Official Answer
Answer
17-7PH (Type 631) is a precipitation-hardening stainless steel that offers a superior combination of high strength and corrosion resistance. In the CH900 condition (cold reduced and aged at $900^{\circ}F$), it achieves a tensile strength exceeding $200$ ksi. Crucially, it maintains its elastic modulus $E$ and resists stress relaxation at temperatures up to $650^{\circ}F$ ($343^{\circ}C$), whereas SAE 1070 carbon steel begins to lose its load-bearing capacity rapidly above $250^{\circ}F$ due to microstructural changes. Furthermore, 17-7PH is resistant to hydrogen embrittlement, a common failure mode for plated carbon steel springs.
