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In order to examine the effects of cleanliness and induction hardening on the very high cycle fatigue properties of low alloy forged steel (40CrMo8) used for crankshafts of Diesel engines for marine use, rotating bending fatigue tests were performed for specimens with different cleanliness (super clean steel or conventional steel) and thickness of hardened layer (0.2 mm or 0.4 mm). The super clean steel showed higher fatigue limit than the conventional steel because the fatigue fracture at non-metallic inclusion was suppressed; however, the induction hardened super clean steel failed in the subsurface fracture mode from the singular microstructure. The induction hardened steel showed lower fatigue limit than the untreated steel. This result was attributed to the fact that the tensile residual stress was generated beneath the surface hardened layer at the fracture origin. Therefore, fatigue limit of the induction hardened steel was determined by the fatigue strength of the matrix at the fracture origin.

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