Answer
During the manufacturing of wave springs, the wire or strip is cold-rolled and then coiled. This process increases the yield strength but decreases the remaining plasticity. The actual stress $S$ in the spring is calculated as $S = \frac{3 \pi P D_m}{4 n^2 b t^2}$. This calculated stress must be compared against the 'Work-Hardened' yield strength, not the annealed strength. For SAE 1070, the cold-work can increase the tensile strength from $100$ ksi to $200$ ksi. However, the 'Residual Stress' from coiling must be relieved through a stress-relief heat treatment (typically $600^{\circ}F$ for 1 hour). If the residual stresses are not managed, the spring will exhibit 'creep' or 'set' during its first few cycles of compression, leading to an immediate loss of designed preload.
