is what you're going to get from that.”
  Truth: Does steel have some looseness that we are concerned about? “Tight” is a strange, ridiculous word to apply to any metal. Metals are not loose, and are not tightened by cryogenic processing. Cryogenic processing hardens the steel by creating a more complete transformation of austenite to martensite, and cryogenic processing increases eta-carbide development of the steel, creating hard, small particles that add to the wear resistance. Movement of carbon in the creation of carbides helps increase toughness. Nowhere is there any metallurgical reference to “tightness.” By the way, “wearability” refers to the knife sheath being able to be worn on the body; the correct term for knife blades is “wear resistance.” We professionals in the metals trade know this—
• Claim: “Corrosion resistance is increased because the grain is tighter.”
  Truth: Here we go again with the tightness. Corrosion resistance is increased in cryogenically treated knife blades because of a more complete transformation to martensite, because of decreased asperity (roughness), and most importantly, the increased amount of chromium carbides. Since there are more chromium carbides, there is a repassivization effect that can increase the corrosion resistance of the surface of the steel.
• Claim: “Stainless steels will generally hold an edge "a little bit better" than carbon steel blades.”
  Truth: This is an example of the gross generalization that haunts our tradecraft. What stainless steels? What carbon steels? If he is talking about high alloy, hypereutectoid (high carbon) stainless steels, they will “hold an edge” not just “a little bit better,” but a by a remarkable, substantial, and striking amount! Since chromium is the hardest metal in the periodic table of the elements, and since high chromium blades contain significant amounts of extremely hard chromium carbides, it’s clear that when properly treated, stainless steel knife blades will outperform plain carbon (standard) steel blades in every category: wear resistance, toughness, corrosion resistance, stiffness, longevity and use. And I didn’t even mention vanadium, molybdenum, tungsten... High alloy stainless steel blades, when properly processed, will absolutely hold the cutting edge many, many times longer than all carbon steel blades.

You might think that in writing this section, I’m nitpicking terms, concepts, and claims, but I’m simply demonstrating how rife our tradecraft is with people who don’t really understand the metallurgy, yet claim they are worth the investment of your money. Please think hard about the logic, information, and experience of every so-called professional. There are a lot of these guys out there doing a passable job, there are others who may do an excellent job, and there are others who are no good at all. Now you have some logic as to why this is so.

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Recipes for Steel Processing

Look around on the internet and you'll see plenty of recipes for heat treating and processing of knife blades steels. You can find them on forums, bulletin boards, hobby sites, and knifemakers' own websites, but you won't find any here. The reason that I don't include recipes is that they can only be generalized, and heat treating and processing, including hardening, tempering, annealing, or spheroidizing is vastly different with each steel type, and with each circumstance.

What are these differences?

• The steel type and alloy is the first determinant factor in process steps. From this very page, and on the descriptions of steels on my Blades page at this bookmark, you can easily see that steels vary tremendously in their alloy content, their use, and their applications even if specifically and only used for knife blades! There is no standard recipe for steels; even the process itself can vary. For instance, some steels require staged quenching; some do not. Some require supplimental hardening experienced only in temper alternated with deep cryogenic thermal cycling; some do not. Some simple steels require only one tempering cycle for one hour, some steels require at least three tempers for the best performance results the steel was designed for. Some steels benefit from four tempering cycles! Some will anneal, some will not, some can anneal just by heating them up and letting them air cool, some require cycling at high temperatures to achieve full softness or spheroidization. There is no standardized process that can be given by a basic recipe.
• The steel manufacturer plays a very large role in steel processing. Even with the same steel classification, designation, and alloy set, the steel varies greatly between manufacturers or foundries. This can be seen in the wide variations of treatment and processing on each manufacturer's data sheets and white papers, and clearly, there is no standard for each steel type. While different sources of steel may have similar generalized ideas of treatment, no universal recipe exists.
• The date the steel was manufactured is seldom, if ever, discussed or considered, and I will flatly claim that this is a significant factor in the processing of the steel. It's not that the steel ages or changes in any way by simply sitting around waiting to be treated and processed; it's that each manufacturer periodically varies their own foundry process and steel alloy content. You can read about this on the disclaimers on their own websites and their own data sheets. They simply tweak their steels, and the method used to handle them varies accordingly. You'll read (from them) that they reserve the right to change these process variables and the steel alloy content, so that their own data sheets are simply a current or dated guide. It's expected that as they roll out new steels, lots, and batches, that they will tune and adjust not only the alloy, but also the alloy's processing steps to refine and improve their product.
• The steel geometry is a considerable factor in processing steps. I've mentioned before that guidelines for steel processing are typically established by processing 1" thick sections; this is standard in the industry, and knives are not 1" in all dimensions. Knives are a very special case, with relatively thin, long pieces of steel that require a unique blend of hardness and toughness, and even that varies depending on the type of grind, the thickness of the cutting edge, spine, and even the geometry of the ricasso area where forces are transferred through the handle to the blade. Even the inclusion of serrations and the geometry of the serrations must be considered in the process results. The tip type, cross section, and geometry, the expected use of the knife, the associated fittings, materials, and mounting method must all be individually considered, and all these factors vary tremendously between knife blades.
• The foundries also offer guidance on processing based on a safe margin of error or safe ranges. What this means is that they don't want a machine shop, fabrication shop, manufacturer, or maker using their steels to suffer from catastrophic failure of their steels, so they give very safe and carefully marginal guides. Of course, they don't want their steels to crack, warp, or deform; they want them to be durable and tend to offer process instructions on the side of toughness, not wear resistance. Most of them don't even suggest deep cryogenic processing, opting for the term "cold treatment," and I'll suggest that this is a generalized term for a reason. Many of them won't detail the range of "cold treatment," and this may be done for several reasons. They may not be sure of established testing of their steels in these various ranges, since the testing of new steels is slow and expensive, and scientific and institutional settings for this type of testing is budget-strapped. It may be questioned if they are up on the current processing technology, or it's possible they don't want to offer a process that would tremendously increase (as much as 8 times) the life of the items created with their steels!. If they did, logically, they would sell considerably less! By the way, the term "cold treatment" is not the current standard, it's only a generalized term; the standard terminology is listed above and is used by The Journal of Materials Processing Technology, The American Iron and Steel Institute, the International Journal of Emerging Technology and Advanced Engineering and other scientific and professional resources.
• The processing environment and equipment is one of the largest factors in this discussion of why a "one size fits all" recipe for steel processing is a useless endeavor. In knifemaking (and in every machine shop or fabrication facility) the environment, the equipment, and the people performing the processing of steels in every step are vastly different. In knifemaking, you've got guys adapting toaster ovens for tempering, not understanding localized heating and thermal transmission, and not even understanding the difference between radiation, conduction, or convection. You've got a wide range of equipment in play, in all different environments in different parts of the world. Variations of timing, movement, and even humidity and ambient room temperature will all play a role in how various steels respond to treatments. Good grief, there are makers using window fans to air quench blades, completely against the foundry's process recommendations for still air quenching, when even a slight draft in a room can have a deleterious effect on the steel!
• Evaluation of process and tuning of the process can also vary wildly. For instance, an inexperienced maker may claim that a cutting edge is fracturing because he thinks it's too hard, when in actuality, it's been overheated in post treatment grinding, and is too soft, making breakage easier because it's ground thin! These mistakes in process control and the understanding and evaluation of issues and events can vary widely among craftsmen, and to offer a standard recipe for processing would assume that the user is capable of evaluating and understanding all of the intricacies of the process in his particular environment. Even when this is necessary, good scientific method must be employed, varying only one aspect of the process at a time to tune it, and recording, testing, and evaluating the results.

What to do? This is not so simple, but it is obvious. While I can't speak for other makers, in my studio and shop, I have an established written log of heat treating to keep accurate records of each step of the process. I've developed specific methodologies based on my particular environment, equipment, materials, and knife geometries. It's logical to have and employ some basic testing equipment that any machine shop would have, and to keep records of evaluations along with the recommended process sheets for each type of steel used. It's critical to have knives tested (to the point of failure if necessary) to understand the results of process adjustments. The experience in these evaluations that can only take place by making many knives with a wide variety of materials. These knives should be field-tested and use-tested in the hands of professionals, and that should be reflected in the type of knife, the wide assortment of knives, and the client basis that uses them. There is no better testing than an satisfied client, and decades of satisfied clients is a sobering goal.

For those of you who are interested in just what steps are done, and what time this all takes, please know that my clients deserve to know exactly what has been done to properly process their knife blades, and they are welcome to the details. For the general public and other knifemakers, just to be clear, a typical processing of a small batch (4-8 knives) of high alloy, martensitic hypereutectoid stainless tool steel knife blades consists of 15 steps, and takes about 95 hours. I also have an extended, detailed and developmental process called "T3" which shows some surprising and beneficial results. It takes 30-33 steps and 172 hours, about a week!

Other knifemakers and all manufacturers and boutique shops, as well as all heat treating firms and individuals are not willing to pursue the excellence of knife blade performance this far, but I'm committed to the very, very best performance out of my high alloy knife steel blades. I also understand that most makers only use one or two steel types, but it makes sense to have intimate knowledge of how those steels respond in every way including long-term exposure to the elements, usage, sharpening, and finish potential, something few makers but all clients consider. Above all, it's important to be able to explain to knife clients what these processes are, what they do, and how they effect the blades that the client desires, because the knife client, owner, and user are really the important factor in this discussion.

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What is this "T3" and "T4" Heat Treating Designation?

If you've seen a lot of my knives (I know you have; I can read my statistics), you have seen my declaration of my heat treating method. In my early knives, I simply used the phrase "hardened and tempered," which is, essentially, what heat treating is. As a professional knifemaker, I simply harden and temper my blades, just like everyone else.

—Except my blades and treatment are NOT like everyone else's; not by a long shot. As I developed and improved my process, I went from conventional heat treatment to freezing treatments, to extended freezing treatments, to quenching nitrogen furnaces, to sub-zero treatments, and ultimately to shallow and then deep cryogenic treatments. You probably realize that from the basic descriptions on this page.

This is how it should be; a knifemaker's techniques should continually improve based on his knowledge, equipment, materials, and practice during his career. As I improved my understanding of these high alloy steels, my treatment protocols and steps have changed. I've mentioned before that I don't give out recipes for blade steel heat treatment, not because I don't want to share this information, after all, you are on the largest, most comprehensive website of any singular knifemaker in the world. I don't give out details because most people wouldn't understand them; it's a science that exists with differing results in different steels, different shapes, sizes, lengths, and thicknesses of the particular blade! This is why heat treating en masse isn't a good idea; sending your knives off to a faceless heat treater to be give the "58 Rockwell Treatment" won't enhance them or offer an individualized level of improved performance in the process. And isn't that what custom knifemaking is about?

Other knifemakers will argue this all day, to no avail, since most don't have the processing equipment or practice that gives them insight to these differences. I've got a special project in the works on that; keep an eye out, as this will interest anyone who is reading this right now. Without revealing the details, I will note some simple designation practices I use in my own treatment protocols.

"T3" is the designation I give to Tertiary carbide formation technique; the "T" is for "tertiary." This is a third level of eta carbide development that happens in deep cryogenic environments, due first to primary martensitic development, but followed by days of spinodal decomposition. In simpler terms, "tertiary" designates at least 70 hours of deep cryogenic soaking. You might wonder if and how I've seen primary and secondary carbide development, and I have clearly seen and experienced this, settling on the tertiary level as being the most reasonable and productive at this point in my work. In upcoming texts, I'll detail how all this has happened, and what to see and expect in other heat treating processes, but that's beyond the scope of this webpage.

The numeric designation, typically "3" or "4" is much simpler. It refers to how many individual tempering cycles were performed on the knife blade. I've written before how critically important these cycles are to the development of carbides in the steel, and how multiple temper cycles will enhance toughness, and in some cases the hardness of the steel. Imagine how exited I was to see dramatic improvements in overall wear resistance, making the steel seem like a wholly different material. Truly, it takes on another allotropic form when proper temper cycles are used, and with all of the steels I use, this absolutely requires multiple temper cycles. I will go into more detail about this in my upcoming work, since tempering is not what I thought it first was, back in 1979, when I made my first knife.

For my clients and other readers and researchers who wonder about the designation, I can sum it this simply: Tertiary carbide development in deep cryogenics, followed by multiple tempering cycles designated by a number. Thus: T3 means tertiary carbide formation in deep cryogenics followed by three tempering cycles.

The reality of how these cycles are employed, and what happens between these steps is a book on its own! And, no, I don't plunge the blades into liquid nitrogen for a couple days like an oaf!

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Decarburization (Decarb)

Decarburization is the loss of carbon from the surface of an iron-based alloy (in our case, steel) as the result of heating in a medium that reacts with carbon.

The number one error in heat treating is decarburization. Carbon can move in solid steel, and when the steel is heated, it tends to migrate to the surface and react with oxygen, producing a scale of black, burned material. This scale is mostly the carbon that was in the steel knife blade, now uselessly sitting on the surface.

When novice or unknowledgeable knifemakers discuss this scale, they often call it "decarb" and tell others that it simply has to be removed, as if it is some normal, expected occurrence. Know that all decarburization scale represents a failure in the process, and no scale or decarburization should be present on any steel knife blade after heat treating. Knife blades should come out of the process only slightly discolored, with no visible scale, and a simple light sanding with fine grit will be all that is required to remove the discoloration. In the properly heat treated blade, usually a fraction of a thousandth of an inch will have to be removed to bring the blade to bright, full polish.

If there is "decarb," the carbon content in the steel has been compromised. There isn't much carbon in steel to begin with; in most knife steels the amount is from 0.6% to 1.6%. The loss of carbon is highly detrimental to steels, and steel knife blades that have decarburization are, in reality, failures of process. You won't see the word "failure" used by knifemakers when discussing decarburization, but it represents a final, lower content of the most beneficial element in steel (next to iron) and is truly a breakdown in heat treating.

You can't tell if your steel has lower carbon from appearance, particular if the blade is sanded, ground, or finished. The knife blade will still cut, it may still seem hard, wear resistant, and tough. What you will have is a lesser performer, overall. This goes on a lot in knifemaking; blades seem good enough for use, but are inferior to a properly treated blade. When you finally do receive a properly treated blade the difference can be astounding. To know how this happens frequently in our tradecraft, read this section of bad heat treating process on my 440C page.

Decarburization happens for several reasons, and it's entirely preventable, so there is really no reason to tolerate this huge error in heat treating practice of knife blades. The first cause is the atmosphere around the blade during critical heating. Heat treating furnaces can be outfitted with nitrogen or argon inert gas to purge out the harmful oxygen surrounding the blade. Another, more common solution is to wrap the blade with a thin envelope of stainless steel heat treating foil, commonly used in all machine shops. This forms an exclusionary environment and atmosphere around the blade, one lacking oxygen or other harmful gasses, and prevents all decarburization. Combine the two (purge gas and stainless foil) and the blade will come out of heat treating with just a slight discoloration and absolutely no black scale at all.

For steels that must be immediately quenched, like O1 oil-hardening steel, the envelope would slow the process of quenching, so it's not used. O1 is heated quickly to its hardening temperature, in a reduced oxygen or inert gas furnace, and held only long enough for complete transformation, and no longer. When I treat O1, the only discoloration is a medium gray darkening of the surface from the oil it's quenched in.

This leads us to the second cause of decarburization and that is extended time at transformational temperature. Steels that are held for far too long at critical temperature will have excessive carbon migration, and thus, heavy decarburization. When manufacturers suggest holding times at austenitizing temperatures, these are suggestions based on standard steel sizes. While you'll have to contact the supplier's data source for clarification on this, many foundries time at critical temperatures are based on 1" thick blocks of their steel! A knife blade is incredibly thin in comparison, so does not need to be at this temperature for as long as a 1" thick block. Experimentation and fine tuning by each knifemaker will define these critical times. What is important to know is that the data sheets are suggestions, and each machining shop or tooling maker is expected to develop his own process.

For the knife client, owner, and user, know that no decarburization should happen, ever, on your blade. The steel should come out of the process clean, and only slightly discolored (from heat only). How to tell? If your knife blade has unfinished areas that are blackened, there has been decarburization. If the knife is completely finished on all surfaces, you may not be able to tell, so ask your knifemaker about decarb. If he claims it's normal, find another knifemaker, or at least one with more skill.

To illustrate how slight discoloration is the only effect from heat treating, consider this—when I heat treat a fully engraved knife blade, in order to preserve the engraving (done before heat treatment) I can only remove about half a thousandth of material after heat treating to bring the blade to full polish. I grind the blade to 1200 grit before heat treating, then I engrave by hand, and after hardening and tempering, I make a light pass with 2000 grit abrasive to remove the color, and polish. Good heat treating practice will preserve fine engraving cuts, because it takes almost no finishing to bring the knife to a high polish. That's good heat treating!

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Adjustments, Fine Tuning, Sophisticated Blade Steel Treatment Regimes

You may have read on this page, my Blades page, and on my FAQ page that there are no secret treatment procedures, regimes, and methods that are unheard of in knife blade treatment and heat treating of specialized tool steels. In those sections, I suggest that the maker should follow each manufacturer's suggestions; that it's easier than baking a cake, in that the knifemaker does not have to mix the ingredients, he simply has to process them. For most knifemakers, and for most manufacturers and companies that heat treat knife blades, it's simply a matter of following the manufacturer's directions, because each foundry has spelled out their specific treatment procedure. In the section above, I explain why recipes should vary a bit depending on the manufacturer, date of steel creation, and expectations of the use and application of the steel.

You are on an advanced page now, so I'll take this further. While it's fine to start with the foundry's recommendations, what if fine tuning of the process is necessary or desirable? Like every professional machine shop, the knifemaker is expected to develop his own treatment regimes, to tune the process a bit, to make adjustments that can yield an improvement on the standard foundry or steel supplier's recommended process.

I'm writing about tuning and adjusting the process, equipment, and steps: tweaking them for the best possible results. The maker should start with the manufacturer's process, and then, if he desires, and if he as the time, interest, and ability, he can make adjustments that can yield a higher performance blade, or a more dimensionally stable blade, or a tougher blade than standard process.

For my own work, I have no issue whatever discussing the fine tuning of the process of heat treating and cryogenic processing of knife blade steels with my clients, and have found that they are quite knowledgeable about the subject. I don't offer this information to the public, here on the website, or to other makers, but I do discuss these particulars with engineers and metallurgists at my suppliers and at the foundries I deal with. It isn't reasonable to offer these details to knifemakers, since all of them have different equipment and work environments than I do. Many of them have mistaken and incorrect process steps. For example, a good deal of these knifemakers don't adhere to the cold temperature drop rate of 4-5 degrees Fahrenheit a minute and they just thrust the blade into dry ice slurry or liquid nitrogen, which is ruinous to the steel. A lot of them don't even reach a martensite finish temperature which leaves the steel with metastable (unstable) retained austenite! Nearly all of them short the cryogenic soaking and aging equilibrium step to a handful of hours maximum, even though it's been proven that this is not enough. Because of these mistaken concepts and lack of study and experience on the subject, it doesn't make a lot of sense to share minute details of process with other knifemakers when they don't even understand the basic principles.

After having made knives for over 40 years, my clients expect that I apply certain adjustments to my process overall, and these can be significant and important improvements based on my particular equipment, layout, steps, and heat treating environment. Of course, all changes and adjustments need to be carefully tested and plotted, recorded and evaluated in the heat treating log that records everything done to every knife blade.

What are some of these adjustments I'm writing about? They start with simple things, really. The tuning of a heat treating oven and calibration of the thermocouples compared to a standardized test instrument is and important one. A minute or two adjustment between various blade thicknesses in a batch that contains both thick and thin cross-section steel blades offers improvement. The block quenching apparatus, temperature, and time compared to rate of cooling fluid flow and circulation can make a difference in the steel quality. Peak drop-off times of temperatures between steps and even how many seconds it takes to move the knife to the next phase of the process is important. Cryogenic soak times are substantial players in the process; there is a substantial difference in soak times of 2 hours, 10 hours, 30 hours, and 60 hours. The temperatures and rates of cooling and heating in the multiple tempering cycles are tremendously important, as is the very specific temperature control of the tempering oven and cooling rate buffers. No single component is more important than another; all are critical steps. The entire process is a synergistic reaction of allotrope conversion and control, leading to a superior result.

As with all of these adjustments and tweaks, the client deserves to know the process, if he is interested and inquires about his knife. The knifemaker's recorded log should track the steps and results; fine tuning of the process is expected by the professional. It is a controlled, sophisticated methodology based in experience, testing, logic, and comparison. It can only happen through decades of practice by the professional knifemaker treating his own works, wearing the attitude of the creative logical experimenter balanced with the control of the technician and scientist.

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Equipment

This section is directed more at knifemakers who may wonder about the equipment used in proper processing of these modern, high alloy, and stainless steels. Knife clients and users may learn a little about why this process is important and the maker's understanding and control of it is even more important.

The equipment used in proper processing of high alloy and stainless steel alloys varies depending on the size, amount, and budget of the individual shop, studio, or maker. I won't go into manufacturing or mass-processing, because you can find all you want about these large pieces of automated equipment on the internet. Manufacturing interests are well-covered in the large tool processing field, and examples and suppliers of this equipment are abundant. It's enough to know that a decent small cryogenic processor can be acquired, brand new, for about $10,000, with some smaller models available and plenty of larger ones to be had for the price of a new vehicle. That's just the cryo processor, not the furnace, quenching stage mediums, or tempering ovens and chillers.

This doesn't work for most small shops, as knifemakers and artists are typically processing dozens of blades a year, and not hundreds. Small volume is the problem, and this is why it's usually uneconomical for the individual maker to outfit his own shop. Small volume equipment is simply not made, not offered for sale, and not available, which is sad, because if someone was making these smaller units, they would probably do well! For instance, furnaces are now available for the small shop and studio, but when I started making knives, they weren't available. Most of us who wanted to heat with electric furnaces were adapting and modifying casting burnout ovens and pottery kilns to do the job. Then, several companies started making knifemaking heat treating furnaces and they are doing quite well today fulfilling this need in our trade.

This means that currently, for the modern small shop, the maker must make first the equipment to do the job. This means custom adaptation of existing or available equipment, equipment borrowed from other industries and professions, equipment not designed for, but adapted to use in the small metals lab, studio, and shop.

There are those who claim that without a "proper" cryogenic processor, the results achieved cannot be as good as shipping the knife blade off to a heat treater who is outfitted with the dedicated equipment. This is not true; if the process is followed accurately and specifically, it doesn't matter what brand of equipment is used to do it; it's about the accuracy and control of the process. Just as it doesn't matter which brand of furnace is used to heat the steel to it austenitizing temperature, just as it doesn't matter what brand of oven is used for tempering: what matters is the rate, temperature, and time.

I will suggest that a reasonable attempt should be made to avoid having an outside contractor heat treat and cryogenically treat knife blades because of the reasons detailed above and throughout this page:

• The maker is uncertain of the process done to the knife
• The blades are often snap tempered, leading to substantial retained austenite and an overabundance of pearlite.
• The austenitizing temperature and timing is unknown as is the decalescence point.
• The cryogenic process aging time is unknown.
• To have the most dramatic advantage, cryogenic processing must be immediate and concurrent, and not delayed for the sake of economy or batch processing.
• The timing of the knife blade processing is dependent on the heat treating contractors batch. Steps like snap temper are performed so blades will become "stabilized" while batch volume is built up for cryogenic processing.
• The tempering process is unknown, and some steels must be triple tempered with deep cooling between tempers. Timing is critical and varies with the size of the blade!
• The most important part: the knifemaker does not have the confidence, understanding, or dedication to learn about the very process that makes handmade knife blades better than manufactured knife blades in every conceivable way.

That last one is a real kicker. Say you are confident that your outside contractor has the equipment, know-how, and reputation of reliable heat treating. You are dependent on his name, his work, his method. Fine, but what about if you have a client who asks you to make a knife that is non-standard? What about a client who wants a knife made of a steel that has a wide range of heat treating options and methods (like D2, ATS-34, or CPM154CM, CTS®XHP, or CPMS30V)? These, and many other high alloy and stainless steels have a variety of treatment methods, all to create different results. You should be able to explain these to your knife client, as someone who orders or purchases a fine custom handmade knife expects more knowledge, more information, more savvy from their maker, as these knives are not cheap! As a knifemaker, at the very least, the tuning and tweaking of the process to achieve a fine, specific result is an ongoing learning affair, and it allows a vast improvement over typical "safe" methods of processing. I use the word safe because it's safe for the heat treating contractor to perform a less aggressive heat treat regime to avoid damaging a knifemaker's blade. This, I will claim flatly reduces the performance of the knife blade and it's clearly described above.

Batch processing and cost factors: for the professional heat treating contractor, consider this: a modest and small cryogenic processor can eat up 5 liters of liquid nitrogen per hour. This may not seem like much, but think about this a moment. Consider that it will require at least 30 hours or more of soak time, and a ten hour cool down at recommended rates to get to this temperature. This means that 40 hours at 5 liters per hour and the processor is chewing up 200 liters of liquid nitrogen. There are about 1.8 pounds to a liter of liquid nitrogen; this translates to 360 pounds of liquid nitrogen. In our location liquid nitrogen is currently costing about $2.00 per pound. So this means that using this particular processor rate, it will cost $720.00 to do one run. A conservative heat treating contractor may charge between $10.00 and $20.00 per blade, so in order to just break even with the nitrogen cost, the heat treating contractor must batch process at least 70 blades. But this is not the whole cost; he must pay for the equipment, the electricity, the facility, the transportation and overhead of the business and his own labor. I'm using the keystone rule to suggest that he needs to do 200 blades at once, just to break even and make a payment on his equipment.

If your blade is in the 200 in the batch, do you think that he will do it when it's optimum in timing and without process delays? This doesn't make sense. This is why outside contractors handle huge lots of knives, many from knife manufacturers, and they include some handmade works in the batch. Not a lot of personal attention to your knife; this is expected, after all, it's not the maker carefully controlling the process of each knife blade heat treatment.

Look, I'm not here to slam heat treating contractors, they perform a critical service to the industry. I'm just showing how this works, so that you know what to and what not to expect in this process. If a ten thousand dollar processor and hundreds of dollars of liquid nitrogen per heat treat is not in your budget, there are other means and reasons to build your own equipment for this procedure.

Most makers are familiar with heat treating, and my own decades of experience has proven that simple tools can do the job quite well. The issue is understanding the process. For instance, in cryogenic processing, it's not important what container is used to hold the knives submerged in or suspended above the liquid nitrogen for dozens of hours during aging, what is important is the rate at which the steel reaches that low temperature. Simply dropping a blade into -320°F liquid from room temperature is sure to be a disaster. You don't need a sophisticated cryo processor to slowly lower the temperature; there are other methods, including building quench staging, buffer chambers, and even building your own small batch knife-sized processor if you are inclined! The knifemaker often has to build his own equipment and methods, and as long as the focus is the process itself, he can control his results to a fine degree. In my own studio, the equipment is built around the process, and the process is built around the desired result.

I won't go into great detail about the technical side of the equipment here; that's a discussion for my advanced book, and every artist and craftsman has his own idea about how he would like to proceed, based on the knives he's building. I will acknowledge that a good background and understanding of physics, electronics, and machinery is essential if you are to take this on. You won't find this information on a knife forum; it's too far-reaching and complicated. As with most things, a little knowledge is a dangerous thing; immersion in deep background study must take place first. I will state that if you have read the entire page up to this point, you already have an understanding of heat treating and cryogenic processing greater than most knifemakers or knife manufacturers!

In my studio, the equipment is accurate, verified, dedicated, and tested, and much of it is either handmade, or adapted from other industries for the specific purpose of making knives. It's also like the rest of my knifemaking: continually evolving, improving, adapting, and changing to suit continual advances in steels, in knives, in materials, and in the requests of my clients, who want the very best knives possible from me. This is a tradecraft and art where the learning never stops, and I'm proud and happy to be a part.

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Misconceptions, Myths, and Lies

• "Cryogenic quenching embrittles the steel." This is wrong. They think of a banana dropped in liquid nitrogen that shatters when it hits the floor. This is not steel. The banana undergoes a sudden, drastic freezing of its water molecules, expanding them into large ice crystals which is a brittle structure. Steel is slowly and evenly cooled, there is no radical change (from a liquid to a solid), and the harder structure of martensite that results is then tempered for the correct and required toughness.
• "NASA created cryogenics." No, not true. people have been freezing things for many decades before NASA. In fact, the Germans were industrially using cryogenics on parts for aircraft engines as early as 1930. The United States started using cryogenics in the 1940s. NASA didn't even exist until 1958, so when someone says this, they are trying to make their product, service, or themselves look cool. Don't believe it.
• Wikipedia states that "Very little research into this technique has been published in the scientific literature, and the papers published to date are contradictory." Wow. Whiskey Tango Foxtrot. This is just a lie, and whoever wrote this was plainly ignorant of the massive amount of research and results performed in this field. Do tell the all the major steel industries about WIKI's open source findings, so we can return to pre-World War 1 status. Sigh.
• "If it were good, the cryogenic treatment of steels would be more widespread." Really? Just because the person who thinks or writes about this is out of the loop, it's no excuse to deny that currently, some of the many items that are considerably improved by cryogenic treatment by manufacturers and the machining industry are: saw blades, drills, cutting pliers, knives, and punch and die sets, milling cutters, taps and dies, gear cutters, broaches, files, scissors, trimmers, slitters, woodworking tools, chain saw blades, drawing dies, and stamping dies. It is widespread. Do some research, for goodness sake.
• "You can make a perfectly good knife without cryogenic processing." This is true, but personally, I am not here to make merely good enough; I'm here to make the best. I can make a perfectly good knife with hypoeutectoid (lower carbon) steels, a perfectly good handle with leather, make a perfectly good knife handle without bolsters, make a perfectly good sheath with single thickness kydex and hollow eyelets holding it together. Perfectly good. And good is, I suppose, good enough. If you think so, you're on the wrong website. Sigh.
• "Cryogenic Processing is only for machine cutters: mills, drills, indexes, etc." This goes along with the claim that "a knife is not a valve seat, not a planer blade, not a forming die or cutting tool." Okay, this is true. But both are steel, and wear resistance in all uses can be improved drastically by this process. Even though you may not use a hand-knife at 200 surface feet per minute, you will cut with it, so why not improve the wear resistance, and toughness, and corrosion resistance as much as possible? Why not have your knife user, owner, and client sharpen knives only half as often? This is a conservative improvement potential in cryogenically treated hypereutectoid steel alloys: doubling the wear resistance. If you were the knife buyer and owner, what would you want? Would you say, "Hey, don't bother with the cryogenic stuff; I'll take mine only half as wear resistant; I like to sharpen more often. And that way, I can cut my blade life in half." ...Ahem.
• "You can't do it unless you're a professional/metallurgist/scientist" The reason most makers don't do it, and some justify their lack of process participation while verbally spanking other makers for attempting it. This is some shallow game that a lot of knifemakers do, and it's just juvenile. They throw insulting terms like "garage cryogenics" and one even claimed the old adage, "sleeping in a garage does not make you a mechanic." This is jealous, shallow, spiteful ignorance. Cryogenic treatment is simply a process, just like heat treating, only different, with different equipment and technique employed. Would the jealous complainer fault the maker (and himself) for operating a professional grinding machine? I know many machine shops that would love to have a knifemaker's grinder, but they are just not skilled enough to use a belt grinder, opting for a bench grinder instead. Isn't the knifemaker trying to be a professional in his field? Doesn't that mean every part of the process? There really is no technological mystery to the process of cryogenic treatment of metals, it's not voodoo science; it does not require a Ph.D., and it's becoming more commonplace every day.
• "It's too dangerous." Yep, fooling with metals and machines that cut them is like playing with baby rabbits. Grabbing a handful of bright yellow orange metal with tongs and thrusting it in a hot bucket of oil is something a child could be trusted with. Turning on a bench lathe that runs at 5 horsepower and has enough torque to twist off a one inch bar of steel in three seconds is a cake walk. Grinding with white hot sparking metals flying off into clouds of dust is namby-pamby kid stuff. Touching a blade to a rag wheel spinning at over 150 miles an hour is something a novice can try.... and it goes on and on. What? I thought we were talking about cryogenics! Yet these things we knifemakers do daily are much more dangerous and deadly, and we learn, understand, and apply safety practice in all of them. So that's just an excuse. By all means, it can be dangerous, so as adults, we know that and take safety precautions and care.
• "Cryogenic tempering:" I saw a website of a company that cryogenically treats razor blades, nylons, and other finished products (what about my underwear?) and the owner of the company called his treatment "cryogenic tempering." What? There is no such thing. And that's the owner of the company! Boy, talk about not knowing your own business. Cryogenics may be applied (in metals) in quenching and in aging, but NOT in tempering. Tempering is reheating of hardened metals to convert a portion of the crystalline lattice into other allotropes, and getting something very cold is not heating... I don't know how this phrase even got started, but by the ignorant. Of course, their company is claiming nylons won't run if you "temper" them with cryogenics, so maybe they are even colder to begin with and liquid nitrogen heats them up.... I'm suddenly disinterested in clothing apparel, no matter who is wearing it.
• "Cryogenic treatment is not a different type of quenching method but is an additional treatment normally used after quenching." Technically incorrect, and this is from an article about cryogenic knife blade treatment in one of the leading knife publications! Cryogenic treatment is part of quenching; as the blade is quenched from its austenitizing temperature, the quench continues into the sub-zero, shallow cryogenic, and in some cases, deep cryogenic temperature range. Then, it is held (aged) for a period of time for the results painstakingly and clearly described above. While some people separate the quenching from the cryogenic soaking, in knife blades this is not particularly necessary, as they are not thick blocks of metal forming dies, and do not suffer quench fracture that would require "snap temper" operations.
• "Cryo treatment is a fad whose time had passed." Really? How much out of the loop are you? This is a claim made by the ignorant, as cryogenic processing is growing every year, and has become a mainstay of many high-tech industries. The truth is that conventional heat treatment is becoming a fad whose time has passed! For instance, as early as 1995, the STANDARD by the United States Air Force for all parts made of 440C in any aircraft are that they are cryogenically processed! A fad? Really?
• "O1 doesn't benefit from cryogenic treatment, so it's not necessary." Wrong, wrong, wrong. While O1 may be adequately quenched in oil and only to room temperature, and it still makes a fine knife blade, read this detailed text above to understand why this comment is so, so wrong.
• "Eta-carbides don't form at shallow cryo, only in deep cryo." Again, a misconception, and it's surprising to note that even a lot of metallurgists aren't up to current knowledge on this one, though the studies (some listed below in the references) clearly explain this. The error that eta-carbides were not formed in shallow cryo was probably based on bad or inconsistent testing, where blades were simply left to reach the shallow cryo temperature, and not aged in a continual, long time period. These carbides definitely do form at -86°C/-125°F (shallow cryogenic treatment and aging). Scientific studies have also shown that most of the benefit of cryogenic quenching takes place in shallow cryo (at -125°F/-86°C) with some steels having further enhancement in deep cryo.
• "Fine eta carbides don't do anything in a knife blade." A knifemaker made this statement, and it's just foolish. Carbide production and benefit is extremely well-known and documented in the cutting trades, and knives are cutting tools. The claim is just wrong, and you can get the details on this page and in countless studies available, for free, all over the internet. This is another example of how knowledge is changing the world through this amazing medium.
• "It can't be proven that cryo helps." Another amazingly ignorant attitude. Read, for goodness sake, educate yourself some!
• "The studies and research are no good because they're performed in (India/Japan/Taiwan/Great Britain/Spain/etc.)." Just because there is no great industrial iron and steel behemoth in the United States any more, this doesn't mean that all other research from other countries has stopped! That's a sad commentary on research in general, and technological progress specifically. Researchers are many races, from many nations. If the studies are clearly described and documented, they are easy enough to assess. Read them, understand them, acknowledge that most of the researchers are not selling a product, bloviating their curriculum vitae, or falsifying facts. It's all pretty clear; don't let racism cloud your mind. It might help to know that some of these researchers have Ph.D.'s in Cryogenic Treatment of Materials. Wow!
• "The idea is nonsense" Sigh; I read this on a post about cryogenic treatment. If this oblivious person simply read this very page, he would more become educated than most people on the physical properties and reactions, and understand why cryogenic treatment is such an important part of metalwork. But the internet is open to all comers, uninformed and knowledgeable, and as the information resources continue to grow, the ignorant will be (hopefully) weeded out or too embarrassed to post.
• "I'm an old metallurgist, and back in the old days, we didn't do this stuff, and I know what is and what was, and what will be and I know all the details...etc." Sadly, I've seen this attitude far too often. First, studies on cryogenics are current and evolving, and a lot of new information has happened in just the last few years (this page was initially written in 2015-2016,and undergoes constant upgrades and additions), and continual studies demonstrate the quickly evolving nature of this science. New materials are being tested every day, and new delineations of study details are constantly revealed and underway. So unless the person claiming to know these things in immersed in current study, it might be best to defer to someone who is, someone who actually applies cryogenic processing to the specific materials, like some of the finest custom knife blades available, being used by some of the top military, combat, and counterterrorism operatives in the world. That way, the knives are made by a skilled craftsman, who, although he may not be a metallurgist, understands the current state of materials technology required by the most demanding users of knives. While you may think this a bit arrogant, know that I defer to all of the published scientific professionals whose works are accredited, peer reviewed, and detailed below. I'm not making some spurious claim, this is science. If someone disagrees with what I present here, their issue is not with me; it's with the sources below. Write to them and straighten them out, please!
  By the way, be very careful about someone claiming to be a metallurgist and offering free advice. Just like knifemakers, they should have their own online curriculum vitae, listing of their accomplishments and study, and they should refer to scientist's and published data to back up their claims, just like any profession. Please note that all of the references listed on this page and below do have just such credentials, and you can easily access them.

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Emails and Inquiries with Answers

As you might guess, I get plenty of emails questioning me about heat treating and cryogenic processing of knife blade steels, since is something I professionally do. Please remember, I don't perform this service for others; I'm not pushing a business in heat treating; I have nothing to gain in the heat treatment of other people's knife blades. The reason I present this information here is like the rest of this website: I do this as a service to my community, to give realistic, valid information to the reader. Whether the reader is a past, present, or future client of mine, whether the reader is another knifemaker, beginner, hobbyist, part time, or full time, whether the reader is a person who has a passing interest in heat treating and blades or he is a dedicated knife enthusiast who wants to know the facts about the subject, the information I present here is a gift to my profession. Anyone interested in knives in service in real world use, knives made in the best possible way our modern technology can dictate deserves to know what is on this page. While the reader can find this information in professional sources, text, documentation, studies, and guides, I've compiled it here with real world knife blade creation, and include misperceptions, erroneous processes, and mistaken ideas, so that you can know the facts and the hyperbole together. Then, the reader can make his own conclusions and decisions based on this information.

As the traffic has grown on this page and many others related to specific blade steels and knife related process, more questions, curiosities, and interest has grown, and that's a good thing. I've decided that, rather than answer some of these questions and inquires on a singular level, I will post pertinent queries here, if they have any bearing on the topic, for others to learn by. I detail this on my page "Learning About Knives." Please know that I don't ordinarily answer questions not related to direct knife orders; I simply don't have time to answer them all. So if you write me and ask a technical question, please don't be surprised if I don't answer you. Sometimes, a query will strike me as important, in that I can offer what I have learned and know and many others who are reading the same pages will find the answers they are looking for as well. Again, I do this as a service to my profession, and you can learn about the service aspects of the professional knifemaker at this link.

Here are some important emails with the answers I've given. Note that names are omitted to protect the privacy of the individuals, and other knifemakers, companies, and entities are also not given, mostly to protect them from embarrassment. I'll add to this section as I feel is necessary; thanks for helping to stop misinformation, wives' tales, and misconceptions in our tradecraft through learning and education.

Further Information:

It amazes me how many knifemakers claim to know what can only be seen with an electron microscope or x-ray diffraction (retained austenite, carbide structure, etc.), while not having the access to, training about, or the very equipment used to make these determinations. But, as I've stated before, it's truly sad that our tradecraft is so filled with misperception, ignorance, and myth. Here's some logical, simple and clear help:

If you are a maker and are using D2, start with the manufacturers’ recommendations. They know their steel and will politely tell you how it is best treated. If you wish to improve on their process, research needs to be done in earnest, through AISI, ASM, ASTM, SAE, and other sources detailing real metallurgical studies. There are some great treatises out there about the process, and I encourage you to purchase them, read them, and study them, and they are not available for free, and this is perhaps one of the reasons makers don't study them; they don't see the need to purchase them, but that is where the information resides.

Forums are not typically the place for professional information (sorry, but true), because the depth of data, information, experience, and process applications can not be presented in a couple paragraphs of a post, or multiple posts for that matter. Take this very page for instance; I suppose it will print out to the equivalent of over 100 pages of text, yet barely scratches the surface of steel knife blade treatment information, and each steel is different! This is why textbook-type resources are the best source of information; most of them are over 600 pages long. For some reason, most knifemakers won't bother to purchase these $100-$400 dollar information-rich engineering sources, much less read them several times, frequently access them for the applicable information, and then apply this data to their own work. I frequently use these necessary resources, but could not convey the scope of study on this website, or this page would be many thousands of pages long!

Again, keep it simple. Start with the manufacturer's recommendations, which are usually the best for basic, reliable performance, and if you wish to improve on them, talk to the metallurgists and engineers at those companies, be prepared for the discussion by studying as much as you can; as most are professionals with degrees and are accredited in their field, and won't appreciate wasting time educating you.

Here's a great formula for success:
Study, study, study, practice,
Study, study, study, practice,
Study, study, study, practice,
Study, study, practice, practice,
Study, practice, practice, practice,
Practice, practice, practice, and then write.

Knifemakers who do little study, minimal practice, and then write on forums are shortcutting a few steps...

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Cold Temperatures and Railroad Rails

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Third Temper Cycle and Data Sheets

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Equipment, Process, and Contractor Limitations

What if you don't have all the equipment to do this? What if you don't trust outside contractors?

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"Special" STS Steel for Knife Blades

Good steel comes in all shapes, sizes and applications. A steel that is designed for and performs well in one application does not translate to a worthwhile use in a totally different application. Here's an email that someone sent wondering about making a knife with deck plate steel.

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440C and Quenching Methods

Here's a great email from a chef who has invested in learning about heat treating in his search for the very best, high performance knives:

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Does it help to quench 440C in oil?

In a follow-up email, I was queried about quenching 440C in oil. Some white papers and data sheets include this as an option of quenching. The steel suppliers don't often describe why this is listed, so I thought it was important to include it here.

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Is carbon steel sharper than stainless steel?

I should have a section on my website called, "Knifemakers say the dumbest things," to illustrate the many wives' tales and myths and misunderstandings in our tradecraft. As a professional, I would be remiss if I let these foolish fallacies stand, and I feel obligated to detail where and who they come from. There is a bit of truth in all myth, but after you read this section, you'll understand where it comes from and why it's a myth. That will be the limit of what you will learn, for there is no truth to this myth at all, just a lack of understanding and lack of skill in knife blade sharpening, and lack of skill in making knives.

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ASTM and Heat Treating Standards

Recently, I had the honor of speaking to a graduate level metallurgy class at a university. They were are great group of minds, and I hope to continue this in the future. One of the participants (Joshua) asked if I followed any standards (like ASTM) for heat treating practice, or if I've honed in on my own special treatment regime in working with high technology knife blade steels.

I thought that the question was important enough to address here, for Joshua and other interested participants and readers for some more clarity. When I answered him, I immediately went into the description of manufacturer's white papers and data sheets on heat treating, and didn't really talk much about ASTM standards. Did you know that there are six different types of standards supplied by ASTM? They are Test Methods, Specifications, Classifications, Practices, Guides, and Terminology. One could write their own book on just how the ASTM works and its critically important place in the safety and use of our modern materials.

There are many standards in our world, because materials need some regularity, so that the uses are predictable and valid. Since we have so many materials in humanity, organizations have been developed to offer a standard or regular way of identifying, regulating, and categorizing how they are worked with, so we know what to expect.

After discussing this with engineers who actually have worked on some of the ASTM's committees and boards, it's clear that the organization offers general guides and specifications, and often refer back to the manufacturers and (in this case) steel foundries and suppliers for the details on how to treat their products. This is because—although the ASTM sets standards—they often encompass a wide range of the material's makeup, percentages, and condition.

For instance, it's standard for 440C to have a range of chromium content, from 16% to 18%, and this is a large difference. Rather than detail each foundry's specific product for a highly specific treatment protocol, the best and most detailed information comes from the particular manufacturer. The manufacturer or foundry offers the starting points for treatments, but even they don't give highly detailed information on steel treatment.

They do this for many reasons, but consider that they don't know exactly what is being made with their steels, so it's expected that each metalworking industry will develop the high details of heat treatment based on the exact use and condition of the steel product being made as well as the expertise of the metals shop, their equipment, budget, and time constraints. I've mentioned these limitations before at this bookmark, but since the foundry or steel distributor doesn't know what happens to his steel, he can only generalize.

Another important point is this type of disclaimer. For educational purposes, this comes from Carpenter Steel® data sheets:

"Disclaimer:
The information and data presented herein are typical or average values and are not a guarantee of maximum or minimum values. Applications specifically suggested for material described herein are made solely for the purpose of illustration to enable the reader to make his/her own evaluation and are not intended as warranties, either express or implied, of fitness for these or other purposes. There is no representation that the recipient of this literature will receive updated editions as they become available."

From this, it's clear that these are generalized starting guides and not rigid, specific, and highly detailed or regulated documents. This is why it's so important for knifemakers to do their homework, refine their process, and keep accurate records and documentation of results to improve their heat treating methods.

One more important point: there is no ASTM standard for hand knives, anywhere. I detail this on my "Knife Testing" page at this bookmark. 

Thanks again, Joshua, for your great question!

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D2, Treatment Protocols, and Cutting Edge Research

At the same University speaking event that I detailed in the previous topic, Matt, a Teaching Assistant, asked about D2 steel. He revealed that there was a lot of variation among knifemakers in the timing and treatment of D2 and there wasn't any specific and regular method for treatment of this steel.

I did detail some of my processing of D2 steel treatment during the session, but I felt it was important to go a bit deeper here, for Matt and other people interested in this very special tool steel. D2 is a finicky, special steel and it's hard to get right. First, D2 should never be exposed to any oxygen during heat treatment; it will readily and quickly decarburize, ruining the possibility of bringing this steel to its zenith in performance.

Matt (and others who want to know), please look over this great study into D2 and cryogenics, done in 2012. I won't include a direct link, since web URLs change, but put this in your search engine:

"Comparison of Effects of Cryogenic Treatment on Different Types of Steels: A Review"
by P.I. Patil and R. G. Tated.

In this study, they stopped the cryogenic equilibrium at 40 hours, and thought that 36 hours was optimum, but my own research has demonstrated a different and improved character of high alloy hypereutectoid steels by bringing this time up to and past 60 hours. Of course, my setup, equipment, range of austenitizing, and procedures are different than in the study. Also, using only the hardness tester at the end of a cryogenic soaking cycle is not the true indicator of what will happen with the wear resistance, since so many more developments happen in the multiple stages of accurate tempering!

This (and other studies) demonstrate that this is truly cutting edge research, so much so, that I'm developing some detailed and specific equipment just to test what happens to these tremendous alloys at these fascinating ranges of treatment.

Graduate level metallurgists and researchers like Matt stand at the crossroads of high technology knife blades, and have the potential to move our tradecraft, science, and art into new realms, never before conceived in modern knives. As I've written before, there is truly a possibility for a knife blade to last 100 years or more, in use. What a great time to be in this field!

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Ridiculous comments deleted from my YouTube Videos

You may be surprised to find that I delete ridiculous comments from my YouTube channel. The reason I do this is that what I present there—just as what I present here—is meant to be educational. When you present data, experience, and information like I do, please understand that I do know quite a bit about what I'm writing, talking about, and presenting. While I can't know everything about, say, every type of steel, I do know about the types of steel that I use in making knives, and am quite well-schooled in the real world of making knife blades. From this very page, you can understand that it is not a simple science, and takes quite a bit of dedication and education to understand the process. Just to give you a clear, small representation, just one of the advanced steel properties textbooks that I reference continually is written by top metallurgists through the ASM and costs more than most other maker's knives!

I don't want to get into deep, detailed arguments on the YouTube channel response string with people who clearly do not have an understanding of the science. Doing so (arguing) will tend to make YouTube just like the forums, where divisive, uneducated, inexperienced, and unknowledgeable forum participants spread misconceptions, misunderstanding, and lies about steel properties. They love to argue, post, and present their opinions without reference, logic, or science, and then hope to have what they deem is a conversation when others respond. This almost always degrades into attacks, much of them personal. It is of zero use to those of us who are trying to actually educate and share their experience.

This is one of the reasons I don't post on forums but rarely, and I absolutely do not recommend them for any viable information. Do not get your information from a forum; the very best makers and researchers do not post there, and you will not find the correct information about steels, heat treatment, and knives there. You will find it in metallurgical science textbooks, graduate level studies, peer-reviewed journals, and serious scientific investigations.

A large part of the issues with the internet are in your ability, as a reader or viewer, to understand and delineate truths from ignorance. There is more ignorance than truth on the internet, so you have to think about what you are reading, who it comes from, what their experience level is, who they make for, and what they present. Always suspect, and check sources! Getting information from anonymous sources or hobbyists is a poor result all around.

Below I'll give some examples of a posting I deleted, and I'll give you some insight—for educational purposes—so that you can understand the ridiculous amount of effort it would take to educate someone who has only casual, peripheral knowledge about steel. You'll understand why I don't attempt the effort, since it would require erasing what the person thinks they know about steel, and then starting over from the beginning, "This is an atom..."

I simply don't have time for this, and neither do you. Please accept my sense of humor at this ridiculousness.

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Here's another one from the same guy:

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Liquid Nitrogen Usage

Here's an email about the expense and use of liquid nitrogen:

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Bohler M390 and Elmax Finishing Problems

Here's an email sent by a knifemaker who has great difficulty in mirror polishing high alloy hypereutectoid stainless steels. The email and my response will make it clear that even the very hippest, coolest, newest and highest alloys have their limited, distinctive, and specific range of use and application. These applications may excel in one area, but may be woefully inadequate in others.

This is why knifemaking is a trade-off, and why some alloys (like the two mentioned below) are best served in thicker, roughly-finished or blasted conditions where high wear resistance takes precedence over corrosion resistance and toughness. This also clearly demonstrates how they should not be used in the kitchen, where blades must be thin, tougher (than other high alloys) and able to resist corrosion. Remember, corrosion starts at the very cutting edge, so this is how important it is.

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Knifemakers, Cracking Blades, and Forum Recipe Parties

I just had to include this; it pops up continually in knife discussion forums, and makers ask me from time to time about problems they have in heat treat. The funny thing about knifemakers is that, if they don't have the necessary equipment to perform a particular and specific procedure in heat treating, they will often ignore the problems that this creates in their process. Then, they focus on another factor, something they can control. Simply, they ignore the actual issue and blame something else.

They don't do this in a vacuum; there are plenty of other hobbyists and part-time knifemakers who will pile onto the problem, all doing the same thing; ignoring the specific and focusing on something else, tweaking and fudging one or two small, unrelated items when the major issue is simply ignored. Since they don't recognize the inadequacies of the major flaw, it doesn't occur to them that it may be happening in their own work. Perhaps, if I explain it using a recent example, it will make sense—or not make sense—which is why it's a problem in the first place.

A knifemaker has a problem with Nitro V, a fairly unremarkable steel. Assigning the name "Nitro" makes this steel sound high-powered and exciting, but in reality, it's a low alloy, ho-hum stainless steel without outstanding alloy content that would make it difficult to heat treat. It may help to understand that it is of lower alloy content than box cutter blade steel: it's hypoeutectoid (lower carbon), and has no volume of any alloying elements that would make it difficult to work or process in any way. Here's a little more information on Nitro V on my "Blades" page, and why it doesn't even rate for a knife blade, in my experience and opinion.

The blades the knifemaker has created with Nitro V have cracked. This is not a singular occurrence; it's happened repeatedly. He is horrified by this (rightly so) and seeks out information about why this is happening. Other makers have also experienced this with Nitro V, and they conclude that the steel type must be at fault. Still, other makers have not experienced the fault, and the debate starts. The forum participants start discussing the minutia of heat treating, and for some reason, focus on the tempering cycles as an issue. They progress to discussing tempering ovens vs. home ovens and start talking about loading up the home ovens with cast iron pots and pans filled with sand and dirt during tempering, so that the hysteresis (fluctuations in temperature caused by the cycling of the heating device) will be reduced.

They do this because they think that this is something they can control. They totally miss the step between quenching to room temperature, and then quenching to shallow cryogenic temperatures. It's not in the conversation; there is not a thought crossing their minds about why QUENCH CRACKING is occurring in the heat treated steel blades.

Someone hints into the possibility of quench cracking. Quench cracking is well-known and understood in metallurgy, machine shops, and metal fabricator crafts and industries. In fact, if there is any crack in any piece of steel at any time during heat treating, the obvious and instant question that would pop up in any metal shop would be "Why is this quench cracking?"

Nobody ever asks, "Why is this temper cracking?"

Seriously, put the term "Quench Cracking" into any search engine and see the hundreds of thousands of results (all helpful) that come up to understand how prevalent this issue is in metal work. Just for fun, put in the term "Temper Cracking" and you'll find that you are redirected to "Quench Cracking" because you have put in the wrong term—really.

This is a very good indicator of the problem, and at least one poster understands that sudden, drastic changes in steel blade temperature at this time is important. The most drastic part of this is when the knife has already quenched to room temperature, and then it's "put in" the very cold environment of dry ice and kerosene, or dry ice and alcohol, or whatever the hobbyist knifemaker has available. The knifemaker may or may not know about the 4-5 degrees Fahrenheit per minute rule, but since these knifemakers haven't got the equipment or process to actually achieve the 4-5°F per minute procedure, one poster suggests that lowering the blade "slowly" into the cold mush will help. He even says this should take a minute or more.

How does a minute to get to -100°F equate to 4-5°F per minute? Seriously, the cooling of the whole blade needs to occur uniformly throughout the blade, and needs to take a minimum of 20 minutes to get to 100°F below zero. As you touch the blade with the very cold liquid, that area cools suddenly and instantly. One of the main reasons for quench cracking is the temperature differences in the part as it's quenched. The blade must be UNIFORMLY and SLOWLY COOLED at a specific and controlled rate. The crack may occur suddenly, or it may occur later, in tempering, in cycling, or even in use, after the knife is sold!

This is clearly "fudging" the science for the sake of not having the right equipment and process, and somehow, these makers think that this will be successful! This happens only because they have gotten away with it in the past, with other blades, with other steels, and haven't noticed the fault. Maybe they were lucky; maybe the knife owner wonders why his blade wears down so easily or why it's cracked, or why it just isn't all that the custom handmade knife blade steel is supposed to be. The truth is most handmade knives are made this way, and while it's accepted among knifemakers, it should be clear to clients that this is going on.

Would you want a metal shop that makes, say, turbine blades for aircraft engines to be fudging the science for the sake of cheapness? By the way, these cracks can be major and sudden, or microscopic and chronic, and will undoubtedly lead to a poor performing blade or outright failure.

No matter; because they don't have an answer for the 4-5°F per minute rule, they focus instead on the tempering. They discuss this like old ladies swapping recipes, with concern about short-cycling their kitchen ovens, multiple stress relieving cycles, worry about hot spots and air flow during temper, and generally ignoring the root cause of the problem.

How very sad for the person who purchases any knife from any of these guys. This is why I write this, so that if you are a knife-buying client, you deserve to know why your homemade knife might not be up to the task, use, and durability that you would expect after you have paid for it.

You might ask why I don't just jump right into the forum and help these guys. I've tried that before, and here's what happens: they don't have the equipment or logic or experience to learn and change, they don't want to hear that from any "big name" knifemaker, they would rather pile on and cause some conflict or confrontation that increases their post count to 42,000. After all, what does Jay Fisher know about it?

I think it's better to post it here, for all to read who want to know the truth. Some knifemakers will read it and learn, but the important thing is that the knife client, owner, and user reads it and learns who may be fudging the process and why.

Thanks for helping stop wives' tales, myth, fallacies, hyperbole, and ignorance in our tradecraft, science, and art.

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AEB-L and different carbides as a selling point?

Someone sent me a statement about AEB-L; it's repeated in several venues on the internet. I don't know exactly where the statement originates; this is not clearly revealed or referenced.

Remember this: AEB-L is the very same steel—along with 420 stainless and other equivalents—that has given stainless steel kitchen knives and working knives a bad name. Because of the low carbon, medium chromium content, and because of no other substantial alloying elements, these steels are low performers overall. It's because of these alloys that so many people hate stainless steels to this day, yet knifemakers continue to use them and continue to praise their performance!

As you read this section, please understand that people will try many things to sell steel, particularly steels of lower alloy content. Also understand that if lower alloy content steels were indeed better performers than higher alloy steels, there would be no use for higher alloys and no reason to have expensive, time-consuming, and elaborate heat treatment regimes of high alloy steels. After all, if the lower alloys were the truly superior performers, why bother with the more expensive types at all? Do you seriously think that high alloys are created only to make life more difficult? Don't machinists, metallurgists, and the industrial world know that high alloy steels are not needed? You might be surprised by what is claimed by even so-called experts and even degreed metallurgists.

When I write about steels, it is from two sources: education and experience. While I'm not degreed, I am highly educated and knowledgeable about my field, and actually consult with and speak at universities and with metallurgical specialists in several fields related to my career and profession. I work as a paid consultant in legal cases about metallurgy, and coordinate, evaluate, and advise litigants in steel heat treatment and cutting instrument cases.

If you are a client, patron, knife user, buyer, or knifemaker, it's extremely important to look at the record, the photos, the experience, and the claims of any source of information, including this one that you are reading right now. On this website, you can clearly see who I am, who I make for and how long I've professionally made knives. You are free to judge my reasoning behind what I present, based on my experience. This is demonstrated here on the website by the thousands of knives and many hundreds of testimonials about the knives I've made for everyone from combat soldiers to counterterrorism agents, from professional chefs to collectors of fine art and museums. Unlike people who just share and post forum comments and rudimentary information, I've actually made these knives, in the real world, and applied the science required to do so.

There are many people posting articles, studies, comparisons, and evaluations on the internet, and some of them even have degrees in metallurgy. Most of them do not have the experience of actually making knives, but go on to post borrowed and referenced work by other people: in text charts, graphs, and photographs. Yet they have never made a single knife!

This is a tenuous activity if the website is actively asking for money or donations, even through internet donation programs and fundraising schemes. Using other people's and researcher's works for monetary gain without their consent is not educational; it's a felony federal offense of copyright infringement!

Compare that practice with one you are reading here—open information, for free, for truly educational purposes—and I don't ask for money, donations, or any kind of support, whatever. You can clearly see the difference. As I've written before, I write not because I need to sell knives; I refuse most work and have a years-long waiting period without ever having written a word of what you see here. I'm just saddened when I see a lack of integrity in our tradecraft and I'm determined to bring logic and understanding out of chaos, myth, and hyperbole in this field. This line of work has been good to me, it's been my sole professional career for over 30 years, and this is my honor to give back to knifemaking and knife enthusiasts. People deserve to know the truth.

Some people posting misinformation are knifemakers, but are not making for top organizations or incredibly serious knife users (like military, law enforcement, counterterrorism professionals, or principle security details) yet boast about their knives, performance, and steel details. Some have great followings of groupies, fans, and admirers; some are begging for additional income by fundraising on their web venues.

My point is that no matter who they are, look at their knives. Look at their experience, logic, and craftsmanship. Look at their record. Quoting from other sources is important, but it does not take a knife expert to simply copy and paste studies from outside sources, and then tweak the conversation to influence the reader.

You can see that I, too, use and reference sources for this very page. The difference between me and most other sources is that I've actually made every single knife you see on this website (apart from a handful of collaboratives), and the knives are in the hands of professional users of the very top caliber. It's not about me; it's about the knives and their owners and users. All I ask is that you logically consider the source of all claims, and their own experience, and not simply what they have read in a book, study, or report.

This particular comparison of steel types is of AEB-L, compared to 440C (shock!). AEB-L steel is the same as 13C26; they are both very simple razor blade steels, made by several manufacturers. This steel was originally patented in 1928. It is a very early, basic stainless steel. I contains only 0.67% of carbon, and 13 percent of chromium, making it a low alloy, hypoeutectoid stainless steel. There are no other remarkable alloy contents, but there is a rather high amount of manganese, making the steel easier to work and machine and preserving austenite (yikes!). As stainless steels go, AEB-L barely classifies as stainless, with 12 percent being the classification point. This means that in acidic, corrosive exposures, it will have much lower corrosion resistance than a higher chromium alloy and will rust. Ever see a razor blade rust after being left wet for a long time? Now you understand the limitation of lower chromium stainless steels.

No matter. This steel is simple to work with, and makes a fairly inexpensive knife blade. Lots of guys use it, lots of companies use it. It's used a lot in pattern-welded damascus blades; I've used it myself in this application since it forges and welds easily because it's a rather low alloy steel with low forging and forming temperatures. It is technically a "strip steel," made for rolls and strips and bands, and it is conventionally produced (not a powder metal). It's a good steel for razors, and making cheap, uniform cutting edges. It is not a high alloy; it's a very simple steel. It's easy to grind, easy to forge and form, and easy to sharpen with conventional rocks (novaculite Arkansas stones, water stones, silicon carbide, aluminum oxide, and ceramics). This is because it's not particularly wear-resistant.

The important point is that if AEB-L were far superior to other steels, it would dominate the entire industry of knife blades, and all other steels would be pushed out of existence and manufacture. I've noted before that there is no ultimate, superior steel; all are different and all have their advantages and limitations.

However, it seems that this steel needs to be hype-sold, because these higher alloys are expensive an costly to make a knife out of, and you can't hand-forge them, and you can't make a pretty pattern-welded decor damascus blade out of them without ruining them, but you can with AEB-L.

What is humorous is the rather lengthy explanation that is being used to sell this nearly century-old poor-performing stainless steel. As usual, it's compared to 440C (everything is compared to 440C), and the text below hints at AEB-L as being somehow superior, due to "special" carbides developed. This is bunk, and I'll present the quote in italics and insert my comments so that you can see what is left out of the text and why and how it can be downright misleading.

Claim: "Few know what AEB-L steel is, and those that do, only have heard that it is similar to 440B or 440A."

Reality Check: This starts out badly. Most metallurgists and machinists and metalworkers are familiar with this steel, after all, it was patented in 1928 and it's been used billions of stainless steel razor blades made in the last 75 years. It's actually very similar to 13C26 and 12C27, also low alloy, low carbon, low chromium stainless steels. This steel and its equivalents have been used on millions, perhaps billions, of poor-performing kitchen knives that have given stainless steel a bad reputation overall. Other steel companies make equivalent steels in many countries of the world. Clearly, this comment is directed at those who don't know about steels, the uneducated and inexperienced that will be swayed by what is to follow:

Claim: "The only similarities between AEB-L and 440B or 440A is the amount of carbon."

Reality Check: This is incorrect.

• AEB-L has 0.67%-0.68% of carbon. 440A has 0.6-0.75% carbon, so the carbon content varies immensely. In nearly every case, the carbon amount in 440A is higher than in AEB-L.
• 440B has a lot more carbon than AEB-L: 0.75%-0.95%! Therefore, the amount of carbon in 440B is quite different, markedly more than in AEB-L.
• Critically (and not mentioned because it makes AEB-L look bad), both 440A and 440B have 0.5% of molybdenum. AEB-L has NO MOLYBDENUM. Molybdenum is a strong carbide former, and adds tremendously to the toughness and wear resistance of these steels.
• Both 440A and 440B also have much less manganese, which makes them more difficult to machine. Manganese is an austenite-stabilizing element, somewhat detrimental to the formation of martensite in elevated levels (like in AEB-L). Austenite is not the desired allotrope in any cutting tool.

Why would this statement be made? Whoever wrote this has negatively influenced his credibility; every venue that repeats the sentence is spreading misinformation.

Claim: "The fact that AEB-L has only 12.8% chromium by weight compared to the 16-17% in 440A and 440B makes the steels quite different."

Reality Check: No kidding. Lower chromium, lower alloy, less corrosion resistant, tremendously fewer chromium carbides—AEB-L is a lower-performing alloy overall.

Claim: "AEB-L is more similar to a stainless 52100 than 440A..."

Reality Check: This is totally ludicrous.

•  52100 is a hypereutectoid alloy steel, with much higher carbon than AEB-L. 52100 has 0.95%-1.10% carbon, a tremendously higher amount than the 0.67% in AEB-L.
• AEB-L is a hypoeutectoid stainless steel, 52100 is a hypereutectoid low alloy steel.
• 52100 also has only 1.3%-1.6% chromium, a tremendously lower amount than in AEB-L. These steels are totally different types!
• 52100 is actually classified as "low alloy chromium steel (look it up!)," because it has a hint of chromium helping it create some limited carbides with improved hardenability.
• AEB-L is classified as a "martensitic stainless steel," which offers much higher corrosion resistance; it's a stainless steel.
• AEB-L is actually in the very same classification as 440A, B, and C: they are all "martensitic stainless steels." (Look it up!)
• 52100 is a steel commonly used in standard (non-stainless) ball bearings, and AEB-L is never, ever used in ball bearings of any type.
• Though 52100 is a great steel for ball bearings, if you want corrosion resistance and you want superior ball bearings, 440C is the desired steel type—that's why it was used on the cryogenic liquid oxygen pumps in the space shuttles.
• This is typical of an attempt to paint one steel as similar to another with totally different properties for the benefit of sales. It's misleading, and it lacks integrity.

Claim: "AEB-L naturally forms what is called the K2 carbide, the harder of the two chromium carbides, compared to the K1 carbide, which is formed in steels such as 440C. The K2 carbide is about 79 on the Rockwell C scale, compared to 72 for the K1 carbide. "

Reality Check: What is "naturally formed?" There is no such thing, and now we've descended to the ridiculous. I've mentioned before how the goal is to compare every steel to 440C because 440C is the benchmark for steel comparison. This is because 440C has a long-standing reputation for high performance and value. Here, a claim of "superior" carbides will lead the casual reader to think that somehow, AEB-L is better, forming a "harder" carbide. The writer of this suggests that "K1" carbide is formed in 440C, but doesn't follow up with the comment, "as well as many other carbides and complex carbide types involving molybdenum and chromium as well as iron, and these carbides are more dense, profuse, and abundant in 440C overall."

Let's get this very clear. There are three basic types of chromium carbides usually formed in high martensitic stainless steels. They are:

• Cr₂₃C₆: a cubic carbide with Rc hardness of about 68
• Cr₇C₃: a hexagonal carbide with a Rc hardness of about 69
• Cr₃C₂: a Orthorhombic carbide with a Rc hardness of about 80

Please look carefully at the hardness numbers; they are misrepresented in the original claim that is the subject of this section. All three of these carbides are formed in 440C. In addition, 440C has a notable amount of molybdenum, and with the extremely higher chromium content, the tremendously higher carbon content, and the addition of molybdenum, 440C forms profuse carbides of many types, including molybdenum carbides (Mo₃C₂) and complex combination carbides (chromium-molybdenum-ferrous carbides). When properly hardened and tempered, 440C forms an extremely tough and hard carbide structure, with higher strength overall as well as higher wear resistance, higher corrosion resistance, and a much higher-performing knife blade! By the way, read more about carbide development on this very page at this link.

 Claim: "Through proper heat treatment, AEB-L has fine, evenly distributed K2 carbides."

Reality Check: Now this gets a little confusing, so please try to follow along— Where is the author of this getting his information? Look up "K2 carbides" in your search engine. I'll wait here.

Waiting...

Waiting some more...

Welcome back. You have found that there is a company named "K2 Carbides®" and references to "K2" carbides in documents about silicon carbide, which is not a chromium carbide. There are also references to fibrosity in carbides that use a designation "K2". K1 and K2 are constants in formulas, but in none of my metallurgical steel references are the names (K1 and K2) used. This is not how metallurgists typically define carbides. They are defined by the chemical formulas listed above. The only other place I see this type of terminology is on hobbyist knifemaker discussion forums, related right back to this string of text.

Where does this designation: K1 and K2 originate? It comes from the book "Metallurgy For the Non-Metallurgist" by John Verhoeven, written in 2005. On page 130, he states, "The first two pure chromium carbides have been labeled K1 and K2 carbides by German-speaking authors."

This is using the German language (on American English venues) to identify two carbides by a simpler means than their chemical formulas. Evidently, Verhoeven wrote this in his book because the book is directed toward knifemakers, and he didn't want to confuse the simple-minded reader.

I'm kidding, of course, but there are some very serious issues with Verhoeven's book. I'm going to go into a detailed critique of this book, but after studying it thoroughly, and discussing it with other metallurgists and engineers, it's clearly written to sell a very particular type of steel to knifemakers. This steel is compared to 440C, and some of the claims are downright horrendous.

While the book is a great general overview of steels that can be gleaned from any beginning to intermediate-level metallurgy course on steel and iron, When it comes to the stainless steel chapter, Verhoeven goes off the rails. He's constructed a diagram (with promoted software offered for sale) that compares 440C with AEB-L. In this, the general concept is that 440C has too much chromium, and too much carbon, and therefore makes too many carbides at the expense of making martensite. This is utterly ridiculous, and then in further reading, you discover that he's conveniently left out molybdenum as an element. He admits that the "440C" he has plugged into the chart (software for sale by developers), is 440C without the molybdenum.

News flash: that's not 440C! And too many carbides? In doing this, the direction of the book is that toughness is increased in lower alloy steels. Yes, we know that this is generally the case; nothing new here. Let's make knives out of structural steel; it's so soft and tough that in never breaks, only bends! Another news flash: toughness is not the number one desired property in knife blades; it's wear resistance!

There are so many problems with this book that only a detailed review and critique will make this clear that this is not good knife blade information to reference. I'll present a detailed review, but just to whet your appetite, know that in Verhoeven's book:

• There is no mention of cryogenic treatment, only "cold" treatments limited to -150F.
• There is no mention of powder metal technology tool steels.
• The book is written with input from hammer-forging knifemakers only.
• The book is written as if all knifemakers and bladesmiths are only hammer-forging.
• There is heavy emphasis on "sensitizing" which doesn't occur in stainless steels without overheating. and then is only a problem with ferritic stainless steels, and they are never used to make a knife.
• There is heavy emphasis on toughness—not wear resistance—as being the most desirable property in knife blade steels.
• There is little mention and inadequate discussion of complex carbide types formed with chromium, molybdenum, vanadium, niobium, or tungsten alloys.
• And the major issue: there is only ONE steel mentioned by brand name in the entire text. Of the many hundreds of steel types, only one manufacturer (Sandvik and one steel type—AEB-L and its equivalents) is mentioned in the whole book! To me, this seems more like a sales document than a worthwhile text to detail steel properties.

Detailed critique of this troubling text is in the works...

But back to the carbides, K1 and K2 (by German authors... ahem). Perhaps this is done for the sake of speedy typing, since making all those subscripts is a pain in coding and typesetting, but this hasn't stopped the use of the traditional carbide chemical designations by scientists, metallurgists, and technical writers. Nonetheless, in high chromium steels:

• "K1" is Cr₂₃C₆: a cubic carbide with an actual Rc hardness of about 68
• "K2 is Cr₇C₃: a hexagonal carbide with an actual Rc hardness of about 69

Compared to the incorrect hardness claims in the original claim above, you can see that hardness isn't much of a concern or delineation here, but the "K2" carbide is a bit harder.

Let's go a bit further and quote from Verhoeven's text, and we discover this comment, "The K1 carbide is perhaps the most important carbide in stainless steels as its formation often occurs along grain boundaries and promotes localized corrosion along grain boundaries, a type of corrosion called intergranular corrosion."

So it's about corrosion? And in looking up this corrosion issue, you'll find that it occurs in sigma phase and is a problem in FERRITIC stainless steels with less than 0.2% carbon! What does this have to do with martensitic stainless steels that have been properly hardened and tempered? Nothing, that's what. Ferritic, low carbon stainless steels are not used to make knife blades in any way, any where!

This whole K1-K2 stuff isn't working for me! What about Cr3C2: a Orthorhombic carbide with a Rc hardness of about 80? It's the most durable carbide and it's not even mentioned! What about molybdenum carbides, chromium-molybdenum carbides, eta carbide formation, and complex carbide development as the main reason for increased wear resistance and toughness? Verhoeven has left all that out.

Let's go on—

Claim: "AEB-L lies almost perfectly on what is called the "Carbon Saturation Line", which means that all of the carbides formed are precipitated carbides, not primary carbides like are formed in 440C, and there is more carbon and a similar amount chromium in solution as compared to 440C."

Reality Check: Whoa, there, partner! Remember that above, I mentioned that this "chart" has displayed 440C without the molybdenum? In other words, that's not 440C, yet it's represented as such for the sake of his argument and point! This is typical of carbide-nonsense created to sell a lower alloy steel and for nothing else. Knifemakers claiming their lower alloys are somehow superior are the bane of our industry; as are suppliers of low alloy pattern-welded damascus decor steel blanks, billets, and blades that repeat this carbide-nonsense. Using the phrase "almost perfectly" suggests that there is some perfection approached here, and this is nonsense.

Since this is educational, let's look over this "Carbon Saturation Line" stuff. The term is from an isothermal transformation diagram and delineates the maximum amount of carbon that austenite can dissolve within itself (when heated to its critical transformation temperature). Above that line, much more profuse and abundant amounts of carbides are formed than ferrites. Below that line, much more profuse and abundant amounts of ferrites are formed than carbides. At that line, the amount of carbides is markedly less than above the line! Yes, being at the "Carbon Saturation Line" is not a good thing for formation of carbides overall. AEB-L is below that line at a full percentage point of carbon. 440C is way above and outside that carbon saturation line. This is why 440C is able to create an abundant and tremendous amounts of carbide, and AEB-L is not! This is why 440C is a hypereutectoid stainless steel and why AEB-L is a hypoeutectoid stainless steel.

This is also why stainless steels like AEB-L and the equivalents have given all stainless steels a bad name in knives overall!

Claim: "Primary carbides are very large. So, through a balanced composition, AEB-L has excellent toughness, edge retention, workability, ease of sharpening, and ease of polishing."

Reality Check: The size of the carbide does not play a role in how a knife blade performs. For more on that, please read about carbide particle nonsense on my "Blades" page. This is right up there with "minimal molecule edge radius" and other nonsense microstructure components as if the knife user is sharpening his blade under an electron microscope and using it to cut bacteria in half.

The term "ease of sharpening" is a huge, hot red flag in steel discussion. When steels are easy to sharpen, they are simply and clearly less wear-resistant. Resistance to wear = resistance to sharpening; this is the physics of all cutting edges. Suggesting "excellent edge retention," and "ease of sharpening" completely lacks integrity—this is an oxymoron.

The very same thing can be said of the phrase "ease of polishing," because easy to polish means easy to abrade, thus lower in resistance to wear (and abrasion). This is why professional and institutional wear testing is an abrasion process... ahem. This is why high alloy steels are so difficult to work with and bring to a high polish; they resist abrasion. The polishing process is abrading with finer and finer grits until these grits and abrasive process produces a surface that has an even surface with scratches too small to be seen by the naked eye. Softer, less wear-resistant steels are plainly easier to polish, and that is often why they are used.

Summary: Guys write this stuff because they are trying to hype-sell their knives, and prefer their particular steel type. They think that if they write up their favored steel type, and put a bunch of misleading drivel in the statement, no one will look behind the curtain and see that the wizard is a lonely old man, putting on a show for the benefit of the smoke and mirrors and awe of the terminology. Meanwhile, they are making blades out of the same steel that caused so many to hate stainless steel knife blades. They are making them with wooden handles without fittings, octagonal (Japanese style) uncomfortable handles with bamboo and plastic, rabbeted, stick-tang weak handles, plain flat grinds, etched surfaces that are impossible to clean, and lots and lots of pattern-welded decor damascus created just for the pretty and not performance.

Worse, much, much worse, is that these pattern welded damascus blade are not food contact safe! They are not safe because they are etched and rough-surfaced and hide bacteria and pathogens, and they do not satisfy the NSF § 7.1 Stainless steel requirement, that alloys with less than 16% chromium must be heat treated by a proper post-weld process. Because they are made of two different steels, there can never be a proper post-weld process, because each steel is different!

All of these steels have to be heat treated correctly, and there are very narrow and specific circumstances, conditions, times, temperatures and stages for proper transformation of each individual steel type. The treatment is a narrow, controlled, and specific process, and that process, when properly performed, only applies to one steel type. In pattern-welded damascus blades, there is no fixed, rigid, and reliable treatment regime since you are dealing with at least two different types of steel! So, no heat treatment will yield a reliable, superior condition of the pattern-welded damascus steel; it's impossible! More about that on my Blades page at "What about Damascus Steel; It's All About the Pretty."

This is why pattern-welded steels of multiple layers are not used in any serious tool application anywhere on the planet apart from handmade knives. This is why there are no damascus pattern-welded valve bodies, drive axles, or injection forming dies. There are no pattern-welded metal shears, ball bearings, or milling cutters.

Apart from damascus pattern-welded steels, whoever wrote this doesn't even mention heat treating or cryogenic processing of AEB-L (which is absolutely necessary for complete conversion), and how carbide formation and final steel condition is entirely dependent on the knifemaker's ability and experience in heat treating.

No matter—even the very best treatment regime will not make AEB-L a better knife blade than 440A, 440B, or 440C. It's less wear-resistant, with no molybdenum, with lower carbon and lower carbide amounts, and not as good a steel than many other high alloy steels, no matter how small the atomic sharpening radius at the electron boundary is. Or is it the radius of the smaller quasi-particles of spinons, holons, and obitrons...? After all, how sharp is sharp... really?

Thanks for helping stop hyperbole, myth, fallacies, sales jargon, nonsense, and ignorance in our tradecraft, science, and art.

Page Topics

High Details and Specific Results of Heat Treating Knife Blade Steels

Someone posted this question on my YouTube channel:

I want to go on a bit more, since this is too expansive for the YouTube format.

Knifemakers want information, and mostly this is available on the internet, but not always. Good information, trade information, can be very expensive and highly guarded. Take, for instance, the realm of heat treating. If a company or entity invests many thousands of dollars in heat treating and procedure, protocols and systems, analysis and improvements, are they expected to simply give this data away for free to help competitors? Of course not. This is how information can have extreme value in the modern world.

What many knifemakers are seeking is simple recipes: mix "A" with "B" and warm to "C" and then cool to "D" and your done. They don't often want to bother with real research in their own field. Knifemaking is not general metalwork; it's a specific, highly controlled process that has a very narrow field interest.

There are no massive tomes that reveal heat treating and specific carbide amounts, indexed to steel types, geometries, and specific knife shapes for every conceivable steel used in the tradecraft. If a company is making jet aircraft turbine blades for the military, you can be certain that these companies know the exact and specific contents of their metallurgy. They also are not going to furnish their competitors with their manufacturing process and guarded procedures. No such information reference exists in knifemaking. There isn't even a degree or certification in knifemaking, and there probably ought to be, since the safety of the knife user may be based on the knifemaker's ability to properly treat a blade!

Sure, some knifemakers have posted their recipes and procedures, but none of them is doing actual wear testing by abrasive methods, metallographic analysis using scientific methodology. For example, on one knife forum, one particular participant is accepting test pieces furnished by knifemakers to perform a toughness test. He doesn't know how the steel is treated, he doesn't know the exact chemical analysis and supplier of the steel, he doesn't even know when the steel was purchased or from who. He doesn't know how fast the steel was moved into cryo, how the cryo temperature was established and maintained, or even if it was. He just accepts bits of treated steel from makers for a "comparison." And this guy calls himself a knife metallurgist! In the universities I deal with, he would have failed his statistical analysis course at the first test...

My point is that scientific testing is a highly, HIGHLY controlled process. All testing needs to be done with strict control and in an extremely narrow framework of process. Simply put, as many variables as possible must be eliminated to reach a specific conclusion. For example, if steel testing is done in heat treating, the amount of minutes a test piece sits at room temperature before cryogenic cooling takes place can change many characteristics in the steel metallurgy. In handling samples for testing, all of the samples must be treated exactly the same, at the same time, in the same place and exposure, with strict controls of the process. Otherwise, a finding does not have validity. Is the D2 steel tested developing more ferrite and cementite because of holding time at room temperature after initial quenching, or is it developing more ferrite and cementite because of a higher tempering temperature surging because the tempering oven door is opened too frequently?

If you think this can get very complicated very fast, you are right. Consider that most knifemakers think a two rivet-handle is good mechanical practice, and a sheath is simply a troublesome afterthought, and you'll wonder why they are worrying about grain bonding and carbide development in high purity alloys.

The answer is, as always, start with the manufacturer's (or foundry's) information on steel treatment, and if you have the time, money, and scientific interest, start making careful and considered adjustments to their recommended procedures based on performance and what you have learned. Document everything! Don't expect a recipe book to tell you that you need to have a maximum of 12 feet between your quenching apparatus and your freezing apparatus so that you eliminate any chance of the occurrence of quench cracking from the volumetric expansion caused by isothermal transformation of retained austenite into martensite during your stride from one piece of equipment to the next! And keep your strides smooth and fast, yet do not incur any wind from your travels...

Page Topics

Listening to and Weighing Steel for Hardness?

People who work with steel can come up with some curious practices. I've heard so many of these that it's a bit ridiculous to admit that somehow, they are, well... interesting.

It reminds me of the old guy I worked with when I was an industrial electrician, who swore that electricity flowed from positive to negative. We got in a heated argument, and he threw out his many decades of experience as proof that he knew what he was talking about. After all, he had been around since Edison. I asked him how it was then, that he couldn't actually fix the DC elevator control circuit we were working on, based on his expansive knowledge. He swore at me, left the jobsite, and I made the repair myself, based on my understanding of how things work. Hint: always think Tesla, not Edison.

Now, I'm laughing, because you could claim I'm one of those old guys who stubbornly insist they know what they are doing, so this hits home. After 40 years of knifemaking, I can stand on my experience to make certain claims, but here's the difference: I'm constantly learning, clarifying, detailing, and presenting what I've learned, and I have quite a few examples of how this applies to the real world. What you see on this website are real knives, each one is a learning experience, each one another step in the understanding of this design, material, and use.

One thing I'm dedicated to is exposing and stopping all the falsehoods and mystical hoopla that surrounds knives, steel, and our tradecraft. In this next email, you can see how people could be fooled, how some of the information is valid, and how ridiculous knife beliefs are.

Please, stop this kind of stuff when you see or hear it, and always try to educate those around you. They'll understand and appreciate it in the end.

Page Topics

Steel Type vs. Heat Treating

It's easy to get swept away with all the advanced alloys on the market. Every one of them has limitations and conditions, and no matter what the alloy set and designation, it's the proper heat treating (hardening and tempering) that sets blade performance. The very best steels are frequently, perhaps usually, poorly heat treated in knifemaking.

I cannot stress how important heat treating is in knifemaking. Everybody is in a big hurry to make knives; very few guys are researching, testing, and improving their heat treating process as part of the field of knifemaking. They are converting toaster ovens to PID control, they are cobbling together heat treating furnaces from found parts and scavenged components. Many of them are "heat treating" in an open forge and tempering by just heating the steel in an open flame! They're shoving blades in dry ice slurry, they're sticking blades in Dewar bottles of liquid nitrogen. They don't have a plan, they are in a hurry, they want to move the process along without logic, research, or result-based decisions about their process.

They get their information from forums, the very worst place to get hobbyist, non-professional information on a highly controlled, precision process. Forums are great, until you realize that they are filled with other hobbyists, even lifelong dabblers in knifemaking that have no actual visual or proven record of making or supplying the very best knives for the most demanding of uses in the military, tactical, or professional fields. And if "old Joe" on the forum claims you can just shove the blade in liquid nitrogen for an hour or two, it must be right, because "old Joe" has 12,000 posts on the forum—and Dizzybob123 can vouch for his work, and everybody knows the "Dizzy" is a great guy and always posts with integrity...

Where is the proof? Where are the knives? Where is the intensive, verified, detailed record of these great works and creations? Where are the references, the logic, and the reasons?

It's hard to take knifemakers seriously when they don't understand the science and results of just one component (heat treating) of knifemaking. For many of these makers, they are continually, obsessively searching for "the right" steel type, the one that will take a lousy heat treating process and result in superior blade. They throw out the alloy letters and numbers with pride, "I only use super steels!"

Then, you find out they don't bother to triple temper (suggested by the Uddeholm sages themselves), because "Old Joe" on the forums says you don't need a triple temper, anyway. They don't even reach full martensitic conversion temperatures, much less soak times for eta carbide formation. After all, they've used the same lousy, inadequate process on other steels, and they've noticed a little bit of improvement by changing the steel type, so there must be something to it—

Maybe they should just buy a factory knife, regrind it, and just let the factory handle the heat treating.

Probably the most significant take-away from this section is that most clients of custom knifemakers are unable to recognize a failed heat treat process, because all they have known in their lives is lousy factory-treated blades (factories cut corners, too). So they think, "Wow, this M4 blade is outstanding!"

Then they pick up a finely treated 440C blade and their mouth drops open as they cut and cut and cut, and cut...How can this be; it's not CPM M4!?

The thing is that all high alloy, hypereutectoid steels can perform better when properly heat treated. Some have finish issues, some have elasticity issues, some have corrosion resistance issues, some have issues with toughness at the thin cutting edge. A superior, accurate, technically proficient heat treating process will improve all of these, on any steel!

The rest is just advertising letters and numbers. Everybody knows the best steel is BR-549.

Page Topics

O1, Cryo, and Bad Internet Information

A comment about heat treating and O1 came in from my Instagram account:

"I was reading your notes (on this page) on sub-zero and cryo of O1, never even knew that was a thing with O1 . . . so much bad information on the web to try to filter out..."

This is the major problem with the modern knifemaking community. The internet is a great source of information, but you do have to have a heavy filter on the source. For instance, I always encourage new knifemakers to never, ever get their information on a forum, even though it's tempting and forums are heavily promoted by search engines. The reason is because they pay for SEO (Search Engine Optimization), and as my 14 year old granddaughter reminds me: "Google runs everything."

It's unfortunate that what started as a vehicle for researchers in colleges—the internet—has become a commercially-driven advertising agency, where money paid to search engine companies is more important than the actual correct information that is researched, documented, and peer-reviewed. Now, even research is sold, requiring a fee to see a reference paper, a college thesis, or a professional study. In its place, people are looking for quick answers.

They want to know, quickly, how to do a heat treat, quickly, and have a good result, quickly. They don't care to dig deep, everyone promises speed and certainty, no one wants to take the time to understand just exactly what is happening. They want a recipe, and the shorter and simpler the details, the better.

They go to forums for this, and there are plenty of anonymous and named hobby-level knifemakers to give them a quick recipe, based on something they learned from another forum member, or maybe even a knifemaker's website. Since videos and forum recipes are short, fast, and easy to understand, they sell well, like dime novels or (for the younger crowd) podcasts thrown together with a looped track of royalty-free music running in the background like a thorn in your ear.

Add to this that steel foundries and manufacturers well-know that the average buyer of a billet or bar of O1 doesn't have the facility or the interest to do an advanced heat treat, so they offer a simple, easy, and fast way to get a generalized heat treat of their steel. It's not excellent, it's not even great, but it's good enough for most jobs, and it only requires a way to heat the steel and quench it. So the standard recipe for O1 is:

1. Heat to 1400°F-1500°F
2. Oil quench
3. Temper at 450°F for one hour for 58 HRC

That's it! And that will give you a blade that is reasonably hard, and wear resistant, and okay for a hobbyist knife or a basic tool.

Here's the thing: let's hope and pray that you are not making knives just to make a hobbyist tool. If you're going to go to all of the trouble to make a knife by hand, why not make the best possible knife by hand? Why bother making a knife that has ho-hum performance, ho-hum finish, ho-hum handle, or a knife that rates on the same level as a manufactured or kit knife? Why bother?

Is it because you want to prove that you can work at the same level as other hobbyists? Is it to glean clicks and likes on Instagram? Is it to prove that you are just like a manufacturer, who can produce a knife 1000 times faster, but you are down for the struggle? If it is, I suggest you don't make knives at all; because you are simply wasting your time, and life is, after all, short.

But if the reason you are making knives is because you want to create something better, something more advanced, something remarkable, something that not only is nice to look at but excels in actual cutting performance, then you have to up your game. The forum answers won't be adequate, the supplier's quick and easy heat treat won't bring the steel to the pinnacle of performance. Your knowledge of steel will have to go beyond what can be supplied by the common sources.

This is one of the reasons I've created this page. On it, you will see why it's important to bring steels into the cryogenic range during quenching. For instance, all steels that have more than 0.3% carbon have a martensite finish temperature below zero, and O1 has 0.9% carbon! O1 has a martensite finish temperature well into the cryogenic range, but it is typical to only quench to about 120°F and then temper. This is a safety factor to prevent cracking, but that's for complex machined objects, not simple knife blades. Yet, no one considers this, because easy is always better in their minds.

If you do some serious research, you'll find that cryogenically quenched O1 can have a wear resistance increase of over 400%! Why wouldn't you try to offer this to a knife user?

Look, it's okay to perform a simple heat treat on basic knife, just as long as the client knows this, and doesn't pay a premium for some perceived handmade excellence. The issue is that cheap knives are common and no reason to make knives by hand in the first place. That starts with the condition of the steel, controlled by the knifemaker, who is striving for a premium product.

What Books or Information Sources do you Recommend for Heat Treating?

I always try to encourage new knifemakers. It's important to welcome people who are seriously interested in knifemaking.

There are many people who want to make knives, but are either stuck in the 19th century practice of hammering low alloy steels, or are trying to make cheap, unfitted, roughly-finished blades handled with a piece of wood, antler, or plastic. I'm always encouraged to discover guys who want to do it right, since they are actually rare in this tradecraft.

You might believe all knifemakers want to make the best knives, but take a good, close look at what most knifemakers are producing. Look at all the blades with poor grind geometry. Look at all of the blades without fittings: no bolsters, fake bolsters, or no guards. Look at all the stick tang (rabbeted tang) knives with a stub of metal shoved in a hexagon of wood. Look at all the two and three-rivet or screwed-on handles, look at the lousy sheaths. As you begin to notice knives, you'll see this same low-effort, low quality type of knife over and over, and this should make you wonder. If the maker doesn't care enough to finish the blade or include bolsters, what kind of heat treating is he performing? Is it like the rest of the knife: poorly made, unfitted, unfinished?

Of course it is. A guy who doesn't bother to finish the steel isn't going to be taking the time and effort to perform a superior heat treat, especially since it's something the buyer of this kind of knife will never see.

When you read the phrase, "I just got these blades back from heat treat," you'll know, for certain, that the maker is either unskilled overall, or doesn't care about the process, since heat treating is the foundation of knifemaking.

The neat thing is that there are makers who are constantly trying to improve their work, and they are not just beginners. It's important that all of these makers get the information they need to understand and excel in the process, and the foundation of the process of knifemaking is in the heat treat. Knives are, after all, cutting tools, and their treatment and resulting microstructure determines every important property they should possess. Otherwise, it's decor damascus time—something you might want to show around and hang on the wall, but a lousy cutting tool.

The first and most important thing I can recommend to guys like C. S. is never, ever get your information on a forum. Forums and discussion boards are filled with misinformation and errors. This is because the people who really are knowledgeable in metallurgy and treatment are not posting there. While a cursory overview of forums seems to offer viable information, this is misleading. Guys who frequent these venues are not reading studies and metallurgy conference papers, they don't reference graduate level textbooks on steel treatments. If they did, they would tell you that there isn't a simple recipe for superior heat treating that can be done in a small garage or shop built in a Tuff Shed®. You won't find machinists sticking a bar of steel into an open flame-filled tunnel of fire, guessing at the temperature by the color, estimating the time for transformation, and shoving the hot steel into a bucket of canola. This is why I say, over and over, that there is no blacksmith in any machine shop, anywhere.

Guys reading this might get tired of reading what good knifemaking is not, but want to know what good knifemaking is. They want to know how to do it right, but without all the learning and studying and interpreting. The first thing to know is that there is no recipe book for proper heat treating. This is because every knifemaker's environment, equipment, and capabilities are different. This is also because steel foundries and suppliers want to sell steel; they want to keep it as simple as possible. That way, no one is discouraged by a complicated process.

For example, if someone told you that in order to take a photograph of a knife, you would need a complete darkroom and lab, and be able to process transparency film, enlarge, and be able to print positives on polyester. This used to be how the best Cibachrome prints were made (and some still are).

Or, you could take a digital photo with your cell phone camera, and post it to your social media account.

Would you start buying up the equipment for the darkroom and lab? Would you be discouraged? Or would you take the easiest route that doesn't cost you a penny?

Steel suppliers know the answer, so they keep heat treatment protocols simple, and this is not the very best treatment. Additionally, a knifemaker can't go to some sourcebook, and come up with a magic recipe that they can heat and cool their steel to that will yield a superior, penultimate solution for heat treating. It's not about a recipe, it's about a process. The process depends on several things:

1. The knifemaker's knowledge and understanding of heat treating and crystalline allotropic transformation.
2. The knifemaker's ability to create a specialized blade shape that will react to the transformation.
3. The equipment available to produce this transformation.
4. The steel alloy needed to respond to the previous three points.

You can see that I've opened up a can of worms here, but let's just talk about the first one. After all, that is what C. S. is asking about.

The first place to go to for information is this very website, this very page. If you're reading this far, you are more than interested, and you deserve to know more.

The next thing I'll suggest is what is one of the better books listed in the References at the bottom of this page: "Steels: Processing, Structure, and Performance," Second Edition, ASM International, by George Krauss, 2015. It's a serious read into many different kinds of steel, and you'll read it over and again, and learn more every time.

Swinging the pendulum the other way, here's a book I strongly discourage: "Steel Metallurgy for the Non-Metallurgist, "by John Verhoeven, ASM International, 2007. This book is referred to over and again by knifemakers, and that's because it's written for blacksmith knifemakers by someone who only has the input of blacksmith knifemakers (read the preface). While it has some good basic information, it never mentions high alloy steels, doesn't cover cryogenics at all, and is written with the premise that all knifemakers are hammer-wielding craftsmen who can't and don't work with high alloys or hypereutectoid steels. There are many false assumptions in the text, like toughness is the most important property of knife blades (not wear resistance or corrosion resistance) and that the ultimate steel is low alloy, low carbon AEB-L. By the way, AEB-L is one of the steels that has given stainless steel such a bad name. Don't bother with this book, it's misleading and too short, and it won't answer your questions adequately. Just know that this book only mentions one steel (in the entire book!) by name, and one company that's selling it—in the entire text! Advertising 101. And the word "cryogenic" doesn't even appear in the index...

The next helpful study is The Effects of Cryogenic Treatment on Cutting Tools, Satish Kumar, IOP Conference Series: Materials Science and Engineering, 2017. This is a great, simple overview of the benefits of cryogenic treatment, and a concise, clear work by Dr. Kumar and others. It's open access (available to the public for free), and the additional references listed there will give you a window to where these facts come from and how they originate. Just copy and paste the information above in bold into your browser, and you will find it.

Please don't be discouraged by the technical writing and nature of these dissertations, papers, and texts on the process. Steel is not a simple thing and as you read, keep a dictionary (online) open so that you can understand each term, phrase, and word. Also understand that English is not always the first language of the writers; there is a lot of great research going on all over the world on steels.

By learning about the process, you will then be able to direct your efforts to the other parts necessary: the blade geometry, the equipment, and the steel itself. First, though, a foundation of understanding about steel will be your guide. We are all limited or rewarded with the knowledge of our art, science, and skill.

Don't Knife Factories Have all the Answers?

I have a short, modest video of a small batch of knives warming after a long cryogenic soak, posted on YouTube. It's a tiny clip, simply demonstrating a moment in the warm-up cycle and the energy exchange as blades go from very cold -320°F (-196°C) to room temperature. It's a tiny part of the cryogenic process, and refers back to this very page of information on the cryogenic treatment of knife blades.

Some people feel compelled to comment, to reveal their own understanding (or lack thereof) of knives, blades, and knifemaking. Here's one of them:

 Let's look closely at this sad comment, so that others can learn from it. If we just ignore these people, and don't call them out, we are contributing to the dumbing down of society. It's time that the internet is actually used as an educational medium, and not as an outlet for ignorance and story. I don't expect this guy to ever learn a thing; his mind is made up, no matter who offers any of that troublesome technical jargon.

The first comment (pointless and time consuming) is a sad commentary on mass ignorance. If this person just bothered to read one textbook, just one of the many studies listed below about cryogenic processing and its advantages, he would have never let this comment come from his bully thumbs onto his keypad. It's sad to think that some people today—in our modern times, with advanced steels, tooling, processing, machinery, and understanding of steels—don't think cryogenic processing matters. He is, simply, wrong; cryogenic processing makes a huge difference in the overall condition and performance of steel.

His next comment is an attempt to insult my knifemaking, and by doing so, insults all knifemakers. To accent "handmade knifemaker" could be seen as an attempt to diminish what I, and thousands of other knifemakers do. His use of the word "barche" identifies him as using the Italian language, as does his name, which I will not bother to use, since he is no source of knowledge or information. A barche is a small boat; a batch is a group of things.

He claims that my knives and sheaths are "strange-shaped." Funny, I have over 500 individual knife patterns, and most of them are recognizable forms. Take, for instance, the chef's knives. Nearly every chef's knife design I make comes from traditional, typical, proven designs, some of which have been around for centuries. He must be commenting only on the knives he sees on the video, which are mostly tactical combat and counterterrorism knife blades. Of course, these would be strange-shaped if you wanted to use them in the kitchen. In any case, all of the knife blades that are shown in the video and the thousand or so that are shown on this website are sold and in the hands of happy owners, so somebody likes the designs, even if it's not him.

The next comment is the most telling. He claims that the procedure was "perfected" by German knife brands. This is typical of someone who is gullible to mass production advertising hype. Advertising by manufacturers is NOT the source of the most advanced technical information. If it was, you would see mass produced knives lasting for generations, requiring very little sharpening, having extremely high value, and you do not. By the way, fine handmade chef's knives do last for generations, require very little sharpening, and retain extremely high value, so there is obviously a substantial difference. This guy has been swayed by the names of Zwilling and Henckels, as if they make the very best knives. They do not, not even close. If you are reading this, you deserve to know why, so let's examine this closely.

The name "Zwilling" and the name "Henckels" refers to the same company, not two different manufacturers. They are not separate; the name of the company is actually "Zwilling J. A. Henckels Knives." They have two lines of manufacture. The "Twin" line (showing the two stick figures on their logo) is made in Germany, and all other Henckels knives (using the name or showing the single stick figure) are outsourced to other countries. Zwilling doesn't identify where these knives are made, and this is omitted probably because they don't want everybody knowing that their knives are made in China, Taiwan, Pakistan, India, or other countries. Otherwise, they would proudly proclaim the source for their knives, right? They do identify their manufacturing interests in Spain, Italy, and Belgium, which makes me wonder about the European Union and their influence on marketplace perspective, but that's another topic. They even have a section of their company that makes Japanese-style knives. They also sell cookware, cast iron, flatware, ceramics, and even recipes. This is, simply, a huge manufacturing firm. That alone means they are limited by their manufacturing process.

If you read enough advertising copy, you'll see that the "Twin" Henckels knives are proud of their "patented stainless steel handle," mentioning the patent over and over. First, this is misleading since the handles are not stainless steel, they are mostly plastic. What they have done is patented the shape of the handle. Look it up; just plug the patent number into the US government patent search information and you'll see it pop right up. The patent was filed in the United States Patent Office (why is a German company issued US patents?) in March of 1996. All the patent is for is the shape—it's roughly described in the patent—and they were so proud of the shape that they didn't want anyone to copy it, at least here in the United States, where patents can be defended. However, none of this matters, because the utility patent was only good for 20 years, so it expired in 2016, so anybody can use their handle shape now. Do you wonder now why I don't get utility patents on my knife designs? What a ridiculous waste of time... but this means nothing, it has nothing to do with the blade or cryogenics whatever. It's just something that stands out in their advertising: an out-of-date handle shape design patent they want to proclaim as a beneficial feature. A bit misleading, isn't it? 

Zwilling J. A. Henckels does not mention cryogenics anywhere in their advertisements. They mention "ice hardening," which is a weak, vague, and non-specific term , a term invalid in all of the steel research, regulation, and treatment industries. The correct modern terminology can be found on this page. Unless they have access to ice that is -320°F (-196°C), then they are not performing DCT (Deep Cryogenic Treatment).

The questions are: what steel are they using, and how are they processing? You'll be hard-pressed to find this out; evidently they don't like to let their customers know what steel they are using, offering only generalities. They do this on purpose. It's not to protect some secret special 600-year-old information about magical knifemaking mojo. Humanity knows quite a bit about steels, alloys, use, processing, conditions, and performance. I can only conclude that they are not proud of their steel choices. After all, if I, as a singular knifemaker, can identify the exact alloy and processing on every knife, why can't a huge manufacturer who has a dedicated advertising department with perhaps hundreds of workers offer the same information? They leave it out on purpose; I hope you recognize this advertising trick.

It's important to get some perspective here. Zwilling's best knives sell for $99.00 US; that's not for a single knife, that's for a pair of knives. These are mass-produced, no matter how many "skilled artisans" they claim work on the knives in their manufacturing plant. If they were really "skilled artisans," they would quit working for Zwilling, since (from my own experience) a truly skilled artisan can make a lot more than a $99.00 pair of knives! But this, too, is an advertising ploy; just remember the sandwich chain here in the US that claims that each sandwich is made by "skilled artisans," and you'll get the scheme.

Back to the cryogenics. Zwilling has a trademarked name for their "ice-hardening" process, and they call it "Friodur®" It's a heat treating process, that—by their own admission—only goes down to -94°F (-70°C). This is standard shallow cryogenics, not deep cryogenics, and that is the first limitation. This means that it's done cheaply, with mechanical refrigeration, and it also means that the steel type they are using does not form extensive eta carbides and has retained austenite -or- it's an extremely low carbon, low alloy steel. This, by those of us that know the material dictates the  process and result, is obvious. They claim this process has 4 steps. Compare that to my deep cryogenic process of 33 steps that takes a week, and you'll see why fine handmade custom knives can cost a lot more.

Henckels and most German knife manufacturers use a steel identified as X50CrMoV15. This sounds impressive, but it's a common alloy. Using the international standard designation makes you think it's extra super special, since our steel idendifications in the US are quite simplified without all the digits and numbers. Lets compare:

Alright, here we go. I've compared X50CrMoV15 to 440C, simply because that is what I and other knifemakers are using for a lot of chef's knives, and because 440C is the standard that other steels are compared to. This is because 440C has been around a very long time, for good reason, and used to make extremely high wear stainless steel tools, parts, and components. More about 440C on a dedicated page.

The first and most obvious comparison is that the carbon in 440C is over twice as much. TWICE! Carbon is the number one beneficial alloy element in steel. Higher carbon means harder and more abundant martensite, and thus, more abundant carbides throughout. Lower carbon means a softer, less wear-resistant, weaker knife blade overall with dramatically fewer carbides and lower overall strength.

440C has 20% more Chromium, an extremely beneficial component of fine steels. High chromium means greater strength, higher corrosion resistance, and a more abundant formation of hard chromium carbides throughout. Lower chromium means lower strength, lower amount of chromium carbides, and a less wear-resistant blade overall.

Both steels have the same amount of molybdenum, which is good, but because 440C has higher chromium and higher carbon, this means more abundant complex carbides can be formed (chromium/molybdenum/iron carbides). This means dramatically greater wear resistance in 440C.

The "German Standard" steel has some vanadium. However, at 0.1%, this is a trace amount, and contributes little to the wear-resistance of the steel at all. Lack of carbon will make formation of vanadium carbides extremely limited in this steel also.

Manganese is much higher in the German steel, and that makes it easier to machine-forge, or hammer out their blades. Manganese in amounts higher than .06% can help in the actual hardening of steels to reduce deformation, but is not an equivalent replacement for regular alloying elements (ANSI, ASA).

The German steel is then a low carbon, low alloy hypoeutectoid stainless steel, similar to the poor stainless steels that have given stainless chef's knives a bad name. It's only better than the poor performers AEB-L and 420 stainless because it has some molybdenum and a trace of vanadium. This is not a particularly good alloy, and this may be why Zwilling J. A. Henckels doesn't crow about it, but only in a generalized way.

One more striking fact about this steel. Henckels claims that their blades range from 56 to 58 HRC, a common advertising trick. They state a range of hardness, leading the customer to believing that the steel can somehow be as hard as a typical knife (58 HRC), when it can't and isn't. The BSSA (British Stainless Steel Association) claims that this steel can only achieve a maximum hardness of 56 HRC. So when you see this "range," you can guess that at its best, this is a low hardness knife blade at 56 HRC maximum, which is rather soft for this application of knife, considering the low wear resistance it offers in this steel type.

What do you expect from a manufacturer who makes $50.00 knives? Now you know why they have to be continually honed, steeled, and sharpened just to remain usable, and why every chef learns how to whip and wipe the blade on a steel rod, just to be able to cut with it! But hey, there's the price and availability, and the centuries of "reputation," which really helps out in the kitchen, as you describe why you have to constantly sharpen the blade due to the ancient reputation...

Now let's look at the final comment, because this really needs to be clear. The unknowledgeable person writes what little he knows about the cryogenic process, "and needs special machinery and technicians to be well executed." He is the victim of mass-marketing advertising drivel.

1. The factories that make these knives do have special machinery, but their machinery is designed for mass production. If you look at Zwilling's own photos and videos of their limited process, you'll see many dozens (or hundreds) of knife blades in automated machinery, some of them covered in frost, to demonstrate their coldness. There are some inherent problems and limitations of processing knives en masse. For instance, if a vacuum furnace is used, the mass of metal that surrounds the blades—the holders, the frames, the furnace walls, the furnace doors, the insulation—all contribute to retaining heat. This is a huge problem in industrial-scale heat treating. The knives are not quenched by being immediately pulled from this hot environment and put in a drastically colder one for a faster quench; they are usually flooded with air blown through the furnace which slowly cools the blades as it removes heat from the furnace walls. Do some reading about vacuum furnaces and you'll find that this is a huge problem in the industry: the removal of latent heat from the vacuum furnace environment. This means slower quenching, and less effective quenching, leading to a higher volume of retained austenite. Even if the racks are immediately removed from the furnace, the racks themselves retain heat, and the knives are left in open air to quench. This is much less effective than pulling the blades from the hot environment, and quenching between two liquid-cooled aluminum blocks at 32°F (0°C). The freezing contact block method results in a more immediate quenching, a greater conversion of austenite into martensite, and a better condition of the steel overall.
2. People who work in a knife factory are frankly factory workers, simply called "operators" in the trade. They are not scientists, not lab technicians, and they are not honing and refining the critical steps to improve each batch (or barche) of knives. They don't test each knife for hardness, they don't measure toughness, wear resistance, durability, corrosion resistance and adjust or refine the process for a multitude of steel types. They are just operators who load the racks and push the buttons. No one likes to reveal this about manufacturing, but if you've ever worked in a production facility, factory, or manufacturing plant (I have) you realize that a process is set up, limited by the machinery that performs the process, and it is never changed. Changing the method—making significant adjustments or variations in the process—would cost many hundreds of thousands of dollars as new machinery, new steps, and new procedures must be engineered, designed, tested, refined, and then built into the factory's operations. It's much easier to hire a good advertising department to come up with an uber-cool techie name to call the cooling process, and write up some fantastic performance expectations based on a 600 year history of the company's name. This is done all the time in knife manufacturing. Conversely, the small shop has the capability to adjust and refine every step of the process at every heat treating batch, depending on their findings of hardness, toughness, strength, corrosion resistance and overall steel performance. The skilled knifemaker is the scientist and technician, the engineer and tool designer, the operator and refiner of all of the equipment and procedures that result in a superior knife blade. This is one of the reasons he is sought out, and why handmade knives can be so much more expensive than factory knives: they simply are that much better.

If this knife manufacturer truly had a process for their secret steel that was far and away advanced beyond what other manufacturers were creating, they would run all of their competitors out of business. All of their knives would erase the competition's, by performance alone. Do you wonder why every $50 to $100 knife performs about the same? Sure, there is brand loyalty; chefs don't want to change a recognizable knife for another that they are not familiar with. Companies like Zwilling depend on repeat sales and market recognition, and work very hard to foster fandom. Not with facts, but with advertising generalities and tradition. Why are we stuck with traditional, low-performing knives, anyway? Wouldn't it be nice to buy one knife, just once, that would last for generations of use?

All of these details relate to knives that are distinctly low-end. Yes, Henckels makes low-end knives, no matter what you may read or hear from their advertising department and chefs who own them. Manufactured knives are a different field than fine handmade works; this is why a fine handmade knife can cost 10 to 100 times more! Think about it; if Zwilling J. A. Henckels could, they would be making $3000 knives, wouldn't they? They know better. They can't and don't make a knife that good, and they know that their market is in the mass production universe. This is why they are not concerned with individual custom knifemakers and their knives; they are much more concerned with Chinese manufacturers, Japanese and Pakistani manufacturers, and big box retailers and Amazon access and sales. They aren't creative; the aren't leading the field, they aren't innovating by experimentation, method, materials, or process. They are a dependable, predictable manufacturer, working in low end hypoeutectoid, low chromium stainless steels, plastic handles, and volume above quality.

Factories don't use high alloy hypereutectoid stainless tool steels; they use low alloy hypoeutectoid stainless steels. Factories don't use radical and extended heat treating methods; their low alloy, low carbon steels don't require it and it wouldn't help them anyway. They also use plastic handles instead of premium and everlasting materials (like gemstone), they limit or simply eliminate bolsters or fittings, they don't finish the steel at all, and they offer a few dozen or so designs, instead of many hundreds. They won't custom make a knife for an individual client or customer, they offer meager or no accessories, but they are priced cheaply.

I hope this helps you understand how and why factory knives are inferior to fine handmade and custom knives, even in the heat treating and cryogenic processing. They truly are different fields, and the only error here is believing that the two can even be in the same range of discussion and comparison.

The truth is most people cannot afford a fine knife, so they are left with simple market choices based on economy. There is nothing wrong with cheap knives, but there is a lot of misleading and incorrect information surrounding knives, limiting the choices and information the public has about knives overall.

 The two fields of knives are similar only in the way that a Ferrari is similar to a Ford Fiesta; they are both vehicles with four wheels. When put that way, it should be simple for most people to understand.

Or maybe an ignorant internet bully would go to Ferrari and insist that they don't know what they are doing, making such fine cars, when Ford has perfected the process.

More about Factory Knives vs. Fine Handmade Knives here.

Quenching Rate of High Alloy Steels

A nice email from someone who was interested in the quenching rates of high alloy, high speed steels and the effect on the resultant microstructure:

It's great that S. is starting out right, by doing some research into the very subject he's pursuing. Too many knifemakers just pop into a forum, read some fast and short steps on heat treating, or glean the very simple and condensed guides on the steel data sheets. If you've read this page up to this point, you should be convinced that the very best treatment of knife blade steels cannot be found on those simple sources. Read any research paper and you'll see how much science is behind the allotropic manipulation of steels and the resultant effects. Most people don't want to study to understand the complexity, they just want to make a knife like baking a cake; a recipe, common tools, and a finished piece they can sell.

But, as S. knows, there's much, much more to the process, and hooray for him, for diving deep into the metallurgy.

He sent me an excerpt of a research paper, titled, "Effect of Cooling Rate in Liquid Nitrogen on the Transformation Rate of Retained Austenite in Steel R6M5," by Popandopulo, A. N.; Zhukova, L. T.; Yaroshenko, A. G. This was published in Metal Science and Heat Treatment, Volume 25, Issue 5, pp.334-337 in 1983.

The article is about R6M5 steel, and while not exactly M2, it is equivalent. The R6M5 is a high alloy steel, with lots of molybdenum and tungsten and some chromium, and is technically a tungsten-molybdenum high speed steel. High speed steels are made to cut other metals at a high rate of surface feet per minute, and can withstand high temperatures.

To answer one of his questions, yes, these results can be translated to other high alloy hypereutectoid steels, since most high alloys respond similarly, such as secondary tempering ranges, martensitic transformation, and multiple tempering cycles.

The conclusions of the paper reveal that:

1. "Increasing the cooling rate in liquid nitrogen from 3–5 to 50–70 deg/min changes the transformation of retained austenite in quenched steel R6M5 from martensitic to bainitic-martensitic." This is not a good thing in knifemaking; we are after as complete a transformation to martensite as possible. Bainite is like pearlite, a mixture of ferrite and cementite, but not layered like in pearlite. It's not as hard and durable as martensite; it's not the preferred result in wear-resistant tools.
2. "The bainitic transformation during rapid cooling shifts the martensitic transformation proper to lower temperatures (−100 to −150°) and thus changes the morphology of martensite crystals." This is not a good result, because it then requires even lower temperatures to achieve complete martensitic transformation. In high alloy steels, it's difficult enough that we have to bring them down to extremely low temperatures for maximum transformation, we don't need to push the range lower, perhaps lower than we can achieve with liquid nitrogen. This will result in higher retained austenite, not good in any cutting tool.
3. "After partial stabilization at +20° for 24 h the austenite is transformed by the bainitic-martensitic mechanism during slow cooling and by the bainitic mechanism with rapid cooling to the temperature of liquid nitrogen, which reduces the amount of austenite transformed." A rather hard sentence to follow, but what I understand is that stabilization at room temperature (waiting before cooling to the cryogenic range) and quenching too fast both have the same effect of producing untransformed (retained) austenite. Not good.
4. "Rapid cooling in liquid nitrogen has a stronger shrinkage effect during subsequent heating in the temperature range of −60 to +120°, i.e., increases the density of martensite lattice defects." Cooling too fast will increase the stress on the microstructure that shows up when it's reheated at the beginning of tempering. This extra stress on the steel is not only unnecessary, lit can adversely effect the microstructure, causing cracking.
5. "The characteristics of the transformation of retained austenite (not stabilized or partially stabilized) must be taken into consideration in selecting the cold treatment for tools made of high-speed steel." Obviously, retained austenite is a negative in tool making. It can transform with mechanical stress at ambient temperatures into martensite, which, untempered, can cause fracture or high wear.

The reason so many knifemakers are stuck on shoving knife blades directly into liquid nitrogen probably originates from the millennia-old practice of shoving low alloy and plain carbon steels into water to quench them. This is fine for low alloys and plain carbon steels, and it's a hard habit to break. The idea that liquid nitrogen is just a very cold water-like liquid, and you just shove the blades in there is wrong, particularly when dealing with high alloy steels. The quenching severity required for transformation is much, much slower in these high alloys, and this older research illustrates some of the reasons why. This is why, in the section above, the 4-5 degrees Fahrenheit per minute rule is so important.

M2 and Other High Speed Steels

Though he says he doesn't use it, let's look at the steel (M2) just to be clear.

There are a lot in knifemaking that think that if they just use a steel that works for high speed use, it's going to work for a knife blade. How many times do I have to say this? Knives are not high speed tools!

M2 is a good alloy for its use, but these high speed alloys are designed to be treated in the secondary hardening range. This means that they have to be tempered at extremely high temperatures 550°C (1000°F) with the goal of producing a high volume of carbides, usually MC and M₆C, dispersed in a matrix of tempered martensite.

To keep this as simple as possible,

• The benefits of the high alloy content in a high speed steel are realized when it's processed to a high secondary hardening temperature, not when tempered to a lower hardness.
• If a knife blade is made of high speed steel and tempered in the secondary range, it can have a hardness to 64 HRC.
• If a knife blade is made of high speed steel and tempered in the secondary range, it will be brittle and easily fracture, particularly at the thinnest area, the cutting edge.
• If a knife blade is made of high speed steel and tempered in the secondary range, it will be much less corrosion-resistant than when tempered at at lower range.
• There is a coarser distribution of alloy carbides and a lower degree of coherency with the matrix at higher secondary hardening temperatures. (—Kraus, ASM)

Because of lack of toughness, knife blades made of high speed steels must be tempered at the lower range, which negates the entire reason for using a high speed steel in the first place!

This is why I don't recommend these steels for knives. A knife must have a good degree of toughness to prevent fracture. Most blades also should have corrosion resistance, and if applicable, they should be Food Contact Safe. M2 is not Food Contact Safe in any condition, and the benefits of using a high speed steel are not realized in knifemaking, since they can't and shouldn't be hardened in the secondary range in knife applications. See the related article above: Bohler M390 and Elmax finishing problems.

One more thing: M2 is an ugly steel, often with waves of weird texture throughout. So, it has to be left flat, or sanded: not a very appealing steel. Yes, I have made a knife or two with it and understand the limitations.

There are so many other steels that make excellent hand knives that don't need or require secondary range hardening to embrittle them and make them even less corrosion-resistant than they already are so the maker can claim a high hardness.

Grain Boundary Embrittlement, Pry Bars, and Knives

I received a very nice, polite email from another knifemaker who trying to navigate his way through steels, knife blades, and applications. Because of the detail, and because I know other knifemakers would also like this information, I thought it would be best to answer it here, where it could benefit more readers.

At this point, I wrote back to him referring to this very page and section, because I think it's important to post this information for all those who may have the same questions.

The first phrase that popped up for me was "grain boundary embrittlement." If you do some basic research, you'll see right away that grain boundary embrittlement is a highly researched topic- for austenitic stainless steels. 300-series stainless steels that have been welded are the main concern, and the issue deals with high temperature welding and temperature cycling encouraging intergranular corrosion in austenitic stainless steels. This is a concern in the nuclear, shipbuilding, and industrial field where corrosion resistance is paramount in welded and high temperature exposed austenitic stainless steels. This has little to bearing whatever on 440C used for knife blades:

• 440C is a martensitic stainless steel, not an austenitic stainless steel; these are different animals altogether, entirely different fields.
• Grain boundary embrittlement and intergranular corrosion are effects of high temperature processing of austenitic stainless steels, and have nothing to do with knife blades or fittings. Knife blades are not made of 300 series (austenitic) stainless steels, and 300 series stainless steel fittings are rarely, if ever welded, and are never thermally cycled.
• If you want to get into more details (I do!), please read more about this in the next section.

So, let's put that concern to rest; the people he consulted have simply confused the type of steel (austenitic vs. martensitic). Communication errors like this happen all the time.

More importantly is the issue of the knife design. I've written before that a knife blade is not a pry bar, but what if a client wants a pry bar/knife combination, and he insists on it? Let's go down this path of logic a bit to its conclusion.

Why don't we see pry bars with cutting edges? Seriously, if we could offer that, wouldn't it be great to have a pry bar that is thin enough to cut? Let's simplify this a bit. A razor-keen wear-resistant tough shovel would be great!

When I was very young, I worked for a while clearing swamp land, cutting vines and roots and digging and hacking, and I used a shovel, a machete, and a rake. It was hard work. The shovel wouldn't cut the roots, because it dulled right away. I had to dig out around the root, and attack it with the machete. Every time I hit a rock with the machete, it would flatten, roll, and dull the blade. If I had a big knife to do the chopping job, I'm certain that I would have broken the blade the first rock I hit. I tried sharpening the shovel, but it dulled straight away because of the sand and small rocks in the dirt. If i would have tried digging with a knife, I'm sure I would have snapped the blade, because of prying...

I know this sounds simple, but a knife is not a pry bar, and neither is a shovel. What is a pry bar?

Probably the best exposures I've had to pry bars are in the Fire Service and in Archaeology. I was a Level 2 NFPA Rated Fire Officer, and was a volunteer archaeologist for 5 years. The best firefighter prybar is a Halligan tool. There is just about nothing a Halligan tool cannot breach; it's simply the best tool made for entry and ventilation, and rescue, where the hand alone can control the action. What are these made of?

Low alloy carbon steel and low carbon stainless steel.

Why? Because they are thick and heavy and weighty, and are not thin enough to cut. If they were made into a thin cutting edge, they would just bend, like a cheap machete or a shovel. Imagine prying open a car door with a shovel or a machete, you'll ruin both of them. But a Halligan tool has the geometry to get the job done.

In archaeology, I worked in ruins that were large cobbles and small boulders, densely packed in fill. To get these out, I used a mechanic's pry bar, also called a combination bull pin or alignment tool. What was it made of?

Low alloy carbon steel and low carbon stainless steel.

The reasons these steels are used is because they are tough, that is, resistant to breakage and fracture. They may be heat treated, but then they are tempered way back so that they are resistant to fracture, and will bend when stressed into the plastic range and not break. The reason they don't break is because they are very thick. There is no thin place on a pry bar, not when compared to a knife.

What about all the media, television shows, and chopping competitions? Don't knives need to be choppers?

Here's my professional career knifemaker opinion: this is all dramatic hype.

The guy clearing brush in a swamp or jungle is using a machete to chop; it's designed for chopping branches, limbs, and even plastic water bottles, should the need arise.

What about game and animal processing? Is someone actually using a big, honking knife to whack on a hanging pig carcass? What? Isn't that what television knifemaking has become? Go look in a packing plant where they process game, and you'll find no one is using a chopper, they are using extremely thin, sharp knives with accurate control to process animals and meat.

What about military combat and weapons? Take a look at my Counterterrorism Knives Page. You'll see what the guys who really do use knives in close quarters combat are using today. While some of the knives are large, they are not chopping through Godzilla's leg to try to sever an artery. And above all, the knives have thin, sharp cutting edges that are hard and wear-resistant; they insist on this.

The discussion then becomes more about geometry than material. You could make a great pry bar out of 440C, but it would be incredibly expensive, and would have to be very thick, and tempered way back so it wouldn't fracture. Any steel used would need this treatment.

If you ground a knife blade on the instrument, you wouldn't want it to break, so you would either leave it too thick and it wouldn't cut well, or you would temper it back and it would easily bend. Safety of the user should be your ultimate concern, and there really is no way to marry the two ideas. Simply put:

• A knife blade is designed to be thin so it can cut.
• A knife blade is designed to be hard and wear-resistant.
• A knife blade is designed to be tough enough to resist fracture, but hard enough to resist wear or deformation.
• A pry bar is designed to be large, thick and strong, not thin with a cutting edge.
• A pry bar is designed to be tough and thus, is softer, and is not wear-resistant.
• A pry bar is designed to be large and hard enough to be stiff, but not brittle.

The two fields (knife or pry bar) are so widely apart that any combination of the tool that attempts to marry them will severely limit one or the other. Another way to look at it is:

• A thin pry bar/blade will bend or break, possibly injuring the user.
• A thick pry bar/blade will not have a thin enough geometry for a cutting edge.
• The hardness and temper of a pry bar will create a weak, soft cutting edge.
• The hardness and temper of a knife blade will create a brittle pry bar, prone to fracture

It sounds like what you are trying to make is an axe. Axes are made of low carbon (hypoeutectoid) plain carbon steels (1040-1060). They are thick and heavy, but won't hold a cutting edge, and have to be constantly sharpened in a convex fashion. You can put an edge on one sharp enough to shave with, but in one chop, the edge is dull and gone.

A knife is simply not a pry bar; no matter what material it's made of. The best steel, the finest heat treatment, and the best finish won't change the fact that geometry is the main design consideration. It's up to the maker to educate his clients as to the limitations of both.

I'm not trying to be critical; as knifemakers we all will face this someday, a client wanting all the attributes possible on his dream knife. So it's really a matter of balance, informing the client what is reasonable in the geometry and design of a cutting instrument. Maybe, the best answer would be to design a sheath, well-made that could accommodate a good, sharp knife, and a small pry bar, too!

For carcass-chopping enthusiasts, I'll add this for the casual media-savvy reader: television is not the source of viable information on actual professional knifemaking. Maybe some day it will be, but not now.

Grain Boundary Embrittlement in Stainless Steel
(aka Sensitization)

In the previous topic, I mentioned a knifemaker's concern about grain boundary embrittlement in stainless steels, particularly 440C. He had been told by metallurgists and educators that this was a concern, and listed it as a reason not to use 440C for a prybar-knife blade. Of course, the word "prybar-knife" shouldn't even ever exist in the human language, and in the section, I detailed the reasons why.

I think it's important to go into more detail about the whole grain boundary issue.

For some reason, 440C gets a lot of negative comment from people who have little familiarity with the steel or its use, even those in the trades of engineering, metallurgy, and knifemaking. I write about it extensively on a special page on the website at this link.

My own personal experience with this steel has been superb; it's truly a great steel, but doesn't "act" like other steels. For instance, it takes a lengthy, detailed process to heat treat and cryogenically process this steel, and small changes in that process can give completely different results. It's a fantastic, proven performer, working in some of the  most demanding applications in the fields of industry, transportation, energy, nuclear, and aerospace.

The expression grain boundary embrittlement sounds like a big, serious shortcoming for steels, and it's easy to get swept away just by the sound of the phrase. Knifemakers like to discuss grain issues ad nauseum, even while the geometry of the blade, the grind shape and depth, the handle mounting and materials, and poor to no accessories should be their larger concerns. There really is a lot of shoddy work out there, but discussing grain boundaries is something that may distract from that conversation. More frequently, imagined shortcomings may be a way for knifemakers to excuse high alloy, high carbon (hypereutectoid), and high performing stainless steels from their use. These steels are clearly better steels for knives, and because most knifemakers do not "do" them well, they look for excuses. A grain boundary issue is often an excuse, because, after all, who is looking at grain boundary when they buy a knife?

For those of you who do care, and wonder what this is all about, here's some useful information.

300-series steels are austenitic; they are comprised of austenite when at room temperature. This happens because they are low in carbon, high in chromium, and high in nickel. Nickel is an austenite stabilizer, but that's not the whole story. As these austenitic stainless steels are heated in the 1050°C-1150°C range (1922°F-2102°F), they take all the carbon they have into solution. However, when cooled quickly from this temperature range, they will form a supersaturated solution at room temperature. But if they are cooled slowly, the carbon is rejected and forms M₂₃C₆ carbide. This carbide (though small in volume) can nucleate at grain boundaries as a particle or even an array of particles. These adversely effect low temperature ductility. This means that the steel is not as ductile (pliable or flexible) at room temperature. Because the M₂₃C₆ carbide needs a lot of "M" (referring to a mixture of metal atoms, which, in this case is iron and chromium), it depletes the region immediately surrounding it of chromium. This changes the electrostatic potential to anodic, and it can cause corrosion at the location where the chromium is depleted. This is called "sensitization." Corrosion is prevented in stainless steels because of the formation of chromium oxide on the surface of the steel when exposed to oxygen.

This effect (grain boundary corrosion or sensitization) happens also in ferritic stainless steels, and less often in martensitic stainless steels, and more notably in 12% by weight chromium and lower carbon-bearing martensitic stainless steels.

Whew! Let's tear this apart a bit to understand it and how it might relate to knives.

• We are talking about (mainly) 300 series stainless steels: austenitic stainless steels. The effect is seen also in some martensitic stainless steels, mainly lower chromium, lower carbon types.
• The sensitization happens because the steel is heated to over 1900°F. Just to be clear, there is no reason ever to heat the working part of a blade of 440C to 1900°F, that's flatly out of its austenitic range of conversion.
• Sensitization happens because the steel is cooled slowly, allowing the precipitation of M₂₃C₆ carbide through migration of carbon and chromium. There is no reason any knife blade should be heated and cooled slowly, in actuality, they are quenched rather quickly, forming martensite.
• Sensitization happens when these steels are held at 500°C-850°C (932°F-1562°F) for extended periods. This never, ever happens with any knife blade, at any time, in any process! If it does, something is being done terribly, horribly wrong! The time frame for this is as short as 100 seconds. Again, holding the steel at about 1000°F for a minute and a half will produce this effect, and this is not part of any stainless steel knife blade treatment anywhere!
• Sensitization mainly happens because of welding; this is the main concern. Knife blades shouldn't be welded, particularly in the blade working area or spine. When knife blades are welded, this is usually on a hidden tang knife, for instance on the tang where a threaded rod is welded.
• Think carefully about welded stainless steel damascus blades now, and you'll realize why these are poor performing decor blades at most.
• Sensitization happens because the the weld is not post-weld treated. Post-weld treatment is essential in all but the lowest carbon plain steels, it's part of the welding process.
• Sensitization creates an area that is low in chromium, so it's less resistant to corrosion, and this also weakens the grain boundary.

Even if a knife blade has a welded area, and it has a potential to be sensitized, such as in welding a 300-series stainless steel rod on a 440C tang, it's a simple thing to perform a post-weld process. Here's how it works: Heat the steel to 950°C-1100°C (1740°F-2012°F). This allows the M₂₃C₆ to re-dissolve. Air cool. This quenches it rapidly enough so that the nose of the time-temperature-precipitation diagram is missed, and formation of the carbide doesn't happen. Then, several tempers of the junction in the lower range tempers back the 440C so it's somewhat more ductile than the blade. These tempers don't affect the 300-series steels as long as they are not heated up past 1700°F.

Note that I'm only writing about welds on tangs, not any part of the blade! If you heat the blade to 1700°F and then air cool, you are quenching and will form martensite, a totally different process. Also, consider that no part of a knife blade is substantially thick, with a large cross-sectional area, so any time a thinner area is heated, it air-cools rather quickly, and will miss the nose of the time-temperature-precipitation curve for M₂₃C₆ formation.

Now, you probably know more about grain boundary embrittlement than you wish to! It's really not a knife blade problem or concern unless you've welded a component on the knife blade, and then a simple post-weld treatment and tempering eliminates it from your night terrors and frightful horrors!

Critics and Complainers (Jay Fisher Haters)

I came across a discussion forum mentioning my name. The forum is made up of people who evidently consider themselves knife "experts," though I don't know exactly what their qualifications are. I did not find one knifemaker posting in the topic, but the aim was clearly to spread some hate and discontent over what you are reading on this very website. While they discussed cutting edges and pulled some of my statements out of context to bolster their points, the reason I'm writing about this here is because of comments they made specifically about cryogenic processing and the claims of improvements over wear resistance which they doubt. (Hint) Don't tell the rest of the metalworking and machining industry that cryogenics doesn't work and that they're wasting their time—

The guy who runs the forum claimed that he wrote me once and invited me to respond, but I didn't answer. He did write me years ago, after doing a hit piece claiming that the only measure of a knife's value was in its ability to cut, and therefore my knives weren't worth what I charged for them. He went on to describe how manufactured knives cut just as well as handmade custom knives, therefore the two were equal in his eyes. Evidently, he's never actually made a knife, at least not one that I could find, and no one who I converse with can find, so one needs to question his authenticity. His insistence on cutting ability being the be-all, end-all in knife value was the basis for my section about comparative logic on my Factory Knives vs. Handmade Knives topic here. Take a look at that and come back for some more straightforward talk.

By the way, please don't tell some of the top military and counterterrorism teams in the world that are using my knives that the ability to cut is the only measure of a knife's value, they're likely to give up their "Ari B'Lilah's" for a scalpel... ahem.

The guy who runs this forum has regularly and repeatedly slammed me and my work over the years, and encouraged and allowed discussions about how my website is wrong, my claims are wrong, and I don't know what I'm talking about. Anonymous posters love to pile on there, after quickly scanning my website for some comment they disagree with, and pulling it out of context to reinforce their point. By the way, not a one of them has one of my knives, not a one has ever stepped up to the plate to buy one, and despite the forum participants suggestions, not a one of them, apart from the forum owner, has ever written me offering a correction or suggestion about what they are disputing on their forum.

These forums are often cliques of high school-mentality losers, gathering together to complain about something they know little. Again, not a one of them is a publicly known, successful, and established knifemaker, metallurgist, machinist, or engineer, and none of them post their professional resume, curriculum vitae, or list of accomplishments in this field, including the forum owner. They anonymously grumble and complain about what I write here.

This is typical cyber-bullying and harassment. Men can be very vindictive, hateful, and cruel, particularly if they don't actually have to face the person they are trying to harass, and this goes on all the time on the internet and social media. They never consider themselves bullies, yet use my (and others') names in their childish cowardice. You might note that I never mention them by name directly; instead I clearly explain why I do what I do and the results. This infuriates a lot of forum posters, since I won't go into the middle of their high school clique and become a victim of their bullying. Not a one of them will ever face me, contact me, or offer any logical response; they are the definition of cowards.

A lot of this comes from the pretense that because they are men, they think they are born with an innate knowledge of knives. Ask any man, and he'll have an opinion about knives, and it often comes from manufactured knives advertisement, movies and media, or from someone in their family or clique. When alternative information is presented to them, the bullying begins. They find like minds—usually on forums—other angry, frustrated cowardly bullies, to agree, and confirm their beliefs. When they act this way, they run off anyone who may actually be a professional, who may actually participate in the tradecraft in an exceptional  manner, who may have some worthwhile experience and knowledge on the matter. Then, they cry and complain that "no big-name knifemaker" ever posts on their forum.

For a professional, facts and reality are the root of understanding any technical idea, application, or tool. This knowledge comes initially from textbooks and research; it does not come from a forum.  However, textbooks are only the start, the professional must then actually apply this knowledge in the real world. In knives, real world testing is not chopping on water bottles, or hacking a two-by-four, or slicing a pig carcass in a staged video competition. Real world testing is having a counterterrorism professional use the knife, the sheath, and the entire rig in close quarters combat. Real testing is having a United States Marine use the knife in the real world situation of hand-to-hand life or death response. It's having a restaurant chef and owner apply the knife every day to provide for not only himself and his family, but also his patrons and employees.

If you are trying to learn more about this tradecraft, science, and art, my advice from being a professional knifemaker for over 35 years and making knives for some of the most demanding users and owners is this: don't get your information from a forum, any forum. Professionals in the field don't post there.

Forums are mostly full of hobbyists, part-timers, and enthusiasts and there is nothing wrong with that. However, the forums are rife with misinformation, hasty and incorrect process, and outright lies. In heat treating, there is an overwhelming perception that speed, haste, and cheap and common equipment will get the job done correctly, and this is usually wrong. For instance, it might be okay to use a forge and a bucket of oil to quench a piece of plain carbon low alloy steel. It might be okay to heat it in the wife's home oven or a cobbled-together toaster oven to temper it. But if you are trying this with high alloy hypereutectoid (high carbon) stainless steels, this is ruined steel. Guys try to use dry ice in quenching, but ignore the fact that it is not even in the shallow cryogenic range, and they don't control the cooling rate at all. There is no aging, no eta-carbide development, no multiple tempering cycles with cryogenic processing in between. Even though modern textbooks on the subject are clear about this, they ignore it. Then, if someone else tries to do exactly what metallurgy professionals recommend, the bullies pile on, telling them they don't know what they are doing!

There are also people who claim to be professionals posting on some forums. Lots of guys like gleaning data from websites and then "presenting" it to forum junkies. The junkies glom onto this as if it's scripture. Am I wrong for asking, "Where are the hundreds of knives these guys have made? Why aren't they posting on metallurgists' and physics scholarly discussion boards with other professionals? Why aren't they doing studies and posting them for peer review? Why are they on a knife forum with hobbyists, trying to get clicks, and asking for website donations? Why aren't other metallurgists: Ph.D.s, professors, and industry professionals posting there?

Forum bullies try to claim that it's wrong for me to say what I believe on my own website, even though I'm a career-long professional knifemaker. They claim it's to protect younger minds may be swayed into the realms of untruth. They claim that the reason I write what I do is because my site is an advertising vehicle, and nothing else. They claim that everything I write is only to make my knives look good.

So, my freedom of speech about the technical specifics of my lifelong professional career should be controlled by anonymous and ignorant bullies on a forum, is that how it should be?

So, here, now, is the time for some clarification. It is true that I get a bit of criticism. The reason I do is because a few people disagree with what I write. Simple enough, and I can accept that they can choose to deny factual references and experience.

The only reason they even know what I think is because I've taken considerable time to post it here on this web site accessible to everyone who has a browser, for free: the largest, most detailed website of any knifemaker in the world and in history. I do this not because I need to sell my knives. I turn down most of the work people request, because I have so much to do already.

As I mentioned about and in various other locations on the site, I don't need to post any of what I do and explain why I do it. I do this as a service to my community of knife enthusiasts, people who do want to know what I think, people who continually thank me for doing so. These are people who send emails to thank me for what I write, who will never, ever buy a knife from me, people who just want to cut through all the nonsense that they read about knives on forums, bulletin boards, factory knife sites, and other knifemaker's websites. I've posted some of their emails on my Testimonials page; please know as you read these that I'm honored and humbled by their comments and goodwill. Also know that the good emails outnumber the bad by about 300 to one, and I've also posted some of the negative ones on my Funny Emails pages for comparison and a laugh.

The main complaint from the forum bullies I mentioned (specifically about this page) is that I make things up. This is, of course, preposterous. If one of these geniuses simply took the time to read, study and clarify what is represented by the scientists, scholars, and professionals on my "References" list below, they would know exactly where this information comes from. This is not merely my opinion; it is what these professionals have determined and proven by specific, controlled studies. Some of the information comes from published books that one has to buy. They've published these studies, they've published textbooks, and they are the reason I make the claims I do about what they have determined. While some logic and practice is presented here by my own experience, my work pales in comparison to what these men and women have determined, discovered, and detailed in their presentations.

Most of the data is available on the internet, in textbooks, studies, and journals (not from knife forums), and it is knowledge and information that was unheard of thirty years ago. Back then, you would have to travel to college and university libraries to read and learn it, and now, for the first time in human history, much of this information is available to the public! What a great time to read, learn, and grow! What a great time to benefit from the studies, experimentation, and discoveries of others! What a great time to move beyond anonymous claims, wives' tales, old misperceptions, and traditions based in limited scope. What a truly great time to be alive!

Yet... you have to read. You have to take the time, invest the time in yourself, sit down, computer, monitor, screen, or book in your face, in the quiet, with a scientific dictionary at your side, a notepad, and sometimes you have to read the material two or three times to fully understand the scope of these professional works and documentation.

Please know that every single day, I read and study, and this takes discipline. I try to write most days, and this takes discipline. I make knives as a metal smith, machinist, jeweler, lapidary, sculptor, leather worker, artist and professional every single day. That takes discipline. It also takes discipline to clarify what is important to my clients, my readers, and the countless strangers who visit this site every day, people with whom I may never meet, never converse with, never know, but who have the discipline and interest to learn what I try to share.

Simply put, you don't make thousands of knives successfully for decades, have pieces in museums, make for some of the top warriors, law enforcement, military, and counterterrorism professionals in the world, for decades, and have novice, faulty, or incorrect ideas and concepts about your trade and craft. It's not my logic or knowledge alone that has created this, though I know I have contributed to my tradecraft and art. It's the people below in my References list, and it's the people who design, refine, suggest, and use my knives in the field, who are the reason I make the knives I do, the way I do, and make the claims I do. Again, you can read the Testimonials to get an idea of their actual experience.

If you are in disagreement, the resolution is simple. I suggest reading before you post anonymously on a forum, from the very sources detailed below. They are not alone, there are literally thousands of scientists daily sharing their new knowledge and studies on the internet and in publications.

Don't get your piecemeal knowledge on a forum, forums are not frequented by professionals in the field of metallurgy. The real guys in the know aren't comparing factory knife A to factory knife B and claiming that the grain structure is damaged (or improved) by a small change in percentage of one of the elements. Real pros are publishing scientific journals, books, theses, dissertations, and then having them peer-reviewed by other real professionals.

Buy a few books, roll up your sleeves, learn, study, and grow the mind that Almighty God gave you. He has blessed you with this amazing device, he has given you the gift of discernment, logic, and curiosity. You are not born with the truth, you must discern it.

If you wonder where a professional knifemaker publishes, know that you are reading one of the very best free sources right now. If you appreciate the truth here, you are the reason I write. If you disagree, please make sure that you have reasonable considerations based on science and physics, backed up by scholarly resources and not some comment by a forum participant who is not actually accomplished in the field. Do continue your research; no one is stopping you from doing that.

Further, if you truly are gifted in the deep understanding knowledge of knifemaking and knives, the oldest implement in our history, then I implore you to pick up a piece of steel and start making. It's been my experience that there aren't enough really good knifemakers to go around. Then, take plenty of photos, offer detailed descriptions, and build your own profession.

Page Topics

Glossary of Terms

alloy (steel)

A substance composed of two or more metals intimately mixed and united. Typically, in knife blades, these alloys are included to enhance mechanical properties, aid in fabrication characteristics, and add specific attributes to the steel. I use a dozen different alloys in my current work; all of them are hypereutectoid and high alloy tool steels and stainless steels.

anneal (annealing)

A treatment of steel to convert austenite and martensite to pearlite, softening the steel, relieving stresses, and making the steel ductile and malleable for easy machining and working. With proper work method, this is rarely, if ever needed in the modern knife shop, and I can count on one hand the times I've done this in 35 years of knifemaking. Annealing is done by heating steel to a predetermined temperature, and cooling slowly over many hours to allow equilibrium phase transformation to take place. The exact time, temperatures, and rate depend on the steel alloy type.

asperity

This is the rough surface or edge of metal, particularly defined when surfaces are polished (or not!). Asperity is improved (reduced) in cryogenically treated steels, and these same steels can be made sharper due to the fineness of the carbide structure created when these steels are cryogenically treated.

austenite (gamma-ferrite)

A crystalline phase of non-magnetic steel created at high temperature conversion, necessary to form martensite, cementite, pearlite, or bainite, depending on the treatment process. More about austenite at this bookmark.

bainite

Bainite is a combination of cementite and ferrite, stronger than pearlite. It's formed from austenite below the temperature that will form pearlite, and above the temperature than which will form martensite. More on bainite at this bookmark.

carbides

Extremely hard particles in knife blade steels. These carbides are sought-after in knife blade steels, they are beneficial to extremely high wear resistance. Some metallurgists believe that they play a more critical role in the durability and wear resistance of steel than martensite. There are many types of carbides, and all of them are formed with carbon and a less electronegative element. In these steels, some iron carbides are Fe₃C, Fe₇C₃ and Fe₂C. Some chromium carbides are Cr₂₃C₆, Cr₃C, Cr₇C₃, Cr₃C₂. Other carbides are molybdenum carbides Mo₃C₂, vanadium carbides, niobium carbides, tungsten carbides and complex carbides that are combinations of other carbides! Some carbides have complicated crystalline structures, some form in interstitial locations of other crystalline lattice structures. With all carbides, their effectiveness depends on how fine they are, how well-dispersed, how high the volume overall that is precipitated. A critical point is that the three elements of chromium, molybdenum, and vanadium have the highest solubility in austenite, therefore they precipitate the highest volume of carbides. This is why these three are big players in high alloy steels.

cementite

Iron and carbon with the chemical compound Fe₃C. It is a brittle, extremely hard ceramic substance. More on cementite at this bookmark.

critical (temperature)

In knife blades and heat treating, this is the temperature at which phase transformation takes place, the temperature when austenite is formed from the base allotrope. Also known as the austenitizing temperature. These temperatures vary depending on the steel alloy. In the old days, all the temperatures of transformation were called "critical."

cryogenic

Simply means: of or relating to extremely low temperatures. Cryogenic references do not have a specific temperature, no matter what you may read on open source definition guides and encyclopedias. Each science and realm of cryogenics is different, but in knife blade discussion, it means colder than sub-zero treatment of blades to impart higher wear resistance, toughness, and corrosion resistance. Further specification must be made, such as shallow cryogenics or deep cryogenics, or specifying the temperature to clarify the context of the idea, range, or discussion.

crystal, crystalline

In this context, a body that is formed by the solidification of the combination of steel alloy elements that has a regularly repeating internal arrangement of its atoms and molecules with strictly defined and identifiable external plane faces.

decalescence

The property of absorbing heat energy without increasing temperature while phasic change is underway in steel. Technically, a decrease in temperature when compared to ambient thermal loading.

decarburization

A very bad thing; knife blade steels are overheated, or heated too long, or heated in an oxygen-rich environment, and the carbon migrates to the surface of the steel, bonding with the free oxygen to form scale. The scale is ground off, and the knife owner does not even know that the steel has been rendered to a less than optimum alloy by carbon loss. Carbon is the most important alloy in all steels, so this is no small error. Read about the horrors of decarburization by an established and experienced knifemaker at this bookmark.

equilibrium

In this context, equilibrium means with all physical structure at rest, in balance, and with changes slow and static, with no dynamic forces. In steel, the phasic changes occur slowly with the physical form at rest, and this is not what knifemakers do, unless we are after full annealing of steels!

ferrite (Alpha-ferrite)

Iron with a body-centered cubic crystalline lattice form, magnetic, soft, a major constituent of mild steel. More about ferrite at this bookmark.

hardening temperature

This is the temperature above the decalescence point to which steels are heated for complete transformation before quenching during hardening process. The hardening temperature (and time that the blade is exposed to this temperature) depends on the steel alloy, the manufacturer's guidelines, the cross-sectional thickness of the blade, and the knifemaker's own experience for the desired result of complete austenitizing.

hysteresis (hysteresis band)

Also called "dead-band". In this context, it's the range of cycling in ovens between the temperature that the heating element turns off after reaching the set temperature, and the oven cools to a lower temperature and then the element turns back on until the set temperature is reached again. This creates a cyclic effect in a range of temperature, and this is called hysteresis. In most ovens and furnaces, this range can be extremely wide, between 50° and 150° F, creating wide swings in temperature and inaccurate control of the process. Attempts should be made with equipment to narrow this band for greater accuracy in the process. In my own studio, switching tempering and drying ovens to PID controllers will result in a hysteresis band of about 1°F! Mechanical freezers can suffer from wide hysteresis bands as well, applied to their cooling control rather than heating control.

interstitial

In this context, the word refers to the holes between larger metal atoms in crystalline lattices where smaller atoms or ions occupy. It also refers to the spaces in the larger molecular arrangement that carbon and small carbides occupy.

lattice

A regular geometrical arrangement of objects constituting volume; specifically: the arrangement of atoms in a crystal in a clear and definite physical and mechanical form.

martensite

Martensite is a very hard, corrosion-resistant, wear-resistant crystalline structure created by sudden quenching transformation from austenite. More about martensite at this bookmark, and further explanation of understanding martensite at this bookmark.

metastable

In the context of steel phases, this means stable for the moment, if no outside forces or conditions act upon the metallic structure. So in knife blades, this is not really reasonable, as force on the structure of the steel (applied by cutting and pressure), aging (inevitable), and temperature changes can all force the metastable material into another phase or condition. The idea is to get the blade into a "stable metastable" condition, so that normal knife use and exposure does not induce changes in the steel knife blade.

normalizing

A treatment in lower alloy steels to relieve stresses caused by machining and forging, involving heating the steel to its austenitizing temperature or somewhat below, and then letting cool in room air or by a fairly fast rate. This cannot be used in high alloy martensitic stainless hypereutectoid steels, because they will quench and harden, but is typically performed in lower alloy or hand-forged blades.

pearlite

Pearlite is a layered structure of ferrite and cementite, formed in steels by slow cooling. It is very tough, but not particularly hard or wear resistant. More about pearlite at this bookmark.

PID controller

PID stands for proportional, integral, derivative, and this is an industrial process controller that is programmed to high accuracy with internal feedback capabilities. What this means is that the controller is not simply a thermal switch, turning heating (or cooling) on and off; it calculates or can be set up to work with the individual application, controlling the rate, timing, error, and expected heat loss (in the case of a heating application) to anticipate the load, process, and needs of an individual device. Without going into specifics, these controllers allow very accurate temperature control, once set up and programmed for the specific use. In the case of my tempering/drying ovens, variations of set temperature create a narrow hysteresis band of 1°F.

precipitation

Described chemically, precipitation is when a solid is formed out of a solution. In steel phase transformation, precipitation occurs as a substance (usually carbide) is produced from a solution. For example, in tempering, the solid solution of carbide particles are formed from the atoms of carbon that are in the solution of the martensitic structure that are not part of martensite's particular crystalline lattice. The atoms of chromium, iron, molybdenum, and other alloy elements combine with the carbon and precipitate into carbide molecules and crystals. Carbon, when "in solution" is not attached to these other elements, so, even though it's actually a solid carbon atom, it is still considered "in solution" until it bonds to form carbides or other materials.

quench

To cool suddenly. In knife blades, this forces transformation of austenite to martensite, and precipitation of carbides, the basis for hardening steel. Quench types, mediums, rates, and temperatures differ depending on the steel type and alloy undergoing quenching.

recalescence

The property of losing energy without a drop in temperature during cooling of the steel at equilibrium during phasic change. The steel actually increases in temperature as the physical structure changes and the steel attempts to reach entropy.

secondary hardening

an increase in the hardness of heat treated steel, particularly in high alloy hypereutectoid steels with strong carbide formers, that happens at a higher temperature range in the tempering cycle due to fine carbide formation. This is most useful and typical on high sped tools that will encounter high temperatures and need to resist softening. The disadvantages of secondary hardening are extreme loss of corrosion resistance, and loss of toughness.

spheroidized (spheroidizing)

A treatment of steel to convert plate-like cementite into spheroid cementite, resulting in extremely soft, malleable, ductile condition of steel for ease in machining and working. This is done by heating steel to a predetermined temperature, and cooling slowly over many hours to allow equilibrium phase transformation to take place. The exact time, temperatures, and rate depend on the steel alloy type.

spinodal

a decomposition mechanism describing rapid un-mixing of a mixture of liquids or solids from one thermodynamic phase, to form two coexisting phases. As an example, consider a hot mixture of water and an oil separating.

stainless steel

A steel containing a substantial amount of chromium, which adds strength and inhibits corrosion. While in the United States of America, we classify stainless steels as generally having more than 10% and up to 13% or more chromium, in Europe and other parts of the world, they classify stainless steels specifically as having more than 10.5% chromium. According to the ASM International, steels containing 11.5% of chromium are classified as stainless, and with 12% chromium they have aqueous corrosion resistance. Since it has been convention in the past to classify steels having as little as 4% chromium as stainless steels, it can be very confusing to classify with only the simple designation of "stainless." Therefore, it's best to describe stainless steels by their grade (austenitic, ferritic, martensitic) and by their trade name or SAE/AISI designation when describing them. Simply identifying a steel as "stainless" does not accurately identify the steel.

steel

A strong, hard metal made of iron and carbon with alloys of other elements all included to produce specific effects and results in the final use of the steel item.

sub-microscopic

Definition: too small to be seen by an ordinary light microscope. In this context (knife steel) it means that a structure is too small to be seen by an ordinary light microscope. Sub-microscopic is then a definition of the size of an object. This doesn't mean the structure can't be seen (actually imaged) by a microscope that uses other methods, such as a scanning electron microscope. The sub-microscopic size limit is about 1500X and the resolution of 0.2 micrometers.

super steel

There is, simply, no such thing. This type of term is non-specific and a sales-directed descriptor, inserted to make one think one steel is superior to others. Like steels with mystical, generalized, or popular name created as business advertisers, this has no place in the context of steel discussion, unless you are discussing comic book characters Superman, Supergirl, Superboy, Superdog... hey, were is Superwoman? How come it's Wonder Woman and not Superwoman? Ahh, I get it: the wonders of women...

tribology, tribological

Tribology is a branch of mechanical engineering and materials science. Tribology is the science and engineering of interacting surfaces in relative motion. It includes the study and application of the principles of friction, lubrication and wear. In knifemaking, tribological studies play a role in determining steel wear characteristics and this is the only scientific, accepted method to determine the relative wear resistance of steel. Cutting tests do not; they are too variable, and knife blades can not be consistently created to any high degree of accuracy. In tribological testing, wear surfaces, indentation, loss of mass, and friction are all considered and calculated. This is ASTM and AISI approved testing of the wear resistance of steels and it is the only recognized standard. More about cutting tests of knife blades on this page.

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