Answer
Stress relieving is a post-coiling thermal process necessary to stabilize the geometry of the wave spring. During the coiling of flat wire into a circular shape and the forming of waves, significant residual stresses are introduced into the material. Without stress relieving, these internal stresses would cause the spring to 'creep' or change its free height over time, even without load. For carbon steel, stress relieving is typically performed at $600^\circ F$ to $700^\circ F$ ($315^\circ C$ to $370^\circ C$). This temperature is high enough to allow micro-plastic flow to redistribute internal stresses but low enough to avoid altering the tempered martensitic structure. This ensures that the spring maintains its 'as-designed' free height and load-at-working-height characteristics throughout its service life.
