Answer
17-7PH (ASTM A564) is a semi-austenitic precipitation-hardening stainless steel that offers superior mechanical properties through a combination of cold reduction (Condition C) and subsequent age hardening (CH900). Unlike 302 or 304 stainless, which rely solely on cold working for strength and lose their temper at temperatures above 250 C, 17-7PH CH900 maintains its elastic modulus and yield strength up to 343 C (650 F). The CH900 heat treatment involves heating to 482 C (900 F) for one hour, which precipitates aluminum-rich intermetallic compounds within the martensitic matrix. This results in a high fatigue limit and resistance to relaxation, making it ideal for wave springs in aerospace engine actuators where thermal stability is paramount.
