Answer
Nitriding is a thermo-chemical process that diffuses nitrogen into the surface of the steel (typically carbon steel or low-alloy steel) to create a hard, wear-resistant 'case.' For spiral retaining rings used in heavy machinery or vibrating screeners, nitriding increases surface hardness to over $60 HRC$. This hard layer prevents 'fretting' and 'galling' between the ring and the groove wall, which can occur when high-frequency vibrations cause micro-rubbing. The process is performed at relatively low temperatures ($925^\circ F$ to $1050^\circ F$), which minimizes distortion of the ring's geometry. The resulting compressive residual stresses on the surface also improve the fatigue life of the ring by inhibiting the initiation of surface cracks.
