Thermo-Mechanical-Metallurgical Modelling of Pearlitic Steels and Railhead Repair Welding

Sammanfattning: The efficiency of rail transport is attributed to low rolling resistance, which comes at the cost of high contact pressures between rail and wheel. Consequently, both the rail and wheel can be susceptible to fatigue crack initiation and propagation due to the resulting high stresses and large number of load cycles in service. Therefore, the mechanical performance of the rail and wheel material is critical for safe operation. Thermal loading, such as that caused by wheel locking or railhead repair welding, can cause gradual or drastic changes in local material behaviour, resulting in significant detrimental effects to the rail and wheel. This thesis presents a phenomenological modelling framework for numerical simulations of the thermo-metallurgical-mechanical behaviour of pearlitic railway steels during and after high-temperature thermal loading. The framework consists of a material model that incorporates cyclic hardening plasticity, phase transformation kinetics, transformation-induced plasticity, multi-phase homogenisation, and recovery of the virgin material state after cyclic melting and solidification. The ability of the model to simulate intricate thermo-metallurgical-mechanical behaviour is demonstrated in quasi-static material point simulations. The material model is implemented in a finite element framework to obtain a simulation-based tool that balances computational efficiency, simulation fidelity, and engineering applicability. Several simulations demonstrate the effectiveness of this tool, including simulations of a wheel flat, laser-induced martensitic patches on the rail surface, and railhead repair welding processes. The simulation-tool is also validated against experimental data, in terms of residual material states after high-temperature processes. The thesis provides insights into the evolution of material phases and stresses under thermal loading in high-temperature railway processes, as well as the redistribution of residual stresses during subsequent operational loading conditions. For instance, simulations of the railhead repair welding process indicate that pre-heating has a minor impact on the quality of the repair, while the welding build-up paths have a significant impact. The simulations also highlight the critical region for fatigue crack initiation by showing how residual tensile stresses are reduced near the surface of the rail or wheel and increased at some distance below during operation for the repaired rail.