Cryogenic treatment is used after hardening to transform retained austenite to martensite and to improve dimensional stability in steels that retain unstable austenite at room temperature. It is not a substitute for hardening; it is an extension of the hardening sequence.
This is especially relevant in tool steels, bearing steels, and high-carbon/high-alloy grades where some retained austenite may remain after quenching.
Reducing retained austenite can improve size stability, support higher effective hardness after tempering, and reduce later movement in service or grinding.
At Braddock, cryogenic treatment would be used when the engineering concern is the control of retained austenite and dimensional stabilization, rather than simply obtaining extra hardness.
Cryogenic treatment is commonly used on hardened steels where retained austenite and dimensional movement are concerns.
After quenching to room temperature, parts are cooled to subzero or cryogenic temperature in a controlled manner. Many commercial treatments range from about -120°F to below -300°F depending on the process and equipment. Cooling too rapidly can induce thermal stress, so ramp control is important.
The cryogenic step converts part of the retained austenite to fresh martensite. Because that fresh martensite is brittle in the as-formed condition, a subsequent temper is normally required.
In some alloys, fine carbide precipitation effects are also discussed, but retained austenite reduction is the primary metallurgical mechanism.
Braddock would control cool-down rate, hold temperature, hold time, and post-cryogenic tempering to stabilize the structure without introducing thermal cracking.
