Dr. Dmitri Kopeliovich
Cryogenic treatment (tempering) is a processing of the material at a temperature below 80K (-315°F / -193°C) resulting in modification of its microstructure and improvement of its properties.
Cryogenic tempering of a steel is carried out as a supplemental process following after the conventional heat treatment procedure (Hardening).
Hardening treatment comprises heating the steel above the phase transformation temperature (upper critical temperature), followed by soaking and then rapid cooling (quenching).
When steel is heated above the upper critical temperature, its structure becomes entirely austenitic, which transforms into martensite (supersaturated solid solution of carbon in α-iron) after quenching.
Austenite-martensite transformation is never complete - a certain percentage of austenite is retained in the resulting microstructure. Austenite has face centered cubic (FCC) structure, which is denser than the body centred tetragonal (BCT) structure of martensite. The densities difference causes internal stresses in heat treated steels.
Austenite is softer than martensite therefore high percentage of retained martensite decreases the steel hardness and wear resistance.
Cryogenic treatment results in the following effects of the steel microstructure: