The Fatigue Behaviors of a Medium-Carbon Pearlitic Wheel-Steel with Elongated Sulfides in High-Cycle and Very-High-Cycle Regimes

The effects of stress ratio (R), loading condition, and MnS inclusion on the fatigue behavior of a medium-carbon pearlitic wheel-steel were investigated by a combination of rotating (frequency of 52.5 Hz, 10(3)–10(8)) bending and ultrasonic (frequency of 20 kHz, 5 × 10(4)–10(9)) axial cycling tests in high-cycle and very-high-cycle regimes. All the S-N curves present horizontal asymptotic shapes and have clear fatigue limits. The fatigue limits (260–270 MPa) for R = −1 obtained by ultrasonic test are almost 140–150 MPa lower than that (400–410 MPa) obtained by rotating bending, and the limit values of R = 0.3 are almost in the range of 195–205 MPa. For rotating bending, the fatigue fractures were originated from the surface matrix of the specimen. Whereas for ultrasonic fatigue, both surface and interior crack initiation occurred, and cracks were all initiated from MnS inclusions regardless of stress ratios. The finite element method was employed to study the influence of MnS inclusions on crack initiation and propagation. The results show that high stress concentrates on the sides of the elliptical MnS inclusion rather than the tip of the inclusion.