Answer
At operating temperatures of $350^∘C$, 17-7PH stainless steel begins to suffer from over-aging and a significant reduction in yield strength, leading to stress relaxation (creep). Inconel X-750 (AMS 5699) is the superior choice for subsea O&G due to its exceptional resistance to chloride-induced stress corrosion cracking (SCC) and high-temperature stability. X-750 undergoes precipitation hardening through the formation of $\gamma'$ phase ($Ni_3(Al, Ti)$), which maintains its mechanical properties up to $700^∘C$. The spring must be heat treated to the No.1 temper (HT) to optimize for relaxation resistance. Furthermore, X-750's lower modulus ($E \approx 213$ GPa) compared to 17-7PH ($E \approx 200$ GPa at temp) requires a slight adjustment in the number of turns to maintain identical load characteristics.
