Cryo hardening is the name of a method in which a finished metal item is first tempered with the usual process before being undercooled and warmed up again. The result is a blade that is very strong and supple, while also offering a high level of corrosion resistance.
Metal products such as knife blades are cryo hardened after the end of the actual production process. Following the usual tempering, the blade is flat-cooled and then warmed up again. This process, also called cryogenic hardening and annealing, is repeated several times.
A blade is cryo hardened in four steps. First, it is heated up to a temperature of approximately 1000°C and then cooled down until it reaches room temperature. After that, the blade is cooled down to approximately -70 to -80°C before being heated to about 200 to 300°C. The last two steps, the actual cryogenic hardening and annealing, are usually repeated several times. The number of repetitions and the exact temperatures might vary between manufacturers and depend on the intended use of the knife.
The steps of the overall cryo hardening process have the following effects: Heating the blade to about 1000°C improves the molecular structure of the steel, which makes it harder and gives the blade better edge retention. By cooling the steel down to regular room temperature, the material is relaxed, which gives the blade a certain elasticity that prevents breaks. Cooling the blade down to -70 to -80°C improves its corrosion resistance, while the last heating step adds more elasticity. This process produces hard yet supple blades with a high level of corrosion resistance.