Official Answer
Answer
At $400^{\circ}F$, SAE 1070 carbon steel begins to lose its temper and will experience significant relaxation (loss of 'clinging' force) over time. This can cause the ring to become loose in the groove, leading to vibration and wear. In contrast, 17-7PH (Condition CH900) is stable up to $650^{\circ}F$. The precipitation-hardened microstructure of 17-7PH prevents the dislocation movement associated with thermal creep at these temperatures. Therefore, for any engine or exhaust-adjacent application where temperatures exceed $350^{\circ}F$, 17-7PH is the mandatory choice to ensure the ring maintains its mechanical integrity and stays seated.
