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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.

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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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