This procedure of precisely controlled temperature profiles avoids any possibility of thermal shock and thermal stress that is experienced when a tool or part is subjected to abrupt or extreme temperature changes. In this process liquid nitrogen is used as a refrigerant. In most instances the cryogenic cycle is followed by a heat tempering procedure. Two main changes in the microstructure of the steel occur as a result of Cryogenic Treatment. These changes are the principal reasons for the dramatic improvement in wear resistance.
Retained austenite is a softer grain structure always present after heat treatment.
By applying cryogenic treatment, retained austenite is transformed into the harder, more durable grain structure - martensite. The amount depends on the heat treating operator and the accuracy of the heat treating equipment. As greater amounts of retained austenite are transformed, and wear resistant martensite is increased, the material obtains a more uniform hardness. These are in addition to the larger carbide particles present before cryogenic treatment. Machining treated material is easier and cleaner.