A group of S45C medium carbon steel specimen was nitrided by plasma nitriding using a mixture of 40% N2-60% H2.The processes were carried out at 500 ℃ for three different time durations of 8,20 and 48 hours.The effect of case depth on fatigue performance of S45C was investigated.The thickness of compound layer for these three time durations were 6 μm,8 μm,12 μm,respectively. Plasma nitriding treatment enhanced the surface hardness and the surface compressive residual stress.The compound layer was composed of the γ-Fe4N phase.Rotating bending fatigue test showed that plasma nitriding treatment increased the fatigue strength of S45C by 190% with respect to the untreated specimen because of the increase of the case depth and the expansion of compressive residual stress in the case depth.The stress intensity factor calculated from the facet area was independent of the number of cycles to failure and was found as at 3.6 MPa·m1/2.These fish-eye cracks were originated from inclusions in the case depth，and were calcium oxide,aluminum oxide and manganese sulfide.
