Factory vs Custom Heat Treating of Knives

Patreon

Thank you Patreon supporters! Thanks to you we can do all the experiments that are in this article. If you want to support knife steel research come join us at Patreon.com/KnifeSteelNerds.

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.

---

[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.