Introduction to Knife Steel Heat Treating from a Metallurgist

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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.