Answer
17-7PH (UNS S17700) is semi-austenitic and undergoes significant work hardening during the cold-rolling and coiling phases. The transformation from austenite to martensite is strain-induced. During the 'on-edge' coiling of a wave spring, the outer fibers of the flat wire experience higher strain, leading to a gradient of hardness across the cross-section. This results in a non-uniform residual stress state that must be relieved during the CH900 heat treatment process. If the coiling tension is not precisely controlled, the resulting wave heights will vary, leading to a load tolerance deviation. Precision springs often require a 'preset' operation where they are compressed to solid height to stabilize the metallurgical structure and reduce subsequent load loss in service.
