Official Answer
Answer
Cryogenic treatment involves cooling the 17-7PH stainless steel to approximately $-73^{\circ}C$ (Condition RH950) or even lower after the initial heat treatment. This process ensures the complete transformation of retained austenite into martensite. Retained austenite is a softer, unstable phase that can transform into martensite over time or under stress, causing dimensional growth and changes in the elastic modulus. For aerospace valves where precise seating is required, cryogenic treatment provides maximum dimensional stability and a slight increase in hardness and yield strength. This ensures that the ring's 'cling' on the shaft and its thrust capacity remain constant over the service life of the component.
