Investigations of fatigue crack propagation in ER8 railway wheel steel with varying microstructures

In this study, fatigue crack occurrences and propagations of high-speed railway wheel steel grade ER8 were investigated through both experiments and FE simulations. Three varying microstructures of the wheel steel including as-received, ferrite- and bainite-containing microstructures, which were obtained by specific heat treatment routes, were characterized. Stress-controlled fatigue tests were carried out for determining relationships between crack propagation rate and stress intensity factor of the steels. In addition, FE simulations of fatigue crack developments on both macro- and micro-scale using representative volume element (RVE) 2D models were conducted with consideration of microstructural features. The extended finite element method (XFEM) coupled with the Paris law was applied in the cyclic simulations for predicting crack propagation. For the microstructure level, flow stress-strain curves based on micromechanics approach and corresponding fatigue damage parameters of different phases in steels were identified and afterwards calibrated with experimental results. Finally, influences of ferrite, pearlite and bainite on fatigue crack resistance in the wheel steel were precisely examined by the RVE model. It was found that fatigue micro-cracks mostly initiated close to interface regions between soft and hard phases. The bainitic phase could obviously retard fatigue crack propagation of steel, whereas the ferritic phase contrarily exhibited deteriorated crack resistance. Moreover, the proposed framework could be acceptably employed for predicting fatigue crack propagation behavior and its rate in railway wheel subjected to microstructure changes occurred.