Fatigue Crack Growth Behavior of U20Mn Bainite Rail

Abstract

a-N curves of fatigue crack growth of U20Mn bainite rail after different heat treatment process were studied (The temperature is cooled from 900 to 20 °C, the cooling rate was 0.5 °C/s, 1 °C/s and salt bath isothermal respectively), the Paris formula of fatigue crack growth was fitted linearly, and the material parameters C and n were measured. The results show that the sample with cooling rate of 0.5 °C/s has the fastest crack growth rate, and the sample with salt bath isothermal has the slowest crack growth rate. The coarse M/A islands with irregular shape in bainite structure with cooling rate of 0.5 °C/s has poor resistance to fatigue crack propagation, which is not conducive to improving the fatigue performance. However, the sample with salt bath isothermal has longer fatigue life. Due to the combination of bainitie lamellar and retained austenite distributed between them, the salt bath isothermal sample can effectively improve the strength and toughness of bainite steel. The sample with cooling rate of 0.5 °C/s is mainly composed of granular bainitie structure, and the fatigue crack growth trajectory is generally gentle without large angle deflection, the sample with salt bath isothermal is mainly composed of bainite lamellar structure, and the fatigue crack growth trajectory is not straight, with a large number of Z-shaped deflection. The fatigue cracks are prone to produce branch cracks at the stress concentration of propagation deflection, and the branch crack consumes the energy of the main fatigue cracks, thus reducing the fatigue crack growth rate and improving fatigue life.

RIGHTS & PERMISSIONS

Wuhan University of Technology and Springer-Verlag GmbH Germany, Part of Springer Nature