Official Answer
Answer
17-7PH CH900 (Condition C, then precipitation hardened at 900F) is an excellent general-purpose material with high tensile strength ($≈$ 1380 MPa), but its maximum operating temperature is limited to approximately 340°C (650°F). Above this, it suffers from rapid stress relaxation. For subsea valve actuators at 300°C, 17-7PH is near its limit. Inconel X-750 (Ni-Cr alloy) is the superior choice for high-temperature stability up to 700°C. It maintains its elastic modulus $E$ and resists creep far better. While X-750 is more expensive and harder to form, its resistance to chloride-induced stress corrosion cracking (SCC) makes it the standard for high-reliability subsea oil and gas environments.
