Answer
Inconel X-750 is selected for high-temperature applications due to its resistance to relaxation and creep. At $700^\circ F$, the Modulus of Elasticity $E$ drops from approximately $31 \times 10^6$ PSI to $29 \times 10^6$ PSI. The fatigue life is determined by the stress range $\Delta \sigma = \sigma_{max} - \sigma_{min}$. Using the Goodman criterion, we must account for the reduced yield strength at temperature. For X-750 in the No. 1 Temper, precipitation hardening (aging at $1350^\circ F$ for 20 hours) is critical to developing the $Ni_3(Al, Ti)$ gamma-prime phase. Failure to properly heat-treat results in rapid stress relaxation at high temperatures, causing the spring to lose its 'memory' and fail to return to its free height, quantified as a loss in load $P$ over time.
