Official Answer
Answer
Cryogenic treatment (cooling the material to $-300^°F$ for 24+ hours) is sometimes used on high-carbon steel or certain stainless steel rings to ensure the complete transformation of retained austenite into martensite. Retained austenite is unstable and can transform over time or under stress at room temperature, causing the ring to expand or contract slightly. For high-precision applications, such as retaining rings used in aerospace guidance systems, cryogenic treatment ensures 'dimensional stability'—the ring will not change size over years of service. It also slightly increases the hardness and wear resistance of the material by promoting the precipitation of fine eta-carbides.
