Answer
Wave springs are manufactured from cold-rolled flat wire rather than round wire. The cold-rolling process increases the dislocation density within the crystalline lattice, significantly raising the yield strength $\sigma_y$ and hardness, but reducing ductility. This work-hardening is critical for achieving high spring rates in small envelopes. The final spring rate $k$ is sensitive to the resulting thickness $t$ due to the cubic relationship $k \propto t^3$. A variation of just $0.01$ mm in thickness can result in a $10-15\%$ change in load. For fatigue life, the surface finish from rolling is superior to drawn wire, reducing crack initiation sites, provided that the subsequent heat treatment (stress relieving) is performed to stabilize the structure without excessive decarburization.
