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Characterization of Crack Growth Behavior of Carburized SCM415 Steel under Cyclic Rotating Bending

Carburized steel was used in severe and cyclic loading conditions such as bearing and structural components. In this study rotating bending fatigue tests were carried out to observe the crack initiation and propagation behavior of carburized JIS SCM415 steel bar whose diameter was 10.0 mm. Transition area origin (TRO) crack on fracture surfaces were observed with scanning electron microscope. The depth of fracture origins was about 0.9 mm. TRO crack was observed around the fracture origin which was nucleated at the edge of carburized layer, and the crack propagated toward the surface and the inside core. Stress amplitude was modified with depth of crack origin, and S-N curve was corrected with modified applied stress amplitude σM. In order to reveal the crack propagation behavior around the boundary between the hardened and the soft core area, stress intensity factor (SIF) on crack front was also computed.