Answer
17-7 PH (AISI 631) is a semi-austenitic precipitation-hardening stainless steel widely chosen for its high fatigue strength and corrosion resistance.
Processing and Metallurgy Sequence:
1. Cold Winding (Condition C): Flat wire is rolled and coiled cold from the annealed condition. Cold reduction achieves severe deformation, transforming the austenite matrix into high-strength cold-worked martensite.
2. Heat Treatment / Aging (Condition CH900): After forming the wave spring, the parts are subjected to thermal aging at $900^\circ F$ ($482^\circ C$) for 1 hour, followed by air cooling.
3. Precipitation Hardening: At this aging temperature, fine sub-microscopic intermetallic compounds of Aluminum ($Ni_3Al$) precipitate within the martensitic matrix. This restricts dislocation movement and raises the yield strength up to $1700\text{ MPa}$ and tensile strength up to $1900\text{ MPa}$.
4. Dimensional Stability: The CH900 treatment stress-relieves the cold-coiled spring while optimizing fatigue limits and mitigating dimensional drift in high-load operating cycles.
