Official Answer
Answer
Load loss in the absence of cracks usually points to stress relaxation or 'creep.' In a recent aerospace valve failure, 17-7PH springs lost 15 percent of their preload after 500 hours at 300 C. Detailed metallurgical analysis showed that while 300 C is within the 'rated' range, the combination of high mean stress (70 percent of yield) and temperature accelerated the dislocation movement within the martensitic laths. The solution was to switch to Inconel X-750, which has a higher creep resistance. Another possibility is 'wear' at the contact points (crests); if the crests wear down by just 0.05 mm in a spring with a low spring rate, the load drop can be substantial due to the $P = k \cdot Δx$ relationship.
