The high cycle fatigue property of carburized 20Cr gear steel was experimentally investigated under axial loading with a stress ratio of zero. The test results show that this steel exhibits gradually decreasing S-N characteristics, and the fatigue strength corresponding to 10 9 cycles is about 455 MPa. Based on the observation of fracture surfaces, the interior inclusion fisheye-induced fracture is the predominant fracture mode in the life regime beyond 10 5 cycles. The fine granular area (FGA)

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... cannot be found clearly around the inclusion. Based on the evaluation of stress intensity factor ranges (∆K) at the front of inclusion and fisheye, the interior crack growth rate (da/dN) equation can be characterized by da/dN = 2.39 × 10 −21 (∆K) 12.32 . Corresponding to the tested specimen, the predicted maximum inclusion size is about 52.02 µm. The predicted fatigue strength corresponding to 10 9 cycles by using Wang's model is slightly higher than the experimental result, but that by using Murakami's model is relatively conservative.