Official Answer
Answer
17-7PH (Type 631) is the industry standard because it combines high yield strength, excellent fatigue properties, and good corrosion resistance with minimal distortion during heat treatment. The manufacturing process involves coiling the material in 'Condition C' (cold reduced) and then performing a precipitation hardening heat treatment at $900^\circ F$ (Condition CH900). This aging process increases the hardness to HRC 41-48. Unlike 300-series stainless steels, which are too soft, or 400-series, which are brittle and prone to corrosion, 17-7PH provides the 'spring back' needed for the ring to snap into the groove and stay there under high centrifugal or axial loads.
