Category: Cryo
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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.
Damasteel Heat Treatment and Properties
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What is the Best Hardness for MagnaCut Knives?
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Chris Hancock, Chad Morris, Edward Voss, Liam Gogley, Erik Turner, Nathan Raptis, Nick Dunham, Joe Busic, Will Red Five Forge, Dmitry Kiyatkin, Edward Edmonds, Asinelli, Mattew Reagan, Nate, Dan Barrett, Malachi Chou-Green, Nats Spawnee, McKenzie Kelsay, Jantz Supply, Adam Webb, Brian Baley, Blade HQ, and Mark Demshock.
Dry Ice vs Liquid Nitrogen vs Freezer – How Cold Do You Need for Cryo?
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Video Version
Here is the video version of the following content:
Testing the Edge Retention of 48 Knife Steels
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Heat Treating Vanax – How Hard Does it Get?
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Vanax
Vanax is an interesting steel because of its good combination of toughness, wear resistance, and corrosion resistance. I wrote about Vanax along with other nitrogen-alloyed steels in this article, to describe how the steel is designed. While the datasheet shows it being capable of 59-61 Rc, I was curious about how hard the steel can go. The steel may be good for kitchen knives, for example, where very thin edges and high hardness is common. And kitchen knife users may appreciate the excellent corrosion resistance of Vanax in the presence of water, salt, and acidic foods.
Cryogenic Processing of Steel Part 3 – Wear Resistance and Edge Retention
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Intro to Cryo and Wear Resistance
In Cryogenic Processing Part 1 I covered the effects of cryo on retained austenite and hardness. In Cryogenic Processing Part 2 I looked at the studies on cryo and toughness. Wear resistance is the most controversial aspect of cryogenic processing of steel. In particular there are claims that the use of cryogenic processing (liquid nitrogen) leads to an improvement in wear resistance that is not found with subzero processing (dry ice). Sometimes it is claimed that cryo can lead to massive increases in wear resistance [1]:
Cryogenic Processing of Steel Part 2 – Toughness and Strength
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Introduction
Part 1 of the Cryogenic Processing series covered the transformation of retained austenite to martensite and the increase in hardness that occurs. That is the least controversial aspect of cryogenic processing of steel. The other two primary properties of steel affected by cryo processing are toughness and wear resistance. Both of these properties can be difficult to pin down as they have high variability. Tool steels are known for their relatively poor toughness which means we are often comparing small numbers.
Detour – Tempering
One important interrelation to keep in mind with subzero and cryo studies is the transformation of retained austenite in tempering. With sufficiently high tempering temperatures all/most of the retained austenite is transformed without any cold treatment. This depends on the alloy content, as low-alloy 52100 will have lost its retained austenite with a 500-600°F temper while high alloy steels need over 900°F. You can read more in the article on tempering. With high alloy steels the loss of retained austenite also coincides with “secondary hardening” which is a high temperature tempering treatment that increases hardness [1]:
Above is a tempering chart for Caldie steel (0.7C-5.0Cr-2.3Mo) which shows both hardness vs hardening temperature and also retained austenite. You can see that at low tempering temperatures (<400°C) the retained austenite is basically constant. You can also see that the hardness decreases with higher tempering temperatures up to about 350°C and then it increases to a peak at around 520°C (950°F). Therefore tempering in the secondary hardening region above 400°C can lead to both high hardness and also the elimination of retained austenite.
Subzero or cryo processing prior to tempering also shifts the tempering-hardness curve to lower temperatures when using the secondary hardening range of tempering [2]:
This means that in general, a lower tempering temperature is required to achieve the same hardness level with secondary hardening. Using the same tempering temperature as without a subzero treatment will lead to a greater degree of tempering. More tempering can be good or bad depending on the situation. Excessive tempering can lead to coarsening of tempering carbides which can reduce toughness. However, if the tempering was insufficient without subzero, the use of subzero processing may increase toughness due to shifting the “optimal toughness” range.
Toughness
In an earlier article where we tested the effects of heat treatment on Z-Wear toughness
Cryogenic Processing of Steel Part 1 – Maximizing Hardness
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Heat Treating and Austenitizing
During heat treatment of steel, the steel is heated to a high temperature called the “austenitizing” temperature where a phase called austenite is formed. Steel has different phases which refer to different arrangements of iron atoms within the steel. Austenite has a different set of properties from the typical room temperature phase of steel. One example of the different properties of austenite is that it is non-magnetic unlike the room temperature ferrite or martensite.
Room temperature iron/steel – Ferrite – Body Centered Cubic Atom Arrangement
High temperature iron/steel – Austenite – Face Centered Cubic Atom Arrangement
After holding the steel at the high austenitizing temperature, the steel is then rapidly quenched which transforms the steel to a phase called martensite which has high hardness. It gains its high hardness because carbon is trapped in between the atoms which makes the room temperature phase martensite as opposed to the soft ferrite.
Normal soft room temperature ferrite on the left and hard martensite on the right
thank you so much for your knife steel research and articles. I started making knives in early 70s and love experimenting with new steels myself. always dreamed that someone would do true scientific research on this subject. I plan on becoming a patreon but am having trouble signing up.