Thermo-mechanical behaviour of pearlitic railway steels

Doctoral thesis, 2024

Railway operations and maintenance activities typically involve high levels of localised stresses and temperatures. The material behaviour of common pearlitic railway steels has previously been studied under thermal and different mechanical loadings, respectively. However, in service the material occasionally experiences these loads simultaneously, which results in a complex thermo-mechanical loading. This thesis attempts to characterise the material behaviour of rail and railway wheel steel when exposed to thermo-mechanical loads. The effect of localised heating events is studied, and the influence of process parameters are investigated.
This thesis work encompasses both slow and rapid local heating events, with the objective of investigating changes in the mechanical behaviour, microstructure and residual stress states. A slow heating cycle was employed to simulate severe block braking on railway wheel material. This was studied through thermo-mechanical cycling to peak temperatures varying between 300 °C and 650 °C, while the thermal dilatation was restricted to different degrees between free expansion and full restriction. Rapid heating and cooling events were studied by using laser heating. Laser sources from PBF-LB additive manufacturing equipment and laser welding equipment were used to simulate local heating events.
It was seen that thermo-mechanical loads result in strongly temperature-dependent material behaviour; however, the effect most notable is the resulting residual stress state in the material after testing. For the slow thermal cycles, as experienced during severe block braking, it can be concluded that overheating of the wheel rim does not have severe consequences for the mechanical properties of the wheel. Quantification of spheroidisation in the wheel rim showed that increased loads at these high temperatures could have a beneficial effect on the material properties. This thesis work made use of novel approaches, such as the laser heating experiments, that can be useful to simulate also other local heating events and thereby characterise the material behaviour.
The findings presented in this thesis provide insight into the material behaviour of railway steels during local heating events under conditions similar to those observed in practice. These insights can be used to inform railway wheel design, rail and wheel maintenance procedures, and component replacement limits. Furthermore, the work has been implemented and used for verification of simulations in parallel activities and provide an experimental basis for the further development of predictive models.