Damage mechanisms of initiating micro-pitting on 42CrMo4 hardened and tempered steel

A deep comprehension of the damage mechanisms involved in contact fatigue should optimize material and heat treatment choice for a specific application. In this work rolling disc-on-disc contact fatigue tests have been performed on a hardened and tempered UNI EN 42CrMo4. The adopted test method creates the best conditions in order to develop micro-pitting on disc surface. Extensive micro-fractographic examinations have been carried out, on the damaged surfaces, through a scanning electronic microscope (SEM). For this steel, loaded with Hertzian pressure of 1000 MPa, the failure mode is always micro-pitting which begins at the surface, and it is not a sub-superficial damaging. If micro-pits develop, they will coalesce in larger craters. By this way, the probability that micropitting will degenerate into sub-superficial destructive pitting is very high.