Answer
17-7PH (Condition CH900) is preferred in aerospace due to its exceptional strength-to-weight ratio and superior corrosion resistance. Unlike SAE 1070 carbon steel, which requires an oil quench and temper (RC 40-52), 17-7PH is work-hardened (Condition C) and then precipitation-hardened at $900^{\circ}F$ ($482^{\circ}C$) for one hour. This process results in a high tensile strength (up to 240 ksi) and excellent fatigue resistance. For applications involving temperatures up to $650^{\circ}F$, 17-7PH maintains its elastic modulus $E$ much better than carbon steel, which begins to lose structural integrity and suffer from creep above $250^{\circ}F$. Furthermore, 17-7PH is resistant to hydrogen embrittlement, a common failure mode for plated carbon steel springs.
