In my collection of heat treatment specifications and research papers is a fair amount of info on the topic of cryogenic treatment of knife steels. Not that I’m a metallurgist or scientist by any means, but I do take a certain amount of pride in my work and I aim to create the best knives I possibly can.
As the blade is at the heart of a high-performing knife, I wanted to find the best possible heat treatment for my knives and “Cryro” seemed to be a worthwhile add-on step to achieve just that.
With the thin edges on chef knives, good edge retention and high hardness is needed to handle the thinner edge geometry and prevent edge rolling. Read my post on “Is carbon steel still better than stainless steel“?
During all my reading, I found a lot of conflicting opinions within the community, so I wanted to do some further fact-checking and try to dispel some of the myths surrounding various cold treatments of steel. This post is more about taking my learnings, and putting them down on “paper” so that I can formalise them and act as a reference for other knifemakers. Read my post on heat-treating K110 / D2 tool steel.
What is a Cryogenic Treatment?
Cryogenic treatment, also referred to as cryogenic processing, deep cryogenic processing, deep cryogenic treatment (DCT), cryogenic tempering, and deep cryogenic tempering, sub-zero, deep freezing or cold treatment of steel (ferrous material) is nothing more than taking the steel and freezing it.
This step is always done after austenitizing and quenching. Generally, one hour of cold treatment for each 2.54 cm (1 in.) of cross section is adequate to achieve the desired results.
NB! Please do not perform a Cryogenic Quench by plunging a hot (austenising temperature) blade into a super chilled quench medium (Liquid nitrogen). The word “quench” as it relates to cryogenic treatments is misleading and should not be used.
- Liquid nitrogen (LIN) treatments are often called “Cryogenic” or Deep Cryogenic treatments DCT, where cryogenic refers to any temperature below (-310°F/-195°C).
- “Subzero” or Shallow usually refers to a mix of dry ice and alcohol / acetone (-110°F/-78°C).
- “Cold treatment” is a general term for colder than room temperature (ie: A household freezer) (0°F/-18°C).
While all methods aim to accomplish the same goal, each will attain differing degrees of transformation. The colder you go the more retained austenite will be transformed to martensite up to a point (+-6% RA). Even with household freezer temperatures, about half of the unstable retained austenite was transformed, meaning you don’t necessarily need additional or expensive equipment when heat-treating steels at home.
Why would you use a cold treatment?
Cryogenic treatment has existed since the late 1930s, making it a relatively new and emerging process. A cytogenetic treatment after quenching is most effective at eliminating retrained austenite for a more homogeneous martensitic structure.
- Reduce the amount of Retained Austenite (RA),
- Destabilization of austenite to untampered martensite,
- Higher Rockwell Hardness (+1 or 2 HRC),
- Increased wear resistance, dimensional stability, fatigue resistance (increase tool life),
- Less prone to developing grinding cracks,
- Improved machineability.
It’s worth noting that some say that toughness can be increased through cryogenic treatments. For knife blades typically a reduction in toughness can be seen due to the shift in hardness and reduction of the more ductile RA.
Martensite Start and Finish Temperatures
The martensite start (Ms) temperature is controlled by the amount of carbon and alloy that is in the austenite prior to quenching. Higher hardening/austenitizing temperatures will put more carbon and alloys “in solution” and thus reduce/lower the Ms and martensite finish (Mf) temperatures.
When the martensite finish (Mf) temperature is below room temperature (typically in chromium/alloy rich steels) there will be some amount of austenite that isn’t transformed to martensite leading to higher amounts of retained austenite, lower hardness, and other negative attributes. Using cold treatments to facilitate continued cooling beyond the normal quench means you get closer to the actual Mf state and “complete” the martensite transformation.
Retained austenite can stabilise if the steel sits at room temperature for long periods of time [8] leading to cold treatments becoming less effective. Tests have shown that leaving the steel for more than 1 hour and up to a maximum of 20 hours impacts the success of transforming RA into martensite. If you are using your home deep freeze (rather than LN) the effect is even greater. It’s recommended that you transfer the steel into your cold treatment with a few minutes and definitely before 1 (one) hour elapses.
When to use a cold treatment in your process?
Cold treatments should not be treated as a separate step to your heat-treatment regime, but rather a continuation of the quench as you try to continue the cooling until the steel reaches or comes close to its Mf temperature.
- After the Quench: To maximize the benefits, the cold treatment should occur directly after the quench and before the tempering cycle. This allows for continuous cooling down to the Mf temp and ensures any retained austenite is not able to stabilize. Make sure the whole blade is at room temperature.
- Snap Temper: Generally not recommended, but if cracking is an issue or a concern especially in the case of clad / san-mai laminated blades (due to uneven temperature distributions and size changes), then a snap temper should be used. Another reason is if time does not allow for further processes that day. Temper directly after quenching, at 90-100°C (190-215°F) for 30-60 minutes and before any cold treatments.
- In between Tempers: Sometimes performing a cryo step between the first, second and even third temper is recommended. The destabilized austenite must be allowed to transform to martensite after cooling and the newly formed, brittle, un-tempered martensite must be tempered. NB! Any cold treatments should always be followed by a tempering cycle.
NB! Retained austenite stabilizes while the steel sits at room temperature, and the longer you wait before cooling the steel further, the less retained austenite will transform during the cold treatment. [8]
DIY Options for the Knifemaker:
- Buy a Dewar: Dewars are more expensive than you would initially think. Occasionally they come up secondhand from farmers or laboratories, so keep an eye out. If you own your own dewar, you can keep a usable amount for up to 6 months if stored correctly.
- Hire a Dewar: Afrox or Air Liquid will hire out a full dewar for a while. There may be an additional charge for usage if a lot of the liquid nitrogen is used up. If you’re inserting your blades and keeping the lid on, very little is wasted.
- Dry Ice Slurry: Some makers resort to using dry ice and acetone/car antifreeze/Kerosene or paraffin/denatured alcohol mixtures for their cold treatments (aka the poor man’s liquid nitrogen). Acetone is a popular choice but comes with a fire/explosive warning and will also eat plastics/polystyrene/styrofoam, so maybe use the other cheaper options. The mixture can reach sufficiently low temperatures (-110°F/-78°C) to convert the retained martensite.
- Household Chest Freezer: While not as effective as the above options, it is one that most people already have access to. This will work best with the lower range of austenitizing temperatures as less RA is created, meaning less needs to be converted.
My tips for the knifemaker:
I recommend using water (post quench) to get the steel to room temperature (not just the surface, but the core). Now water can be a dangerous medium to put a blade into due to cracking… So make sure it’s already at a temperature that you can safely handle with your bare hands before cooling it further. I have 1-inch thick aluminium plates that do a great job at sucking the heat out of blades. Read my post on my water cooling plate quench build.
Once at room temperature, I clamp the blade (or multiple blades) between 10mm thick aluminium plates to ensure the blades stay straight all the way through final tempering.
As most knifemakers don’t have access to equipment to control the cooling rate, a hybrid approach uses mechanical refrigeration to do an initial cooling of the steel to some sub-atmospheric temperature that is well above the desired cryogenic range. At that point, the steel can be transferred over to your main/ colder medium.
The blade doesn’t have to be submerged in liquid nitrogen. Hanging the blade above the liquid is sufficient to cool the steel to the required temperatures (if using a well-insulated container like a dewar). If the blade is half in the LN, the steep temperature gradient between the two areas, can induce thermal shock and risk cracking the blade.
In Conclusion:
If your end goal is to eliminate excessive amounts of retained austenite and to obtain the steel’s maximize hardness, introducing a cold treatment into your heat-treating regime is a valid and worthwhile step for the knifemaker.
It can work for both carbon (simple) and high-alloy (stainless) steels with equipment that is readily available (home freezer) or dry ice slurry, without having to resort to liquid nitrogen.
Disclaimer! Safety is a key concern whenever working with high heat and extreme cold. This article serves as additional information for knifemakers but you should always wear adequate personal protection equipment (PPE) and follow local safety codes and standards.
References:
- Cryogenic Processing of Steel Part 1 – Maximizing Hardness
- Sub-zero Treatment of Steels – Technology/Processes/Equipment by Linde Gas
- Deep cryogenic treatment of cold work tool steel by M. Pellizzari and A. Molinari
- Cryogenic Treatment of Tool Steel by Tushar Mamodia & Rahul Maghanti
- Cryogenic treatment and it’s effect on tool steel by T. Yugandhar, P.K. Krishnan
- Comparison of Effects of Cryogenic Treatment on Different Types of Steels : A Review by P. I. Patil and R. G. Tated
- Sub-zero treatments of AISI D2 steel: Part I. Microstructure and hardness
- Dry Ice vs Liquid Nitrogen vs Freezer – How Cold Do You Need for Cryo?
- Cold and Cryogenic Treatment of Steel
Post Header Photo by Osman Rana.
