The subsequent quenching process of placing the workpiece in a refrigeration container and cooling it to below zero (usually -70 to -196°C) is also called cold treatment. The martensitic transformation in steel materials is generally a variable temperature type, that is, the martensite transformation starts at the temperature M.
As the temperature decreases, the amount of martensite increases and stops at the end temperature M of martensite transformation, regardless of the length of stay at a certain temperature. The lower the sunburst point of the steel, the more austenite will remain in the workpiece after quenching and cooling to room temperature.
Retained austenite is a non-equilibrium phase with low strength (low hardness) and thermodynamic instability. In order to improve the hardness and stabilize the structure of quenched steel, it is cooled to room temperature after quenching and then continued to cool to below zero.
Therefore, the essence of cryogenic treatment is the continuation of quenching and cooling. Tools, molds, precision ceramic bearings, measuring tools and high-precision carburized and quenched parts made of high-carbon and high-alloy tool steel can be cryogenically treated to improve their service life and dimensional stability. The lower the temperature, the better the effect.
Since these parts generally have considerable residual stress and microcracks in the quenched state, in order to prevent cracking, cryogenic treatment is generally not arranged after quenching, but after appropriate tempering.