On the mechanism of fatigue failure in the superlong life regime (N > 10(7) cycles). Part I: influence of hydrogen trapped by inclusions

The fracture surfaces of specimens of a heat-treated hard steel, namely Cr- Mo steel SCM435, which failed in the regime of N = 10(5) to 5 x 10(8) cycle s, were investigated by optical microscopy and scanning electron microscopy (SEM). Specimens having a longer fatigue life had a particular morphology beside the inclusion at the fracture origin. The particular morphology look ed optically dark when observed by an optical microscope and it was named t he optically dark area (ODA). The ODA looks a rough area when observed by S EM and atomic force microscope (AFM). The relative size of the ODA to the s ize of the inclusion at the fracture origin increases with increase in fati gue life. Thus, the ODA is considered to have a crucial role in the mechani sm of superlong fatigue failure. It has been assumed that the ODA is made b y the cyclic fatigue stress and the synergetic effect of the hydrogen which is trapped by the inclusion at the fracture origin. To verify this hypothe sis, in addition to conventionally heat-treated specimens (specimen QT, i.e . quenched and tempered), specimens annealed at 300 degreesC in a vacuum (s pecimen VA) and the specimens quenched in a vacuum (specimen VQ) were prepa red to remove the hydrogen trapped by inclusions. The specimens VA and VQ, had a much smaller ODA than the specimen QT. Some other evidence of the inf luence of hydrogen on superlong fatigue failure are also presented. Thus, i t is concluded that the hydrogen trapped by inclusions is a crucial factor which causes the superlong fatigue failure of high strength steels.