Mechanism of Superlong Fatigue Failure in the Regime of N>10~7 Cycles and Fractography of the Fracture Surface

摘要

In order to elucidate the mechanism of superlong fatigue failure in the regime of N > 10~7 cycles, the fracture surfaces of specimens of heat treated hard steel, SCM 435 and 0.46 medium carbon steel were investigated by optical microscope, SEM and AFM. It has been revealed that specimens having longer life have a particular morphology beside the inclusion at fracture origin. The particular morphology looks optically dark by the observation of optical microscope and it has been named the optically dark area, ODA. The ODA looks a rough area in the observation by SEM and AFM. The relative size of ODA to the size of inclusion at fracture origin increases with increase in fatigue life. Thus, ODA has a crucial importance for the mechanism of superlong fatigue failure. It has been assumed that ODA is made by the fatigue due to cyclic stress coupled with hydrogen which is trapped by the inclusion at fracture origin. To verify the hypothesis, specimens annealed at 300℃ in a vacuum (VA specimens) and quenched in a vacuum (VQ specimens) are prepared to desorp the hydrogen trapped by inclusions. The specimens VA and VQ, had a much smaller ODA than conventionally heat treated specimens. Thus, it has been concluded that hydrogen trapped by inclusion is the crucial factor which causes superlong fatigue failure of high strength steels.