Answer
17-7PH (Type 631) is a semi-austenitic precipitation-hardening stainless steel used for wave springs requiring high strength and corrosion resistance. In the 'CH900' condition, the material is cold-reduced to Condition C and then age-hardened at $900^\circ F$ ($482^\circ C$). This process precipitates fine intermetallic compounds of aluminum ($Ni_3Al$) within the martensitic matrix, significantly increasing the yield strength to approximately $190,000$ to $240,000$ psi. This high yield-to-tensile ratio allows the spring to undergo significant deflection without permanent set. In aerospace applications, this metallurgy is preferred over standard 302 stainless because it maintains its mechanical properties at temperatures up to $650^\circ F$ ($343^\circ C$), providing excellent relaxation resistance compared to carbon steel.
