Answer
Stress relaxation is the decrease in stress (and thus load) in a spring held at a constant deflection over time. At $500^{\circ}F$, even though 17-7PH is within its operating range, the thermal energy allows for micro-plastic deformation via dislocation climb and grain boundary sliding. The amount of relaxation is a function of the initial stress level and time. For instance, a spring stressed to $100$ ksi at $500^{\circ}F$ might lose $5-10\%$ of its load within the first 100 hours. If the assembly depends on a specific preload to prevent vibration-induced wear, this loss can be catastrophic. Engineers must compensate for this by either over-designing the initial load or selecting a more creep-resistant material like Inconel X-750 for high-temperature service.
