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Sub-Zero Cryogenic Treatment of Knife Steels

In my collection of heat treatment specifications is a fair amount of research publications on the topic of cryogenic treatment of knife steels. Not that I’m a metallurgist or scientist by any means, but I do take pride in my work and I certainly aim to create the best knives that I 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 value-added step to the overall process.

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.

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 it?

  • Reduce the amount of Retained Austenite (RA)
  • Destabilization of austenite to untampered martensite
  • Higher Hardness (+1 or 2 HRC / Wear resistance

What is Cryo / Cold Treatments

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

  • Liquid nitrogen (LIN) treatments are often called “Cryogenic” or Deep Cryogenic treatments, where cryogenic refers to any temperature below (-310°F/-195°C).
  • “Subzero” 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).

The colder you go the more retained austenite will be transformed to martensite. Even with household freezer temperatures about half of the unstable retained austenite was transformed, meaning even a freezer can transform some the retained austenite when heat treating tool steels.

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 Mf temperature is below room temperature there will be some amount of austenite which isn’t transformed to martensite leading to higher amounts on retained-austenite, lower hardness, and other negative effects. Using cold treatments means you get closer to the actual Mf and “complete” the transformation.

When to use a cold treatment in your process?

  • Before Tempering: To maximize hardness, the cold treatment should occur before the tempering cycle so that any retained austenite is not able to stabilize. If using this window for your cold treatment, make sure the blade is at room temperature all the way through. The risk of cracking is more prevalent at this point, especially in clad / san-mai steels due to uneven temperature distributions and size changes.
  • Snap Temper: If cracking is an issue or a concern, then a snap temper should be used, where the steel is tempered 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. Any cold treatments should always be followed by a tempering cycle.

My Personal Tips:

If using a cold treatment before tempering, I recommend using water to make sure the steel is fully at room temperature. 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 the blade with your bare hands. I have 1inch thick aluminum plates that do a great job at sucking the heat out of blades.

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 steels 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.

Dewars of liquid nitrogen are available for hire from Afrox for a very reasonable amount. We planned a weekend of heat treating where we hardened 26 blades and were only charged the rental fee as we used so little of the liquid nitrogen.

NB! 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.