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Video

Here is the video version of the following information:

Heat Treating Steps and Equipment Types

There are three major steps of heat treating performed by a custom knifemaker or a factory, and an optional fourth:

1. Austenitizing – heating the steel up hot and soaking
2. Quenching – Rapidly cooling the steel
3. Tempering – reheating the steel to a lower temperature such as 400°F (200°C) for 1-2 hours. This is typically performed 2-3 times (cooled to room temperature in between).
4. Cryogenic or cold processing is an optional step that can be performed after the quench or in between tempering steps. It involves cooling the steel to as low a temperature as possible.

Fundamentally these steps can be performed with a wide range of equipment, and that equipment can have an effect on the results. Below I have written about some of the basic equipment setups.

Austenitizing

The austenitizing step is performed somewhere between 1450 and 2250°F (785-1230°C), so this is not happening in a kitchen oven. The biggest difference between small batch heat treating done by an individual knifemaker and a factory is the size of the furnace. Knifemakers will use a benchtop unit such as those made by EvenHeat or Paragon. Therefore the number of knives that are in the furnace at once is much greater in a factory furnace. There are pros and cons to both situations apart from just the number of knives that are heat treated at once. A furnace is not perfectly evenly heated, it is going to be slightly colder at the door, for example, and there will be some thermal “gradient” and relatively hot and cold spots. Typically there is a “control” thermocouple that measures the temperature and the furnace is constantly adjusting to hit the target temperature. If you are heat treating one knife at a time that is directly next to the thermocouple you can be sure the temperature the knife sees is very close to what the thermocouple is measuring. With a loaded up furnace there will be some variation in temperature. That could be only a few degrees or in extreme situations there could be a 20-30°F+ difference between blades in the center of a furnace vs the “corners.”

Custom knifemakers do not always use furnaces. It is relatively common for knifemakers to use a forge which is relatively uncontrolled and the goal is to pull the blade after it is heated into the right range. Some knifemakers will try to hold a specific temperature within the forge or improve the uniformity of the forge by using a “baffle,” also called a “muffle.” This can be as simple as a large piece of pipe. And less common, though still done by some, is to use a torch to heat the edge or the entire blade. Again, this is relatively uncontrolled compared with a furnace.

Quenching

Quenching also varies quite a bit. Steels are rated by “hardenability” in terms of how fast they need to be quenched. The general categories are “air hardening,” “oil hardening,” and “water hardening.” An air hardening steel can be quenched with pressurized gas or even just in still air. Oil hardening knives are quenched in oil, and water hardening steels are quenched in water. Factory knives tend to use air hardening steels, in part because they are better suited to large batch heat treating, and in part because all stainless steels are air hardening. With custom knives, “plate quenching” has become popular in the past couple decades. Plate quenching involves placing the steel between two plates (usually aluminum) and the plates draw heat out of the blade. This is faster than sitting in air and also helps maintain flatness during quenching. The most common quenching method for factories is a pressurized nitrogen gas quench. The pressure of the gas is typically rated in “bars” where a “1 bar” quench would be atmospheric pressure, and 2 bars would be double atmospheric pressure. The higher the pressure, the faster the quench. However, this is not the only factor that matters. How “loaded up” the furnace is will also dictate the quench rate. If many blades are stacked with each other, that increases the volume of each and thus it takes longer to cool it down. You can easily imagine how a 4 inch thick block of steel takes longer to cool than a 1/8″ thick blade. And if a relatively small number of blades are in the furnace, and they are well separated, that will mean a faster quench rate even with the same pressure quench. The heat treaters usually want to load up the furnace as much as possible for efficiency. Here is a random YouTube video that shows vacuum furnaces:

Slower quenching means that hardness and toughness is reduced. Below shows a published study [1] on M2 high speed steel tested with 2 bar, 6 bar, and 9 bar quenching in a vacuum furnace. They tested the hardness and also toughness with a bend fracture test. With higher quench pressure, both properties were improved. With slow cooling, carbides are precipitated at the grain boundaries. This depletes carbon from the matrix, reducing hardness. And those precipitated carbides are brittle and reduce toughness, especially since they are present on grain boundaries.

Data adapted from [1]

In a study on Uddeholm Dievar tool steel [2] they compared toughness between air cooling and oil quenching. They found the same 48 Rc hardness with both conditions but the oil quenched steel had 25% better toughness. They took micrographs of both conditions, and you can see the grain boundaries in the air cooled steel. The dark grain boundaries are visible because of the carbides formed along those boundaries.

Image from [2]

Vacuum furnaces are not the only method used by production knife facilities but are by far the most common. One counterexample is Paul Bos heat treating, which is located at Buck Knives and also performs all of the heat treating for Buck. See the video below. They use a conveyer belt furnace and the steel comes out the other side and cools in air. This type of cooling is generally faster than a pressure quench in a vacuum furnace because of how much time it takes to cool down a fully loaded vacuum furnace.

Tempering

Tempering has similarities to austenitizing in that we are heating to some temperature and holding there. The main difference is that tempering is from a much lower temperature, typically in the range of 300-1050°F (150-565°C). This means that it can be performed in less specialized equipment. Many custom knifemakers will use a kitchen oven or toaster oven.

Cold or Cryogenic Processing

Cryo processing generally involves the use of liquid nitrogen. This can be simple, like dipping a knife into a liquid nitrogen Dewar, to fancier setups that spray liquid nitrogen, or cool down a refrigeration unit with liquid nitrogen. There are also refrigeration units that can get very cold without liquid nitrogen, though are generally at a somewhat higher temperature. Liquid nitrogen is about -320°F (-196°C). Dry ice is also relatively common for subzero temperatures, it is about -109°F (-78°C). I have also tested the use of household freezers, which are in the range of 10°F to -15°F (-12 to -25°C). This is not as effective since it isn’t as cold, though it can still effect the transformation of steel.

Measured Properties Comparing Commercial and Custom Heat Treating

Below I have collected several examples of measured properties between commercial heat treating and small shop heat treatments. This type of data is not widely available so hopefully it is enlightening.

Hardness

The final hardness of the steel comes from several factors. Austenitizing from a hotter temperature, or tempering from a lower temperature, usually means higher hardness. Cold or cryo processing can increase the hardness another 0.5-3 Rc depending on different factors. However, even with the same austenitizing, tempering, and cryo setup, the quench rate will affect the final hardness. This is one of the biggest differences between typical factory and typical custom heat treating.

For the MagnaCut datasheet I measured hardness for a whole range of austenitizing and tempering combinations with “plate quenching” with no cryo, a household freezer, and liquid nitrogen. We also sent steel to Peters Heat Treating, which has typical factory vacuum furnaces that use a pressurized gas quench. They tested with a “2 bar” nitrogen gas quench. You can see that the hardness after the pressurized gas quench by Peters (bottom) is typically 0.5-1 Rc lower than when I plate quenched individual pieces.

Different steels can be more or less sensitive to these differences. MagnaCut has 2% Mo in it which increases its hardenability (thus it is less sensitive to quench rate). However, AEB-L has no Mo and thus it can benefit more from a fast quench. Of course an oil hardening steel would be very soft even with a high pressure quench in a vacuum furnace. For air hardening steels I have heard reports of up to 2 Rc difference between a plate quench and a 2-bar gas quench in a vacuum furnace.

Of course with oil hardening steels they require an oil quench. This is more difficult to do in large batches than air hardening steels. However, factories that are setup for oil quenching will have a relatively similar quench rate to a custom knifemaker. Both are quenching in oil so there isn’t much difference.

Toughness 

I have toughness data for vacuum furnace heat treating of MagnaCut from three different heat treaters that use vacuum furnaces, which I have compared against my own experiments with plate quenching.

The “2-bar” and “Unknown pressure” heat treaters were targeting the same hardness with the same temperatures. I am not sure the reason for the discrepancy in hardness. For toughness, both are a similar offset below the “Plate Quench” numbers because of the slower quench speed. Presumably they had a similar quench rate. The slower quench rate led to a reduction in toughness for a given hardness. The faster 10-bar quench also used a higher austenitizing temperature and a higher target hardness, so the faster quench is not the only reason for higher hardness. However, for a given hardness this heat treatment was closer in toughness to the “custom” heat treatment using a relatively rapid plate quench.

Carbon and Low Alloy Steels

Below are results from heat treating of 52100 with a heat treatment performed by knifemaker Warren Krywko and also two heat treatment facilities that can do oil quenched blades:

The austenitizing temperatures and quench rate were similar in the case of 52100 since all were quenching in oil. Therefore the hardness and toughness was similar whether it was done in a small shop or by a commercial heat treatment company.

Forging and Thermal Cycling by Custom Knifemakers

One way that custom knifemakers will try to make their heat treatments better than factory knives is in forging and thermal cycling of their blades prior to the steps we outlined above (austenitizing, quenching, and tempering). While knifemakers typically will claim that forging, and especially thermal cycling, is for the purpose of refining the grain size, instead the biggest differences are from affecting the carbide structure prior to austenitizing. The steel manufacturers typically want the steel to be as soft as possible for easy machining and grinding by the end customer. The very soft steel is achieved by having a relatively coarse annealed microstructure. If instead the carbides are smaller and the steel is a bit harder in the annealed condition, then the hardness-toughness balance can be improved somewhat. By modifying the normalizing and annealing process (called “thermal cycling” by knifemakers) you can achieve these results. Click here for an article about normalizing and annealing. This thermal cycling could likely be replicated to some extent even in a factory setting, but typically the time, logistics, and cost make this less feasible when hundreds or thousands of blades are being produced. Below I have the same 52100 chart as before but this time I added in results from 52100 that was forged and thermal cycled by my father Devin Thomas. To be clear, it was the modification to the carbide structure through a different normalizing and annealing process that led to the change in properties, not the forging. You can read about the process that was followed in this article on 52100.

Some steel manufacturers will have a coarser starting structure than others, leading to differences in how the steel responds to heat treatment. In my study on 80CrV2, I found that one steel manufacturer’s steel needed significantly higher temperatures than the others to achieve the same hardness. Notice how the “Buderus” material is softer than the steel labeled “Jantz” or “AKS” which were also heat treated as-received from the manufacturer. The “KSN cycling” used my own recommended normalizing and annealing. The cycling procedure I performed did not start with forged steel, showing that the improved microstructure can be achieved without forging.

We found that this result lined up with the starting microstructure:

Buderus-annealed 80CrV2

Jantz-Supplied 80CrV2

80CrV2 with “KSN cycling”

The smaller carbide size also led to improved toughness for a given hardness with 80CrV2. Notice that toughness was similar for each condition but at higher hardness when the starting carbide size was smaller:

Forging and Annealing Stainless Steels

While “thermal cycling” is much, much more common with simple carbon and low alloy steels, there are improvements to be had even with stainless steels by refining the carbide structure. Below are results for AEB-L and MagnaCut:

You can see that in both cases we had relatively small improvements to the hardness-toughness balance by improving the annealing process that was used for those two stainless steels. You can read about how those were optimized in this article on forging and annealing stainless steels.

When Custom Heat Treatments are Worse

In many cases the difference is pretty small between a “basic” heat treatment and one with more involved thermal cycling. Usually with more complex and “dialed-in” heat treatments the potential improvements are small, on the order of 10-20%. When I refer to a “dialed-in” heat treatment I am also referring to other aspects like optimizing austenitizing and tempering temperatures. I have many articles on my website for various steels, such as when I found that CPM-CruWear has better toughness with a low temperature temper of 400°F/200°C despite the fact that the datasheet recommends the high temperature range (~1000°F/540°C) instead. However, these types of temperature optimizations can be (and sometimes are) done by either factories or by individual custom knifemakers.

When using complex or “fancy” heat treatments, the chance of making the steel worse is usually greater than the chances of making it better. I think every buyer should be skeptical of knifemakers that are claiming their heat treatments are significantly better; without quantitative, controlled testing methods the knifemaker is unlikely to know whether an improvement has been achieved. I have heard many knifemakers claim improvements based on changes they made that basically have no chance of improving anything. And some of those modifications are actually making it worse. For Knife Steel Nerds I have tested heat treatment changes I was sure would improve the steel, but instead found worse properties. For example, I did a rapid triple quench heat treatment of 1084 using salt pots. The hardness ended up the same, but I was shocked to see that the toughness had been significantly reduced, especially since the grain size was small in all three tested conditions. The normal furnace heat treatment is labeled “Furnace in air,” the short salt pot heat treatment is labeled “Salt pot, 3 mins,” and the triple quench is the same label with “(x3)” added. In this case the best result was with the basic and boring heat treatment.

But the above examples I have given on improved (or worsened) custom heat treatments all assume furnace heat treating with well controlled temperatures. There is a much more common and pernicious form of custom heat treating which is largely accepted in custom knifemaker circles: forge heat treating. The biggest problem with forge heat treating is that the temperatures are not well controlled. Typically the knifemaker does not even know for sure what the temperature is. There are some methods, like using a magnet, that are used to try to be more consistent, but none are as good as using a furnace with well controlled temperatures. Some steels are more sensitive than others, and some methods are better at being consistent, but none are perfect. Knifemaker/metallurgist Juha Pertulla did an experiment with forge heat treating of 1075 steel where he found that holding the steel for only one second beyond reaching nonmagnetic had already led to increased grain size and reduced toughness. Using 80CrV2 with its vanadium addition made the steel less sensitive to overheating. In my own experiments with forge heat treating I also found 1084 steel (similar to 1075) to be sensitive to overheating:

I have had multiple knifemakers argue with me that I am wrong about the unreliability of forge heat treating. Three of them have sent me steel they heat treated to show me how their forge heat treatments are consistently high in performance. Every case I have tested so far has resulted in poor toughness from overheating. Here is one example:

When Custom and Factory Heat Treating is the Same

Many custom knifemakers do not perform their own heat treating and instead send to commercial heat treaters. This can be for a variety of reasons, but three common scenarios are: 1) the knifemaker doesn’t have the cash for investing in heat treating equipment, 2) the knifemaker doesn’t trust himself to learn how to properly heat treat and/or trusts the commercial heat treater more, or 3) the knifemaker works at a high enough volume that a commercial heat treater makes more sense. In these cases there is unlikely to be much difference between the heat treatment performed on the knives for an individual knifemaker and those performed on “factory” knives. Of course, the lines are sometimes blurred between “custom” and “factory” knives where knifemakers may outsource many steps beyond heat treating. There isn’t anything inherently wrong with this it just means we can’t always neatly categorize things.

Edge Retention

Many years ago I analyzed a set of CATRA edge retention data from a major knife manufacturer and wrote two articles about it: Part 1 and Part 2. Later I obtained my own CATRA tester and did a large set of tests on a wide range of steels. I have had some knife enthusiasts argue that because I was performing my own custom heat treatments that my tests should not be accepted out of hand. Usually this is because they think a steel over- or under-performed relative to where they had already decided this steel should be on the chart. To study this I compared a range of steels that I had tested to the knife manufacturer’s dataset:

The relative position of each steel was basically the same whether it was my tests with “custom” heat treatments or the knife manufacturer with “factory” heat treatments. However, you will notice that my results were consistently somewhat higher than the knife manufacturer, averaging about 17% better. This is not because my heat treating was superior, but rather due to differences in the design of the blades, the edge geometry, and how the blades were sharpened. The “behind the edge” thickness was greater with the production-made test knives. Also, the knives that I tested were sharpened with an Edge Pro which keeps the edges very “triangular” without rounding. The factory sharpened knife edges instead have a more convex shape, which makes them behave more like an edge sharpened to a more obtuse angle. The factory knives were given a polishing step at the end of sharpening. My knives were sharpened to 400 grit, relatively coarse, which gives the steels enhanced slicing edge retention. I have tested the effect of edge finish on CATRA edge retention in the past:

Another clue is seen when we plot the percent difference between my “custom” knives and their factory knives vs the total CATRA edge retention:

If we ignore Maxamet, the percent difference is greater with lower edge retention steels. I promise I do not have secret heat treatments that can make 55 Rc 420 steel cut 50% longer. Instead it was because of the differences in sharpening and edge geometry. With the low wear resistance of 420, how much cardstock it cut was largely controlled by its cutting ability (sharpness and edge geometry) rather than its wear resistance.

To illustrate just how important the edge geometry is, here is a chart showing the effect of total edge angle vs CATRA edge retention:

You can see that AUS-6 with a 27 degree edge (13.5 degrees per side) cut about 400 mm of cardstock. This matched the performance of the significantly more wear resistant CPM-154 with a 34 degree edge, which is only 3.5 degrees per side greater. And that same 400 mm is measured with the very wear resistant S110V with an edge angle of about 41 degrees. If CPM-154 is given a 27 degree edge instead, it matches Maxamet with a 41 degree edge! So small differences in sharpening can make a big difference in the result of an edge retention test.

Custom knives are often given thinner edges than factory-made knives. In general, when superior edge retention performance is measured with custom knives, it is due to better edge geometry rather than any kind of super, unmatchable heat treatment. When a custom heat treatment does perform better with identical edge geometry and steel it can usually be attributed to higher hardness. In my own CATRA experiments, the effect of hardness is greater than other changes to the heat treatment. This is in contrast with toughness measurements where I often find significant differences when changing heat treatment variables.

Seattle Ultrasonics Kitchen Knife CATRA Study

For one more dataset that shows the importance of edge geometry I can show the “Quantified Knife Project” of Seattle Ultrasonics. I performed the CATRA tests for this project. We measured the cutting performance of a range of low- to high-end factory knives and one custom made knife. For the initial cutting ability (how much cardstock cut in the first back and forth strokes), the only difference in performance was edge geometry and sharpening. In this case the correlation with edge angle is very strong as we would expect:

However, even when we plot edge angle vs CATRA edge retention, we find overall the trend with edge angle is still very strong:

For the majority of the knives, the strong effect of edge geometry completely washes out any potential analysis for the effect of steel type and heat treatment. This is despite the fact that there is a wide range of steel from 56 Rc low wear resistance stainless steels (1.4116) in many of the European knives to ~60 Rc VG10 in many of the Japanese knives. The main exception to the trend are those values at ~820 and ~1050 mm. Those are for the steel advertised as “FC61” steel which is a proprietary name. I have seen speculation that the FC61 is similar to AEB-L or 13C26 (relatively low wear resistance), but this very high performance would seem to point towards it being something more wear resistant. However, not many of the knives were below 20 degrees so there aren’t many other comparisons.

Another specific knife I want to point out is the custom Moritaka knife, the only custom knife that we tested. It was one of the best performers at ~650 mm. This was in Blue Super steel which in my testing with a 30 degree angle was only at 338 mm. So the superior results for this custom knife were not from a super heat treatment or magical forging but instead from a smaller edge angle. Of course it should be noted that the very low edge angle also makes this edge more delicate; it is more likely to chip in use. Everything is a tradeoff.

Sharpenability, Edge Stability, and “Mushy Edges”

In my prior CATRA studies which have included over 50 steels, I have only had significant issues with sharpening a small handful of them. This is in part due to the use of CBN abrasives which makes sharpening even high vanadium steels very easy. Instead, the difficult to sharpen knives were from heat treating, not steel. Those difficult-to-sharpen knives had high retained austenite, which meant that deburring would leave behind a ragged edge which wasn’t very sharp. High retained austenite comes from austenitizing at too high of a temperature, especially in combination with slow quench speeds and when cryo isn’t performed. You can learn more about retained austenite in this article on cryogenic processing.

Both small batch and commercial heat treaters can have issues with retained austenite. But there are a couple areas where commercial heat treaters are at a disadvantage. One is the slower cooling rate we have been discussing throughout this article. Slower cooling stabilizes the retained austenite which makes less of it transform during cold and cryo treatments. The other is that commercial heat treaters often like to perform cryogenic steps in between the two tempering cycles, where cryo is less effective. The first tempering step also helps stabilize austenite just like slow cooling does. They prefer to do it between tempering steps because the cryo processing is less likely to lead to distortion, warping, or cracking. But it is less likely to do so in part because the cryo processing isn’t doing as much. To be clear, some knifemakers doing their own heat treating will also do cryo in between tempering, and some commercial heat treaters will perform cryo directly after the quench when it is more effective.

High retained austenite also makes steel behave as if it is softer than it is. Two knives with the same hardness will behave differently if one has significantly more retained austenite than the other. One area is in sharpening as already mentioned, but the high RA knife will have edges that deform more easily.

With proper selection of austenitizing temperatures and cryo processing, retained austenite can be limited to levels where it doesn’t affect the performance of the knife. Some steels have more issues than others. For example, I found that the datasheet-recommended austenitizing temperature for CPM-S110V was quite high. Steels like Vanax and LC200N are difficult to heat treat beyond 58-60 Rc so the heat treaters are forced to austenitize at the very peak of the curve where it is easy to “overshoot.” For example, here is a comparison between Vanax and Elmax steel with a range of austenitizing temperatures, using cryo and a 300°F (150°C) temper:

You can see that while you can use a pretty wide range of austenitizing temperatures with Elmax and achieve 61+ Rc (and then temper down to target hardness), the range for Vanax is much narrower. The peak hardness is around 1975-2000°F but already at 2025°F the hardness has dropped. When there is a hardness drop from austenitizing too high this is the point where there is too much retained austenite, and in some cases the very peak can also be questionable. So Vanax is difficult to heat treat for 60+ Rc while also avoiding excess retained austenite. When the composition of the steel is slightly different from batch to batch, the austenitizing temperature where peak hardness is reached will vary slightly. This in combination with variation in temperature within a furnace, and between furnace batches, which means that it is usually better to be a bit conservative with the temperature and go a bit lower than the absolute peak.

Corrosion Resistance

The carbides precipitated during a slow quench are also detrimental to corrosion resistance. Of course, other heat treating variables also matter. A higher austenitizing temperature means more chromium carbide is dissolved, putting more chromium in solution for better corrosion resistance. And it is relatively common in industry to use the high tempering range of 950-1050°F (510-565°C) which reduces corrosion resistance. This can make the very high corrosion resistance Vanax behave more like a “normal” stainless steel. I have an article on my corrosion resistance testing here. This high temperature tempering range can be used by both custom knifemakers and commercial heat treaters but I see it somewhat more often with commercial heat treating.

Vanax tempered at 400°F on left and 1000°F on right. Sprayed with 1% saltwater for 24 hours.

Who Knows More About Heat Treating?

In many cases the knowledge of the person performing the heat treatment is more important than the equipment being used. A heat treatment without any issues gives you 90-95% of the potential performance. It is when there is some problem with the heat treatment where the performance is noticeable to the end user. I have written about this in an old article called “What a Good Heat Treatment Can and Cannot Do.”

When a knifemaker or knife company outsources their heat treatment to a commercial heat treater, they typically ask for a target hardness and leave it up to the heat treater. The commercial heat treater presumably has significant experience with heat treating, especially with obtaining consistent results, avoiding catastrophic issues, and with troubleshooting common problems. However, many heat treatment facilities lack metallurgists, and when it comes to measuring properties, the hardness test is by far the most common. As I have shown multiple times in this article, hitting the target hardness is only one tiny piece of the puzzle. Some heat treatment facilities are better than others.

When it comes to custom knifemakers doing their own heat treating, the level of knowledge varies widely. Of course it goes without saying that the average knifemaker is not a metallurgist. In general, I would say that the average knifemaker has knowledge of the basic steps of heat treating, but their knowledge of the mechanisms behind each step is relatively poor. I reacted to knifemaker explanations of heat treatment in this video. If the knifemaker knows how to heat treat correctly and how to follow a datasheet they are usually fine. It is when they are trying to diagnose issues or to modify the heat treatment that they get in trouble without knowing how things “work” in the steel. I have an article and video about the basic steps of heat treating and how to follow a datasheet. For those that want to learn more about what is happening in the steel and how to modify heat treatments for different purposes I have many articles on my website as well as my book Knife Engineering. There are a few knifemakers that have put in the time required to understand heat treating and gained hands-on experience to become well-informed, excellent heat treaters.

Differential Hardening and Hamon

This article is already too long but I did want to mention specialized heat treatment techniques that are almost exclusively performed by custom knifemakers. One is developing a hamon, which is often for artistic purposes rather than strictly performance. Various types of other “differential hardening” techniques are used for specific performance goals like edge quenching or tempering back the spine with a torch. These are famously done to pass the 90 degree bend test required for Journeyman or Master Smith performance testing.

Summary and Conclusions

We went over a lot in this article so I will try to sum up. One of the biggest differences between commercial and custom heat treating is the quench rate. The slower quench rate in many vacuum furnaces used by knife production facilities and large custom knife makers can often lead to a reduction in hardness and toughness. A faster quench that can be accomplished with other equipment gives better properties. The degree to which this matters can be debated, but there is a potential for small batch heat treating to be superior. The potential for improving edge retention with heat treatment is somewhat overhyped. Hardness is the main area where edge retention can be improved, and this can generally be achieved with commercial heat treating. The biggest differences between “good” and “bad” heat treatments come when there is a problem in the heat treatment. It is the heat treaters who can avoid these problems (and diagnose them when they come up) that perform the best heat treatments.

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[1] Gonçalves, Cristiane, André Slaviero, Rafael Mesquita, André Tschiptschin, and Paulo Haddad. “Effect of cooling rate during quenching on the toughness of high speed steels.” Journal of ASTM International 8, no. 4 (2011): JAI103483.

[2] Taljat, B., J. Tušek, D. Klobcar, P. Boscarol, and Giorgio Scavino. “Heat and surface treatment of hot-work tool steel for optimum in-service performance.” In The Use of Tool Steels: Experience and Research: Proceedings of the 6th International Tooling Conference, vol. 1, pp. 67-80. 2002.

The post Factory vs Custom Heat Treating of Knives appeared first on Knife Steel Nerds.

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Intro

I have many articles about all of the nitty gritty details of heat treating and the metallurgy behind every step. However, there may be some cases where knifemakers are afraid of all of the terminology and science and think heat treating is too complicated for them. When it comes down to it, the steps of heat treating are not particularly difficult. When you follow a recipe for how to make cookies you don’t need to know the science behind every step, but following them will still get you cookies at the end. An expert would know what went wrong if your cookies were too crunchy, too puffy, spread out too much, etc. And how to modify the recipe to change the flavor and texture of the cookies. However for most of us we will just follow the recipe. You can do the same thing with heat treating knife steel! So for this article I will tell you how to follow a datasheet. I will include some links to articles with more information about what happens in each step, but you can get to those when you are ready. Another great place to learn more about heat treating is my book Knife Engineering: Steel, Heat Treating, and Geometry.

Video

There is a video version of the following content, which demonstrates some of the things described in the article. The article has some more detail that the video doesn’t have.

What Are We Trying to Accomplish When Heat Treating?

When you purchase knife steel you are getting very soft steel in a condition that is easy to grind, drill, machine, or cut. This is called the “annealed” condition. However, soft steel is not good for a knife as the knife edge would easily deform. It would not “hold” a good edge. So we have to perform a series of heat treating steps to get it to high hardness for good knife performance. We are targeting a range of hardness usually between about 58 and 63 Rockwell C (Rockwell C is the unit of measurement). Sometimes the hardness unit is abbreviated as “Rc” or “HRC.” The higher end of hardness gives better edge retention but has lower “toughness.” Toughness is a measure of resistance to breaking.

Furnaces for Heat Treating

The basic piece of equipment for heat treating is a furnace. A forge can also be used but it isn’t my favorite method. I have recommendations on how to properly heat treat with a forge here. There are several manufacturers of furnaces and some knifemakers will make their own. I use EvenHeat furnaces, I have a KO 22.5, an LT 22.5, and a Salt Bath 709. The KO model is the high temperature model that can reach 2350°F. I previously had the LB model which has a somewhat larger chamber (width and height) with the tradeoff that it reaches 2200°F. The LT model is designed for tempering up to 1200°F. A regular furnace can also be used for tempering but they aren’t “tuned” and calibrated for low temperature, and you have to wait for the furnace to cool down before tempering. You can also use a conventional oven for tempering though the temperature will be more variable.

One of the big decisions for a heat treating furnace is 120 or 240V. The higher voltage EvenHeat models are roughly twice as fast and also have larger chambers. This makes heat up time twice as long with the low voltage and it takes longer to “rebound” after opening the door and inserting steel. It is better to use 240V if possible, and if you don’t have the capability for doing that you should contact an electrician. Another important decision is the depth, or length, of the furnace. It is usually a good idea to get one a bit longer than you think you would need. If you ever wanted to make a longer blade such as a bowie or kitchen knife you will need a relatively long furnace. Also the temperature is usually somewhat different at the ends of the furnace, especially right next to the door. So having some space between the blade and the front/back is a good idea.

Steps of Heat Treating

The main properties you achieve after heat treating will be obtained after three main steps: austenitizing, quenching, and tempering. For stock removal makers these will be the main steps you perform, while for a forging bladesmith you will perform a couple other steps after forging to set up the steel for these three steps. You heat the steel up hot (austenitize), cool it rapidly (quench), and then reheat to a low temperature (temper). Not very complicated.

Austenitizing

Austenitizing is the high temperature step soaking step, generally 1450-2250°F depending on the steel and desired hardness. Typically a higher austenitizing temperature means higher hardness, as shown on the following table from the MagnaCut datasheet:

You can read about what happens during austenitizing in this article: https://knifesteelnerds.com/2018/02/28/austenitizing-part-1-what-it-is/

Atmospheric Protection – Foil or Coatings

One thing that a datasheet often assumes is that you know about protecting your steel from the atmosphere and oxygen. Oxygen leads to scale formation and also decarburization, which leads to a layer of soft steel under the scale. With low alloy steels austenitized under 1600°F or so the scale and decarb may not be bad enough to protect the steel as long as you plan on removing some material after heat treatment. There are also coatings that can protect the steel. I recently tested a few of them though I haven’t published those results yet.

For high alloy and stainless steels the most typical method is to use “heat treating foil.” The two common types are 309 and 321 foil, which are stainless steels. 309 is rated up to 2240°F while 321 is rated up to 2000°F, though the 309 is more expensive. You create a foil envelope to place the knife in and fold each of the sides. Some people include talcum powder to prevent sticking though I haven’t typically had issues with sticking apart from high temperatures (>2000°F) and long soak times. Some people recommend putting paper or something else to burn up in the foil packet but this isn’t necessary. Foil is most often used with high alloy and stainless steels which don’t require an oil quench, because removing the foil before quenching in oil is difficult.

Cross-section of AEB-L steel heat treated in a furnace at 1925°F without any protection. The “bright” layer is the “decarb layer” which no longer contains carbon. It is over 0.2 mm thick.

Preheating

Many datasheets recommend “preheating” steps where the steel is heated up to an intermediate temperature before increasing the temperature up to the final target. I described preheating in this article. This process can be done with multiple furnaces or by soaking at the preheating tempreature prior to a ramp to the final temperature. This process is to help the steel achieve a uniform temperature so that it doesn’t transform unevenly. However, knives are thin enough where this isn’t typically an issue. I recommend holding the furnace for 30 minutes at the target final austenitizing temperature instead. You can read why I think so in this article about small knife furnaces and temperature distributions inside them.

Soak Time

After the steel has reached the austenitizing temperature it needs to be held at that temperature for some period of time, called a “soak.” The soak time recommended in a datasheet is supposed to start after the steel has reached the temperature. Sometimes I cheat and start the timer after the furnace has “rebounded” to the temperature (it cools down from opening the door and inserting a cold piece of steel). Once the steel has reached the same color as the furnace it has reached the temperature. You are only seeing the surface but steel is highly conductive and knives are thin so once the surface has reached the temperature it is only seconds before the center has as well.

You will notice that the MagnaCut chart above shows different soak times for each austenitizing temperature, as higher temperatures will often mean a shorter required soak time. The thickness of the steel also matters, the datasheet for MagnaCut says “Min soak time” and if the steel is thicker than 1/8″ (3.3 mm) it may need a few more minutes.

Some knifemakers are afraid of soaking because they think that holding it at temperature will grow the grain size. It is true that holding steel longer will grow the grains but temperature matters much more than time. Holding a steel for 30 minutes is not going to hurt it if the temperature is correct. Undersoaking is a more common issue than oversoaking.

Quenching

The quenching step is what actually hardens the steel. The austenitize dissolves carbides so that carbon is “in solution” prior to quenching but the rapid quench transforms the steel to the hard phase of steel. If the quench is too slow the steel will not reach its full hardness and will have poor properties.

Steels are broadly categorized into three quenching groups: water hardening, oil hardening, and air hardening. Low alloy steels and simple carbon steels typically require a water or oil quench. These are steels like 1095, O1, 80CrV2, and 52100. High alloy and stainless steels are “air hardening” and do not require a water or oil quench to harden after austenitizing. These are steels like A2, D2, 440C, CPM-154, S30V, MagnaCut, and Vanadis 4 Extra.

With low alloy and carbon steels the speed of the quench necessary is controlled by the thickness of the steel (the “cross section”) and the steel. The speed of quench can be broken down into 1) water and brine, 2) fast oil, 3) medium oil, and 4) slow oil. If you get only one oil I would recommend a fast oil like Parks 50 because high hardenability oil hardening steels like O1 can still be quenched in Parks 50, but a low hardenability steel like 1095 would not work with a slow oil. You can buy Parks 50, sometimes generically called Quench 50, from a couple different places including Dubois. I have an article comparing different oils and cross-sections here. I learned that a steel like 1084 can be quenched in a range of different oils at only 1/8″, but at thicker sizes like 1/4″ the type of oil definitely matters. That article also ranks common low alloy steels by “hardenability” (how fast of a quench they need) to help you pick the oil for different steels and cross-sections. With oil and water quenching you “cut” into the liquid with the knife and move up and down rapidly to break up the “vapor jacket” that slows down cooling from the oil/water boiling on the steel surface. Moving side to side can lead to warping.

It is popular for knifemakers now to quench air hardening steels through a “plate quench,” often with aluminum plates. The steel is placed in between aluminum plates so that heat is drawn out. This is faster than sitting in air and helps keep the knife flat (if the bevels are not yet ground). This can also be performed without removing heat treating foil.

Tempering

After quenching, the steel is very high in hardness, but is also brittle. During tempering the steel is reheated to a lower temperature to increase toughness and reduce brittleness. The hardness of the steel is also lowered somewhat (see the MagnaCut chart from earlier or the chart below).

Tempering Chart for 154CM

Typically steel is tempered at least twice, and each temper is usually 1-2 hours long. Tempering at a higher temperature usually decreases hardness though there is an exception called “secondary hardening” in high alloy steels when tempered between about 750 and 1100°F (400-600°C). You can read about the mechanisms behind tempering in this article. Tempering can be done in a conventional oven or in a heat treating furnace after it cools down. I have some tips on using a heat treating furnace for tempering in this article. EvenHeat also makes a furnace designed for tempering.

More complete tempering chart for 154CM that shows “secondary hardening” above 750°F (400°C)

Cooling After Tempering

There are some people that recommend a rapid quench in between tempering steps. This can be done if you are in a hurry. Air cooling to room temperature in between is sufficient. If you are curious as to why you have to cool in between rather than holding longer you can read the article I linked to under the section labeled “retained austenite”.

Cold Treatments

There is an optional “cold treatment” step during heat treating that can increase hardness. Some datasheets recommend this in between tempering steps but I prefer it directly after quenching. In effect it is an extension of the quench down to a lower temperature. A delay between the quench and the cold treatment can lead to less effect of the cold treatment. The MagnaCut chart from before has hardness values from quenching to room temperature, performing a cold treatment in a household freezer, or a cryogenic treatment in liquid nitrogen. No real hold is necessary at the low temperature, the steel just needs to reach the temperature. Typically 30-60 minutes is enough. I have an article on cryo treatments of AEB-L where I discuss a lot more about how cryo works.

“As quenched” hardness of AEB-L with no cold treatment, a freezer, or liquid nitrogen from different austenitizing temperatures.

You may have noticed that hardness can drop if the austenitizing temperature was too high; Using 1900°F (1035°C) austenitizing temperature with AEB-L only resulted in around 0.5 Rc increase in hardness by using liquid nitrogen. From 1975°F (1080°C) the hardness went up from 60.5 Rc to 64 Rc.

You can use most any dewar for holding liquid nitrogen. An off-brand dewar from Amazon or Ebay works just fine. The important specs are the size of the container, usually in liters (mine is 10L), and the diameter of the neck. The most common diameter is 50 mm (~2 inches) but this can be restrictive for wider blades. However, the larger the neck the faster the dewar loses nitrogen, so typically you need to buy a relatively large dewar to get a larger diameter neck.

Extra Steps After Forging – Normalizing and Annealing

When steel is forged it is no longer in the annealed condition. It may not be soft enough for cutting, drilling, etc. And it is not in the best condition for performing the austenitize, quench, and temper. The basic process that needs to be followed is a normalize and anneal. Normalization involves heating to a relatively high temperature (usually somewhat higher than an austenitizing temperature like 1650°F), and then air cooled. This gives the steel a uniform microstructure and grain size. Annealing is done from a temperature typically a bit lower than austenitizing (like 1400°F) and then slow cooled. Annealing makes the steel soft and sets it up for the final heat treating steps. You will also notice below an optional step labeled “grain refinement” that I generally recommend skipping as I have not found an improvement with my testing. This step is relatively common with knifemakers but not in industry and is not recommended in datasheets.

I have an article that describes this process and provides recommended temperatures for normalizing and annealing different steel. For those few brave knifemakers I also have an article on how to anneal stainless steel and high alloy steel after forging it.

Heat Treating in a Forge Instead of a Furnace

The article up until this point discusses heat treating using a controlled temperature furnace. This is my preferred way of heat treating and provides consistent results every time. Some knifemakers like to heat treat with a forge with less control over the exact temperature. I developed a method for minimizing error in forge heat treating as it is very common to overheat the steel and get poor properties. You can read my recommendations in this article.

Every once in a while a knifemaker contacts me and says I am wrong about forge heat treating and that he can get it perfect every time. Below is an example of tests I performed on some of that steel. The toughness was terrible. If you follow my instructions in the linked article you will increase your chance of success.

What Temperatures Do You Choose From the Datasheet?

Datasheets will often given ranges for possible austenitizing and tempering temperatures. The first thing you should look for is a “recommended” heat treatment. You can modify from there of course but this gives a good starting point. Here are a couple examples:

From the MagnaCut Datasheet

From the Vanadis 4 Extra Datasheet

If the datasheet only gives a range for austenitizing I would start with something in the middle of the range. MagnaCut datasheet says 1950-2200°F and halfway in between that would be 2075°F (1135°C), close to the 2050 recommendation. Vanadis 4 Extra says 940-1180°C which would be 1060°C (1940°F), the same as their recommendation for “large sections.”

For tempering a good starting point is usually 400°F (200°C) or 1000°F (540°C). 400°F is good for most steels, though some high alloy and high speed steels will recommend the 1000°F temper. Often these steels can also be tempered at 400°F but the austenitizing temperature would also need to be adjusted based on that change and that would take some work to figure out.

Another thing you can do is check the back of my book Knife Engineering. I give recommended temperatures for most steels in there.

For many datasheets there are many temperature combinations that could work and it is mostly about selecting a target hardness. 58-63 Rc is a good range to target for many knives; use the higher end for thin slicing knives (and careful customers) and lower for knives that need higher toughness. 60 Rc is a good round number if you want a starting point and aren’t sure.

Elements of a Datasheet

Every datasheet is a bit different but I will break down the various parts of datasheets with the MagnaCut datasheet:

Page one of this datasheet (and often multiple pages of other datasheets) is mostly information about the steel. That top left paragraph describes the steel and its general properties. Next it shows micrographs where you can see that MagnaCut has a much finer microstructure than CPM-154. Below that is a table with information about the carbide types in MagnaCut vs other steels. Then a “Tool Steel Comparagraph” shows bar charts of properties of MagnaCut vs various comparison steels. “Typical Applications” serves to tell potential buyers in different product categories when they might want to look at MagnaCut. At the top right you find the composition. Below that is some physical properties which most knifemakers don’t need. Then some test results showing how MagnaCut stacks up in toughness, edge retention, and corrosion resistance.

On the second page we find the actual temperature recommendations that we have been discussing. First is the forging and annealing temperatures, which of course you don’t need unless you actually forge the steel.

Stress Relieving

Below that is a category of treatments called “Stress Relieving” which we have not discussed thus far. These treatments are optional but can be useful in certain scenarios. During grinding and machining various stresses are built up in steel that can lead to increased movement and warping during heat treating. Performing a stress relief can help with this, where it says, “Annealed Parts: Heat to 1100-1300°F…” Stresses can still be built up during grinding of heat treated steel which required a temperature just under tempering. This is because we don’t want to overtemper the steel, of course.

Size Change

This datasheet also lists the estimate size change after heat treating the steel. The heat treated structure is slightly larger than the annealed structure of ferrite. So typically we expect a small increase in the size of the part after the heat treatment is complete. Using a higher austenitizing temperature will reduce the size increase somewhat, and could even lead to shrinkage. Using cryo would lead to a larger size. This is from retained austenite (higher austenitizing temperature) or from converting more retained austenite to martensite (cryo). You can read about what all of that means in my cryo articles.

When Different Datasheets Contradict Each Other

I have a whole article on the heat treatment of 80CrV2 where I found virtually every datasheet to recommend something different. For example, Swiss Steel Group has what I would call a fairly standard recommendation where it says to austenitize between 800 and 830°C (1475-1525°F). Then Bestar has a recommendation for 840-880°C (1545-1615°F). And the New Jersey Steel Baron datasheet recommends 1465-1480°F. In terms of “thermal cycling,” Bestar offers no suggestions for normalizing and annealing, Swiss Steel Group recommends annealing at 680°C (1250°F) with a furnace cool, and NJSB recommends 1650°F for 10 minutes air cool, 1500°F for 10 minutes and air cool, and 1350°F for 10 minutes and air cool.

Sometimes these datasheets are generated by steel suppliers and not by metallurgists working for the steel company. This appears to have been the case for the New Jersey Steel Baron datasheet. The 1650-1500-1350 normalizing steps are not standard and cannot be found in any other datasheet. The 1350°F in particular could be skipped entirely as it wouldn’t really do anything. And the 1465-1480°F austenitizing range is on the low end for a steel like this. Many of the NJSB datasheets seem to have these same recommendations regardless of the steel – 1075, 1080, 1084, 1095, 15N20, 5160, 52100, 80CrV2, L6, O1,  W1, and W2 datasheets are all virtually identical. I would avoid these datasheets.

Bestar is also an odd case as for a couple of their steels they recommend oddly high austenitizing temperatures. Generally it looks like their datasheets are usually reliable apart from these few cases. In this case the heat treatment would “work” but would be in danger of reduced toughness from austenitizing too high.

Usually datasheets are not as different as in the case of 80CrV2. Standard steels like this one vary somewhat more when compared to a steel made by a single company. They are the only ones that make it so they are the only ones to make a datasheet. I wouldn’t pay too much attention to heat treating guides from knifemakers. Sometimes they are based on good information from datasheets or my experiments, but sometimes they aren’t.

As I said previously, another thing you can do is check the back of my book Knife Engineering. I give recommended temperatures for most steels in there. Usually I found a trustworthy datasheet to have charts on tempering. And in many cases I have hardness-toughness data from my own tests to confirm what a good heat treatment range is. I can’t always promise to have the 100% perfect heat treatment, but I know it is a good one. And in some cases like with 80CrV2 I have heat treatment guides for specific steels on my website.

Where to Find Datasheets

Datasheets can be found in a variety of places. The first and most obvious is from the manufacturer itself. Here are a few websites:

Crucible – https://www.crucible.com/products.aspx

CPM steels S30V, S90V, 10V, etc. Standard steels like O1, A2, D2, M2, M4, 440C, 154CM

Some of the standard tool steels are hidden in the old Tool Steel Selector – https://www.crucible.com/eSelector.htm

Niagara Specialty Metals also has a collection of Crucible datasheets.

Carpenter – https://www.carpentertechnology.com/blog/blade-alloys-101

CTS-XHP, 204P, BD1, standard steels similar to the Crucible lineup

This website is kind of big and difficult to navigate so I linked to an old “blade alloys” page with links to many of them. Carpenter used to have more old datasheets for various standard steels that seem to be gone now, unfortunately. Some of those were replaced with Latrobe Steel datasheets.

Bohler – https://www.bohler-edelstahl.com/en/

M390, N690, S390, etc. They also have many standard grades but each is given a meaningless Bohler designation like K110 is D2.

Another big and ungainly website. You will want to look at “Cold Work Tool Steels” and “High Speed Steels.” Stainless steels are on two different pages under “Corrosion resistant and non magnetic steel” and also “Plastic Mould Steels.”

Uddeholm – https://www.uddeholm.com/us/en-us/

Elmax, Vanax, Vanadis 4 Extra. They also have many standard grades but each is given a meaningless Uddeholm name like Sverker 21 is D2.

Uddeholm strip steels like AEB-L, 26C3, and 15N20 are on a different website with limited heat treating information – https://www.uddeholmstrip.com/

Alleima (formerly Sandvik) – https://www.alleima.com/en/products/strip-steel/strip-products/knife-steel/hardening-guide/hardening-programs/

13C26, 12C27, 14C28N

Alleima has recommendations for a “belt furnace” or a “batch furnace.” A belt furnace is literally a conveyer belt furnace which most knifemakers do not have. This is why those heat treatments have relatively short soak times that must be changed based on stock thickness. I prefer to use the batch furnace recommendations, since that is the type most knifemakers are using and the longer recommended soak time is less sensitive to changes in stock thickness.

More Obscure Sources for Heat Treating Information

The ASM Heat Treater’s Guide is an awesome expensive book that has heat treating information on all of the old standard grades. However, there is a free app (used to be on iOS but now only on Android) which has the recommended temperatures from the book, though it is missing all of the charts and micrographs: https://heat-treater-s-guide-companion.en.softonic.com/android An older edition of the book is also available to borrow on archive.org: https://archive.org/details/heattreatersguid0000unse/mode/2up

Tool Steels by Gill and Roberts – An old awesome book, the best edition is the 1980 4th edition but only the 3rd edition is online – https://babel.hathitrust.org/cgi/pt?id=wu.89089662902&seq=6 Unfortunately in 1944 the standard names for steels like D2, A2, M2, etc. did not exist yet so you have to know how to read steel compositions.

Tool Steel Simplified by Palmer and Luerssen – another old book by Carpenter metallurgists which has some good information on a few old standard grades. It can be borrowed for free online – https://archive.org/details/toolsteelsimplif0000fran

There are a lot of other old books but this is enough for now.

Summary

The three major steps of heat treating are austenitizing, quenching, and tempering. These steps will control the properties of the steel.

After forging there are steps needed to soften the steel and set it up for the three main final steps. The main two steps are normalizing and annealing. Some knifemakers put too much emphasis on the “thermal cycling” steps to try to reduce grain size. The final three steps, especially austenitizing, will largely control the properties. Many steel datasheets do not include normalizing or even annealing temperature recommendations; they are available on my website in the “thermal cycling” article or in my book Knife Engineering.

Follow manufacturer datasheets to get good temperatures. There are also heat treatment guides for several steels on my website. Also the back of the book Knife Engineering has recommended austenitizing and tempering temperatures.

 

The post Introduction to Knife Steel Heat Treating from a Metallurgist appeared first on Knife Steel Nerds.

Another rather large heat treating study! This one took quite a bit of time, effort, and money. If you want to support further research visit Patreon.com/KnifeSteelNerds and become a Patreon supporter. All of the money I receive that way goes to knife steel research. And you get some perks like seeing articles and videos early, and at a high enough tier you get a free Knife Steel Nerds mug!

Video 

Here is the video version of the following information:

Should Stainless Steel Be Forged?

Stainless steels are generally more difficult to move under the hammer than simple carbon and low alloy steels. They are typically more expensive. The forging range is usually narrower; you have to stop forging at a higher temperature or it may fracture. But of course stainless steel has the major advantage that it is corrosion resistant. There are many myths around forging stainless and carbon steels that are used to justify the use of only simple steels. These myths scare some knifemakers away from using stainless steels that might otherwise try them.

Is Stainless Steel Improved by Forging?

There is an old tradition in the knife world that says that only carbon steels should be forged. Some have gone so far to say that stainless steels do not “benefit” from forging in the same way that carbon steels do. This is somewhat difficult to refute, as the benefits of forging are often overblown to begin with. I have an older article on forging vs stock removal you can read here. However, I argue that high alloy tool steels and stainless steels have more potential benefits to forging than low alloy steels. The reason is because with simple steels all of the carbides are dissolved at forging temperatures and re-precipitated later; this makes the carbide structure easier to control with thermal cycling alone. High alloy steels have carbides that do not dissolve without melting the steel itself so there is more possibility of improving that structure through further working. Of course all steel purchased by knifemakers has already been forged and/or rolled from an ingot, so we are often talking about a relatively small amount of further forging.

D3 tool steel forged to different degrees thickness starting from a 10″ round ingot [1]

And the carbide structure has directionality to it, leading to different properties in the longitudinal and transverse directions (along the rolling direction and perpendicular to it). In my forged vs stock removal article I gave reasons for why forging blades to shape rarely leads to superior toughness, but if such a benefit was to be gained it would be more pronounced in high alloy and stainless steels.

M7 high speed steel with different degrees of reduction [2]. You can see that the carbide bands are elongated along the rolling direction.

What is a High Alloy Tool Steel?

Most of the information in this article will relate to not only stainless steels but also high alloy tool steels like A2, D2, CPM 3V, etc. Stainless steels used in knives are simply a subcategory of high alloy tool steels. Low alloy tool steels and simple carbon steels behave somewhat differently like 1095, O1, 52100, 80CrV2, and others. The line between low and high alloy can be somewhat fuzzy, some give it as 5% total alloy content. So if the chromium, tungsten, molybdenum, etc. add up to more than 5% it is high alloy. For our purposes a high alloy tool steel is any with at least 3% chromium. These steels are air hardening and their carbides dissolve at higher temperatures. This does not mean all of these steels will be annealed with exactly the same temperatures, hold times, cooling rates, etc. but similar ideas will apply to them.

How Hot to Forge Stainless and High Alloy Tool Steel

A common mistake with forging of stainless and high alloy tool steels is heating the steel too hot. It is a common misconception that because the steel is more difficult to move under the hammer that more temperature is necessary. When steel is overheated, the grain boundaries melt first, leading to steel breaking apart when it is forged. Some knifemakers mistakenly assume they must not have been hot enough, and try even hotter! The temperature at which grain boundaries melt is roughly the same with stainless steel as it is with simple carbon steels, and sometimes lower. Datasheets typically recommend 2100°F (1150°C), though some will recommend lower like 1900-2000°F (1035-1100°C). 2100°F/1150°C seems to work for most knifemakers I speak to. However, many knifemakers are used to lower carbon steels like 1084 or 80CrV2 which can handle higher temperatures and they are not used to dialing the forge down for higher carbon steels. This is why “cast iron” has very high carbon content (>2%); higher carbon means lower melting temperature so it is easier to melt the cast iron before casting. High carbon steels that are more commonly used in forging like 26C3, White #1, Blue Super, and ApexUltra are also more sensitive to overheating since they have relatively high carbon contents (>1.2%).

How to Normalize Stainless Steel

Stainless and high alloy steels are not normalized. The goal of normalization is not grain refinement but rather dissolving all of the carbides before air cooling. As I noted above, with most stainless steels the carbides do not dissolve until melting. Perhaps we could come up with some kind of creative treatment that could improve/change the microstructure prior to annealing but this is not common in the steel industry. Normalizing is not necessary and we will be skipping it with stainless and high alloy tool steels.

What Are We Trying to Accomplish During Annealing?

Annealing is the step we perform between forging and the final austenitize and quench. The steel is annealed by the manufacturer before you get it. For stock removal makers, annealing is not necessary except in rare circumstances. The purposes of annealing are multi-fold:

1. Soften the steel so it is ready for machining, drilling, bandsaw cutting, etc.
2. Prepare the steel for good response to austenitizing so that we don’t need excessive hold time or temperature.
3. Maximize the final properties after heat treatment including hardness, toughness, etc.

Problems with Annealing Stainless Steel – Long Times, Scale, and Decarburization

One issue with annealing stainless steel is that the recommended annealing procedures often require cooling from 1600F+ down to 1000F at 25 degrees Fahrenheit per hour (~15°C/hr). That takes over 24 hours! Not only does this take a long time but most knifemakers do not have furnaces equipped with inert gas or vacuum so this means that scale and decarburization are major concerns. Without any protection you could end up removing a significant amount of carbon from the steel. To mitigate this it is best to wrap the steel in foil during annealing. I found double wrapping to help some as well. This is not a surefire way to prevent any scale or decarb as the times are very long. Therefore it is best to leave some material to remove after heat treating to ensure all of the scale and decarb is removed. I have not experimented with coatings for annealing but they may also work.

Why AEB-L is a Good Stainless to Start With

The first steel I experimented with was AEB-L. It checks a lot of boxes for forging bladesmiths as it is a very fine carbide steel with properties that can be similar to low alloy and simple carbon steels. Some have called it “stainless 52100” for its fine carbide size and excellent toughness. It is also relatively low cost, as some bladesmiths have sticker shock buying expensive stainless steels when they are used to buying 1084 or 80CrV2 for less than $5 per pound. AEB-L’s relatively low carbide content also means it is somewhat easier to forge than other stainless steels; I have an article on which steels are most difficult to forge here. Having only chromium carbides means it is easier to grind and finish than some of the more exotic stainless and high alloy tool steels with very hard vanadium carbides. One potential downside to AEB-L is it is not typically available thicker than about 1/4″ (6 mm), so bladesmiths that like to forge from heavy stock, round bar, etc. will be out of luck.

My New Experiment with AEB-L

We started with 1/4″ AEB-L which my father, Devin Thomas, hot rolled down to 0.130″ (3.3 mm), which is a bit less than a 50% reduction. The temperature used for rolling was around 2100°F. I then annealed it in different ways and measured the annealed hardness, hardness after quench and temper, and toughness after quench and temper. The different annealing procedures I tried will be given in the sections below. The final austenitize I used was 1925°F (1050°C) for 15 minutes, plate quench, cryo in liquid nitrogen, then double temper at 350°F (175°C).

Traditional Slow Cool Annealing

I have an earlier article that discusses the mechanisms within steel that occur during annealing. The typical method is for heating the steel to a temperature where the steel is austenitic (the high temperature, nonmagnetic phase of steel), but typically lower than austenitizing before quenching. For a simple carbon or low alloy steel this is somewhere in the range of 1350-1450°F (730-790°C) while austenitizing before quenching is typically 1475-1550°F (800-845°C). This lower temperature means more carbide is present, then during slow cooling the soft ferrite forms while feeding carbon to those carbides and growing them, resulting in a “spheroidized” structure (round carbides).

Carbides increasing in size during slow cooling as the ferrite grows into the austenite

There are a couple differences with high alloy and stainless steels. For one, the temperatures for annealing are typically higher, with 1600-1650°F (870-900°C) being most typical. The steels do not transform to austenite until higher temperatures, necessitating the higher temperatures. Typically the hold time is longer at this temperature as well, such as two hours. Following that hold time, the cooling rates are also slower. These are “air hardening” steels so they are more prone to hardening if the cooling rates are not sufficiently slow. This is partially why low cooling rates like 25°F/hr are recommend in datasheets, though that is also true in many datasheets for low alloy steels. So for AEB-L I wanted to try 50°F/hr and 100°F/hr and see how the resulting properties compared with the as-received steel from the manufacturer. The temperature you must cool to for ensuring full transformation depends on the cooling rate and the steel. However, most annealing procedures will recommend a temperature below which no more transformation is likely to occur, such as 1000°F. I used 1100°F for the 50°F/hr anneal and 1000°F for the 100°F/hr anneal. If the steel is fully transformed it doesn’t particularly matter what cooling rate is used below that temperature.

The hardness after annealing was at first a bit surprising because even with the fast 100°F/hr the hardness was significantly lower than what is delivered by Uddeholm (As-received). Perhaps Uddeholm uses an even more rapid form of annealing.

AEB-L As-Received by Uddeholm

50°F/hr anneal

100°F/hr anneal

This hardness difference appears to be confirmed by the metallogaphy, as the carbides in the two anneals I performed have somewhat larger carbides. I next compared the resulting hardness and toughness with prior toughness testing done with the as-received steel.

The same 1925-350 heat treatment with the as-received material (labeled “Stock Removal”) is the 60 Rc point. So the 50°F/hr anneal resulted in slightly lower hardness with equivalent toughness, and the 100°F/hr anneal resulted in slightly better hardness and toughness. This is mildly surprising based on the annealed hardness and somewhat larger carbides in the annealed condition. But overall this is a good result, with the 100°F/hr anneal being relatively fast, having very low annealed hardness for ease in working, and then excellent hardness-toughness after final heat treatment.

Isothermal Hold Annealing

Another similar way to anneal is to do a hold at a lower temperature rather than a slow cool. After heating to 1600°F like with the prior annealing, I held at either 1300°F (700°C) or 1200°F (650°C). Holding at a higher temperature is roughly equivalent to a slow cooling rate, while holding at a lower temperature is equivalent to a faster cooling rate. I held at 1300°F for 4 hours and 1200°F for 6 hours, as I was concerned it would take longer at the lower temperature (based on published transformation curves for stainless steels).

I also decided to try a technique that is common with low alloy steels which is to do “grain refining” cycles prior to the 1300°F anneal. I heated to 1600°F for 30 minutes and air cooled, which I did twice before the same anneal. I did not find grain refining cycles to improve properties in 1084 in a prior experiment, but I thought it wouldn’t hurt to try it again. I labeled this condition “cycled.”

These were somewhat closer in hardness to the as-received condition. However, oddly the hardness of the 1200°F condition  was lower than the 1300°F. Yes I held it longer at that temperature but the transformation was completed in both cases (as will be shown in the metallography), so that should not have been a factor.

Isothermal Anneal 1300°F

1200°F Isothermal Anneal

The 1200°F anneal looks like it resulted in somewhat finer carbides thought they don’t look that different. And both look relatively similar to our slow cool anneals. There are probably subtle differences if we did a full statistical analysis, but the real differences are probably more apparent through the annealed hardness and the final heat treated hardness and toughness.

The properties look pretty similar between the three and I’m not sure there are any “real” differences between the three. They are all around 60.5 Rc and ~35 ft-lbs. Maybe the grain refining cycles or the lower 1200°F led to a slight improvement but those small improvements may disappear if we did the experiment multiple times and averaged the results.

Temper Annealing

Another type of annealing is quite different than the others which is temper annealing, which I have written about before. In fact this is the general type of annealing that I recommended for high alloy steels in Knife Engineering. With normal tempering of martensitic steel, the higher you temper the softer the steel gets. If you temper hot enough you get annealed, soft steel. However, normally we do an austenitizing and quench step from a lower than normal temperature prior to a very hot temper. I recommended this annealing treatment because of older studies that showed a superior grain size and toughness with high speed steels after a temper anneal rather than a more conventional anneal. But the tempering times for optimal properties were very long, 12-24 hours depending on steel and tempering temperature. Also I began to be worried that perhaps the anneal was better only for high speed steels, since they use very high austenitizing temperatures where grain growth is more common. Perhaps the temper anneal resulted in more stable carbides that don’t dissolve as readily at the high temps, therefore they still pinned the grain boundaries. For most steels where grain size is not as difficult to control, this benefit would be lost.

So based on those prior reported experiments I austenitized all of them at 1600°F for two hours prior to plate quenching. This also has the benefit of maintaining the same austenitize as the prior annealing experiments. I then tried three different tempering conditions: 1300°F 4 hours (Q13), 1400°F 4 hours (Q14), and 1400°F 24 hours (Q24). Long anneals were optimal for the high speed steels but I wanted to have shorter anneals which is why I also tried the 4 hour tempers.

The 1300°F anneal (Q13) is definitely harder than we would desire. The 1400°F for 4 hours (Q14) is better, and the 24 hour anneal (Q24) is definitely soft enough. Maybe we could do something in between 4 and 24 hours with 1400°F instead.

Q13 – 1300°F 4 hour temper anneal

Q14 – 1400°F 4 hour temper anneal

Q24 – 1400°F 24 hour temper anneal

Only the Q24 looks properly annealed with medium size spheroidized carbides throughout. The Q13 condition specifically has relatively large regions which still look like martensite, or only very small spheroidized carbides.

Comparing the toughness, Q24 and Q14 are very similar, while the Q13 was significantly worse. The average hardness I measured with the Q24 was around 60 but I got several readings that were lower; overall it seemed to test less consistently. Maybe that means the carbides were too coarse, again pointing to an optimal hold time being somewhere between the 4 and 24 hours I tested with 1400°F. The Q13 ended up being slightly lower in hardness which is just randomness. I did another set of small hardness coupons and it was more similar to the Q14. My hypothesis for why the Q13 had worse toughness is related to the inconsistent microstructure. Maybe those martensite regions led to large grains because there weren’t carbides available to pin the grain boundaries.

AEB-L Annealing Summarized

When looking at all of the conditions as a whole, the overall finding was that if we have properly annealed the steel the properties are relatively similar. We got good (low) annealed hardness with most of them, and the resulting hardness and toughness were also pretty similar. My favorite of the conditions was the 100°F/hr anneal. The overall time to anneal is around 8 hours, meaning you can anneal it overnight after forging the steel. And it had the best combination of hardness and toughness after the final heat treatment of 1925°F austenitize and 350°F temper.

Annealing MagnaCut

I also wanted to look at a steel that may require a slower cooling rate than AEB-L so I also tested MagnaCut. Of course I developed MagnaCut so I was more interested in it. But the reason why MagnaCut was a good steel to look at is because of its 2% Mo. Molybdenum increases “hardenability” of steel so that larger sizes can be air cooled and still fully harden. This also affects annealing. Below I have Time-Temperature-Transformation (TTT) curves for two steels after they were austenitized at a low temperature for annealing:

D2 Annealing TTT after austenitizing at 1600°F

M2 annealing TTT after austenitizing at 1625°F

It can take a minute to understand the charts if you have never looked at them before. The curve to the left shows how long it took for the steel to start transforming, and the curve to the right shows how long it took for the transformation to end. For D2 with 0.74% Mo, at the fastest transformation temperature (~1400°F) it took less than 30 minutes to fully transform. For M2 with 5% Mo (and 6.5% W), it took over 3 hours to fully transform at the same temperature. This is largely cause by the Mo difference. So I was concerned that a 100°F/hr anneal may not work with a steel like MagnaCut with 2% Mo. This should then translate to the many stainless steels with less than 2% Mo (440C, Elmax, M390, S90V, etc.), and to the several stainless steels that also have around 2% Mo (S30V, S35VN, S45VN, S110V). The most common steel with even more Mo is 154CM/CPM-154 with 4% Mo though its datasheet says it can fully transform at 1300°F after 4 hours so there is a decent chance it can also use an intermediate cooling rate.

We did the same experiment of hot rolling MagnaCut from 1/4″ down to 0.130″, I performed a few different annealing cycles from 50°F/hr or 100°F/hr and measured the annealed hardness. However, I also wanted to experiment with the annealing temperature prior to cooling so we looked at that for this steel. We will get to that after we take a small detour to discuss how those annealing temperatures are chosen.

How to Select an Annealing Temperature

Most steels have an available datasheet that lists a recommended annealing procedure. For example, the MagnaCut datasheet recommends the following for annealing: Heat to 1650°F (900°C), hold 2 hours, slow cool no faster than 25°F (15°C) per hour to 1100°F (595°C), then furnace cool or cool in still air to room temperature. So if you follow my recommendation and use the faster cooling rate of 100°F/hr you can simply use that recommended temperature (1650°F/900°C), and then cool at the faster rate.

But what if the steel does not have a datasheet, or the datasheet doesn’t give an annealing temperature? At that point we need to find the temperature at which the steel has transformed to austenite. With a simple carbon or low alloy steel that can be done with a magnet because austenite is non-magnetic. However, there is another point where steel becomes nonmagnetic called the “Curie point,” and high alloy steels reach that point before they transform to austenite. So another way we can try to determine it is to heat the steel to different temperatures and quench and see the point where hardness increases. I did this with both AEB-L and MagnaCut:

You can see that AEB-L shows a rapid increase in hardness between 1400 and 1475°F which is the region over which it transforms from ferrite to austenite. After it transforms to austenite, carbon goes in solution, then you can quench and the hardness is much higher after quenching. However, if you quench after heating to 1400°F, you didn’t transform to austenite and the hardness stayed low. It looks like we could anneal AEB-L from as low as around 1500°F, and the 1600°F I chose for my study should be pretty safe.

With MagnaCut, however, the transition is more difficult to see because carbon doesn’t go into solution until a higher temperature. It looks like the steel may have started transforming to austenite around 1550°F but this actually led to the steel slightly decreasing in hardness. The steel started increasing again around 1650°F and I think this means it is likely where austenite finished transforming and is probably a reasonable temperature to anneal from. But I wanted to see how the annealing temperature would affect the annealed hardness and the final heat treated properties.

There were several surprises in this experiment to me. One is that the temperature mattered much more than the cooling rate, at least for those two cooling rates. The other surprise is that the change in annealed hardness was quite linear. There was no big jump in hardness from annealing at too low of a temperature, even at 1550°F. The “as-received” hardness for annealed MagnaCut is around 22 Rc which is similar to the hardness measured for the 1600°F anneal.

The next experiment I wanted to do was ensure that we were getting good properties from annealing at 1650°F since the annealed hardness continued lower up to 1750°F annealing temperature. And the as-quenched hardness we measured from MagnaCut before didn’t really go up significantly until 1700°F. So I heat treated coupons from the 1650, 1700, and 1750°F anneals with both cooling rates. This also allowed me to compare properties with the two different cooling rates:

The hardness after heat treating was slightly higher with the faster annealing rate, which we would expect to see from having somewhat finer carbides. There wasn’t much difference between 1650 and 1700°F though there was a small drop by increasing to 1750°F. So I would probably recommend sticking with the 1650-1700°F range for annealing MagnaCut.

The toughness was slightly higher for the 50°F/hr anneal in line with its slightly lower hardness. Both the 50°F/hr and 100°F/hr anneals resulted in somewhat higher hardness than the as-received condition. You could austenitize somewhat lower or temper somewhat hotter to reduce the hardness if desired. The forged and annealed MagnaCut had a slightly higher hardness-toughness balance than the as-received material. However, toughness testing can be somewhat variable and I have managed to get ~16 ft-lbs with 62 Rc as-received before. I think the toughness may be slightly better or slightly more consistent with the forging and annealing. Probably not enough to justify forging blades instead of doing stock removal, but I have also heard some bladesmiths claim that you will “ruin” stainless steel by forging it yourself and that is certainly not the case if you do it right. I also don’t think that toughness results of forged low alloy and simple carbon steels are enough to justify forging. Knifemakers should choose forging or stock removal based on other factors.

Summary – General Stainless and High Alloy Tool Steel Recommendations

Don’t forge too hot – 2100°F is a good target temperature. A common mistake is thinking that stainless steels need higher temperatures to forge. They do not.

Don’t forge too cold. Stainless and high alloy tool steels need higher minimum forging temperatures. They become “hot short” (brittle) at higher temperatures than simple carbon steels. Depending on the steel and the datasheet, this minimum is given in the range of 1650-1750°F (900-950°C).

Protect the steel from atmosphere during annealing – Vacuum or inert gas is best but for many knifemakers this means double wrapping in foil (not double folding but double wrapping), and leave some extra steel to grind away.

Choose the right annealing temperature – Typically in the range of 1600-1650°F for most stainless and high alloy steels but can sometimes vary. Check for a datasheet if a temperature recommendation is given and that temperature should work fine. A 2 hour hold at the temperature is typical. If no datasheet temperature is available you may be able to use the method shown above with AEB-L and MagnaCut where I found the temperature at which the hardness goes up, indicating that austenite formed.

Cool at 100°F/hr – This is significantly faster than recommendations given in most datasheets but gives a better balance of speed (~8 hours instead of 24+, can anneal overnight) and final properties (higher final hardness and toughness). There may be some steels that cannot handle this faster cooling rate if they have very high Mo contents. If the steel ends up higher than 25 Rc at the end it may need slower. As a side note, 100°F/hr was also found to be optimal with ApexUltra, a low alloy steel, so maybe we are on to something here.

Cool to 1000-1100°F, after that the cooling rate can be faster. You can experiment with whether the final cooling point can be higher than 1000 or 1100°F by annealing both ways and seeing if the hardness is the same. Once the transformation is done, further cooling doesn’t matter. I probably wouldn’t go any higher than 1200°F in any case. This really only matters if that extra hour or two will significantly impact your workflow.

So to summarize, anneal at 1600-1650°F for two hours, slow cool at 100°F/hr to 1000°F. Before that don’t forge too hot or too cold, and protect it from atmosphere during annealing.

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[1] Roberts, G A, and Robert A. Cary. Tool Steels. Beachwood, Ohio: American Society for Metals, 1980.

[2] Roberts, George Adam, Richard Kennedy, and George Krauss. Tool steels. ASM international, 1998.

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