In order to clarify the effects of nitriding and hybrid surface modification process combining nitriding and induction hardening on fatigue properties of SCM435H steel, high cycle fatigue tests were carried out with a rotational bending machine at room temperature. Observations of fracture surfaces and measurement of hardness and residual stress distributions were carried out to investigate the fracture mechanism. It was revealed that hybrid surface modification process generated a compressive residual stress field and hardened even at center of the specimen. Fatigue strength of hybrid surface modified specimens was much higher than that of substrate and nitrided specimens. This was because a transition of fracture mode from internal to surface fracture; fatigue fracture of nitrided specimens occurred at inside of the hardened layer, in the case of hybrid surface modified specimens, however, fatigue crack initiated at the surface of the specimen with higher hardness and higher compressive residual stress.
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