Answer
Inconel X-750 (UNS N07750) is the industry standard for spiral retaining rings in subsea environments where hydrogen-induced stress cracking (HISC) and chloride stress corrosion cracking are prevalent. This nickel-chromium alloy is precipitation-hardened and maintains its mechanical properties in cryogenic temperatures up to $1300^{\circ}F$. For subsea use, it is typically processed to meet NACE MR0175 standards. The modulus of elasticity for Inconel X-750 is $31 \times 10^6$ psi, which is higher than standard 302 stainless steel, allowing for higher retaining force. The heat treatment involves a solution anneal followed by age hardening to achieve a hardness of 32-42 HRC, providing a balance of high yield strength and enough ductility to withstand the installation stresses of being expanded over a shaft.
