Answer
Nitriding is a thermo-chemical process that diffuses nitrogen into the surface of the spring material, usually carbon steels or alloy steels like 4140, to create a hard, wear-resistant 'white layer' and a diffusion zone. For wave springs subjected to high-cycle fatigue, nitriding introduces beneficial compressive residual stresses on the surface. These stresses counteract the tensile stresses experienced during deflection, effectively increasing the fatigue limit. The formula for the modified fatigue strength is $\sigma_{e'} = \sigma_e + \sigma_{comp}$, where $\sigma_{comp}$ is the magnitude of the compressive stress. However, if the nitrided layer is too brittle, it can crack under high strain, acting as a stress riser that leads to core failure; therefore, the nitriding depth must be carefully controlled to roughly 10% of the wire thickness.
