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.

Experimental investigations were carried out to better understand the rolling contact fatigue mechanisms in nitrided layers of the 33CrMoV12-9 steel grade. Surface-initiated pitting failure mode was reproduced on a twin-disc machine to analyse crack growth and compressive residual stress behaviour within the nitrided layers. Metallographic examinations, 3D observations by means of high-resolution X-ray computed tomography and residual stress analysis were realised on nitrided 33CrMoV12-9 specimens before and after rolling contact fatigue tests. The study revealed that if the initial compressive residual stresses associated with the surface treatment are released during the process of rolling contact fatigue, pre-existing superficial cracks propagate in the nitrided layers along the intergranular carbides. These precipitates induced by the nitriding process therefore act as preferential crack propagation sites.