Microstructure and Fracture Toughness of U75V Heavy Rail Steel Under Different Heat Treatment Conditions

U75V heavy rail steel was studied as the research object, and the microscopic structure, fracture morphology and three-point bending fracture toughness under the conditions of rolling and heat treatment were analyzed by the universal testing machine, strain gauge, OM, and SEM. The influence of microstructure on fracture toughness was revealed, providing reference for studying the heat treatment process of pearlescent heavy rail steel. The results show that the surface strain changes of U75V heavy rail steel in the heat treated and rolled states are similar, but the elastic deformation to plastic deformation of U75V heavy rail steel in the heat treated state lags behind that in the rolled state. The fracture toughness K_IC values of heat treated and rolled U75V heavy rail steel are 45.122 MPa·m^1/2 and 42.048 MPa·m^1/2 respectively, and the fracture toughness in the heat treated state is higher than that in the rolled state. The fracture surface of the rolled U75V heavy rail steel is cleavage fracture, while the heat treated U75V heavy rail steel is quasi cleavage fracture. The interlayer spacing of pearlite in the rolled and heat treated U75V heavy rail steel is 272.2 and 148.4 nm, respectively. The pearlite layer in the heat treated U75V heavy rail steel is fine and there are significant pearlite clusters, which is not conducive to crack propagation, making the fracture toughness of the heat treated U75V heavy rail steel higher than that of the rolled state.