Influence of grain boundary cementite induced by gas nitriding on the rolling contact fatigue of alloyed steels for gears

International audience; It is well-known that rolling contact fatigue life can be increased by modifying gear surfaces. For instance, gas nitriding is a thermochemical treatment enhancing superficial hardness, and introducing compressive residual stresses but nitriding alloys can lead to the precipitation of cementite at grain boundaries. In this study, experiments were performed on a twin-disc machine to investigate the influence of the microstructure of nitrided layers on rolling contact fatigue failure modes. Nitrided layers with small grains, thin and short grain boundary cementite filaments showed better resistance to surface originated pitting compared with nitrided layers with coarse precipitates, for similar mechanical properties in terms of hardness and residual stresses. Moreover, examinations carried out on cross sections throughout crack networks revealed the influence of these precipitates on the propagation of rolling contact fatigue cracks, which is at the origin of the differences in performance between thin and coarse microstructures