Answer
17-7PH (Condition CH900) is a precipitation-hardening stainless steel that offers excellent strength and fatigue properties up to $650^{\circ}F$ ($343^{\circ}C$). It is cold-reduced to Condition C and then aged at $900^{\circ}F$ for one hour. Beyond this temperature, its mechanical properties degrade due to over-aging. In contrast, Inconel X-750 (per AMS 5699) is a nickel-chromium alloy that maintains its spring properties and creep resistance up to $1300^{\circ}F$ ($704^{\circ}C$). For aerospace exhaust valves, Inconel X-750 is required because the relaxation rate of 17-7PH at those temperatures would lead to a loss of preload, resulting in seal failure. The trade-off is cost and a lower Modulus of Elasticity ($E \approx 213$ GPa for 17-7PH vs $211$ GPa for X-750, further decreasing at temp).
