Heat-to-Heat Variation in Fatigue Strength of SCr 420 Carburized Steels

Fatigue properties and fatigue fracture mechanisms were examined for three heats of SCr420 carburized steels. Carburizing, quenching and tempering were performed at four different conditions and fatigue strength was evaluated up to 108 cycles under ratationg bending at room temperature in laboratory air. The heat-to-heat variation of fatigue strength was about twice larger than the variation due to the difference of heat treatment, according to the diversity of the failure modes : failure from the specimen surface, fish-eye failure originated near the case-core interface, or fish-eye failure in the case, and different types of fish-eye origins as non-metallic inclusions, intergranular cracking, or stage I cracking. Fatigue life of the carburized steels would be predicted by the mode I crack propagation period originated from inclusions or intergranular cracking when these were the nuclei of the fish-eye ; the period for the formation of the stage I type cracks would have an important role when the fish-eye was started from this types of cracking.