Danneggiamento per pitting di acciai bonificati, cementati e nitrurati

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: an 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 best realize such conditions able to develop micro-pitting 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 micro-fractographic analyses were performed on the damaged samples by the use of a scanning electronic microscope (SEM). For hardened and tempered steel, failure mode was micro-pitting, 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, micro-pitting 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, micro-pits may develop. Damaging rate can significantly increase by coalescence of small closest pits into a larger crater so much micro-pitting phenomena do not arrest, but it can degenerate into destructive pitting and spalling.