Official Answer
Answer
17-7PH (AMS 5678) is a precipitation-hardening stainless steel that offers superior yield strength and fatigue resistance compared to 300-series alloys. In the CH900 condition (cold reduced and aged at $900^{\circ}F$), the material achieves a high tensile strength exceeding 200 ksi. This allows for higher operating stresses and lower relaxation at elevated temperatures up to $650^{\circ}F$. Unlike 302 stainless, 17-7PH maintains its elastic modulus $E \approx 29.5 \times 10^6$ psi more consistently under thermal cycling, making it ideal for critical aerospace actuators and valve seals.
