Answer:
Cryogenic hardening is often referred to cryogenic treatment and cryogenic processing. Using the term "hardening" is not correct. Cryogenic treatment does not increase the hardness of metals (I.E. rockwell hardness).
Rather it changes metals on the molecular level so that they perform better. Cryogenic treatment is used for a variety of reasons including improving dimensional stability, stress relieving, increasing wear resistance, and eliminating walk and creep.
A typical cryogenic treatment lasts for several days. Metal and steel parts will be put in a cryogenic processor at room temperature. The temperature will slowly be taken down to -300F below. Then it will hold there for 12 to 24 hours depending on the company doing the treatment. After that, the processor will have the temperature raised back to room temperature very slowly. Finally, the parts will be post-tempered to finalize the cryogenic treatment.
Cryogenic treatment re-aligns the micro-structure of the metal to improve its structural stability. Retained austenite will be converted to martensite which allows for a tougher piece of metal. The eta-carbides in the metal will become precipitated which allows for a much higher degree of wear resistance.
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