wiki- Subzero Treatment

• Definition:
  Subzero treatment is a heat treatment process in which quenched steel is cooled below 0°C to transform residual austenite into martensite, thereby enhancing hardness and wear resistance.
• Details:
  After quenching, some austenite may remain in the steel, reducing hardness and dimensional stability. By cooling with dry ice or liquid nitrogen to temperatures as low as –196°C, this residual austenite is converted into martensite. The result is improved hardness, stability, and durability in the finished blade.
• Comparison:
  Compared to conventional quenching and tempering, subzero treatment provides higher hardness and greater wear resistance. However, it also increases processing costs and requires careful handling to avoid cracking due to extreme cooling.
• Practical Use:
  This treatment is often applied to premium kitchen knives made from high-carbon or alloy steels, as well as in precision tools and molds. For knives, it results in longer-lasting sharpness and better performance in demanding kitchen environments.
• Cultural Note:
  While widely used in modern metallurgy worldwide, in Japanese knife-making subzero treatment is regarded as a fusion of traditional craftsmanship with advanced technology, enhancing the global reputation of Japanese blades for both artistry and performance.

Related websites
Sub-Zero processing