Sub-Zero Cryogenic Treatment of Knife Steels

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 certainly 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, 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.

Why would you use a cold treatment?

  • 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 etc etc

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. If you plan on using high-temperature tempering secondary hardening, then there seems to be no reduction in toughness. [1]

What is a Cryogenic, SubZero, Deep Freezing or Cold Treatment?

The Cryogenic, sub-zero, deep freezing, or cold treatment of steel is nothing more than taking the steel and freezing it. This step is always done after austenitizing and quenching the blade.

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).
  • And “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.

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 temperatures will put more carbon and alloy “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.

When to use a cold treatment in your process?

  • After the Quench: To maximize the benefits, the cold treatment should occur directly after the quench and before the tempering cycle. This allows for continuos cooling down to the Mf temp and ensure 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. Temper directly after quenching, at 90-100°C (190-215°F) for 30-60 minutes and prior to any cold treatments.
  • Inbetween Tempers: Sometimes performing a cryo step between the first and 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.

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 laboritories, 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 you a full dewar for a period of time. 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 actually 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 mans liquid nitrogen). Acetone comes with a fire/explosive warning and will also eat 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.

My Personal Tips:

I recommend using water to make sure the steel is fully at 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 before doing so, make sure it’s at a temperature that you can safely handle with your bare hands. I have 1inch thick aluminum plates that do a great job at sucking the heat out of blades.

The blade doesn’t actually have to be submerged in liquid nitrogen to be effective. 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 using a dry ice slurry, then dunk that sucker.

Once at room temperature, I clamp the blade (or multiple blades) between 10mm thick aluminum plates to make sure the blades stay straight from this point, all the way through final tempering.

Allow the blades to rest for at least 1 hour between tempering cycles. This allows the steel to undergo the transformations that it needs to and relieve stress.

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:

Post Header Photo by Osman Rana.