Micropitting Damage Mechanism on Hardened and Tempered, Nitrided, and Carburizing Steels

A deep comprehension of the damage mechanisms involved in contact fatigue is basic to optimize materials and heat treatments for specific applications. In this work, rolling disc-on-disc contact fatigue tests have been performed on three different materials: a hardened and tempered UNI EN 42CrMo4, a nitrided UNI EN 42CrMo4 steel, and a carburized UNI EN 18NiCrMo5 steel. The test method adopted was optimized to better realize such conditions able to develop micropitting on different materials. The main aim of this test campaign was to investigate and compare damage on surface, in terms of initiating and propagating mechanisms. Extensive microfractographic analyses were performed on the damaged samples by the use of a scanning electronic microscope (SEM). For hardened and tempered steel, failure mode was micropitting, started from surface, but evolved into pitting because of crack path extending more deeply beneath the surface. Pits always originated on top surface and propagated as like a common fatigue crack. In the case of carburized steel, micropitting initiated at oxides at most surface grain boundaries, i.e., those sites characterized by greatest brittleness. Superficial cracks initiated and propagated inside the white layer for the nitrided steel. Once initiated, micropits may develop. Damaging rate can significantly increase by coalescence of small closest pits into a larger crater so much that micropitting phenomena do not arrest, but it can degenerate into destructive pitting and spalling. Significant differences, however, exist for ductile surface or hardened one.