Answer
17-7PH stainless steel in the CH900 condition offers excellent high-strength properties up to $650^°F$ ($343^°C$). It is precipitation-hardened, achieving a high tensile strength through a combination of cold reduction and aging. However, for temperatures exceeding $700^°F$, Inconel X-750 is required due to its superior resistance to relaxation (creep). Inconel X-750 is a nickel-chromium alloy made precipitation-hardenable by additions of Al and Ti. While 17-7PH may show a load loss of over 10% after 100 hours at $750^°F$, Inconel X-750 typically maintains 95% of its initial load. The processing for X-750 involves a solution anneal followed by double aging (No. 1 Temper), which optimizes the gamma-prime phase for maximum creep-rupture life.
