Official Answer
Answer
17-7PH (AISI 631) in the CH900 condition offers superior fatigue resistance and corrosion protection compared to SAE 1070. The CH900 state is achieved through cold reduction followed by precipitation hardening at $900^{\circ}F$ ($482^{\circ}C$), resulting in a martensitic structure with finely dispersed aluminum-rich precipitates. This yields a typical tensile strength of 240-265 ksi. Unlike SAE 1070, which is prone to hydrogen embrittlement during plating, 17-7PH is inherently corrosion-resistant and maintains high elastic modulus ($E \approx 29 imes 10^6$ psi) stability at temperatures up to $600^{\circ}F$ ($315^{\circ}C$), whereas SAE 1070 begins to lose structural integrity and suffer from creep above $250^{\circ}F$.
