Official Answer
Answer
15-7 Mo (Condition CH900) is a semi-austenitic precipitation-hardening stainless steel similar to 17-7PH but with $2\%$ Molybdenum replacing $2\%$ Chromium. This modification provides higher strength and better resistance to localized corrosion. In spiral retaining rings, 15-7 Mo offers a higher Modulus of Resilience ($U_r = \frac{\sigma_y^2}{2E}$), allowing the ring to be expanded more during installation without taking a permanent set. This is particularly useful for rings that must pass over a long shaft or a larger-diameter section before reaching their groove. The material is typically aged at $900^{\circ}F$ to reach its peak hardness, resulting in a tensile strength of approximately $225$-$240$ ksi.
