Deformation behaviour and microscopic investigations of cyclically loaded railway wheels and tyres

Railway wheel and tyre steels were investigated by specimens from defined volume sections representing the local microstructure of original components. As a consequence of the industrial heat treatment and the size of the components, microstructural gradients exist in the wheel rim and tyre cross-section. The ferrite fraction of the ferritic-pearlitic microstructure and the cementite lamellae spacing vary depending on the distance to the running surface (tread). Stress-controlled load increase and single-step push-pull tests were carried out at ambient temperature with servohydraulic testing systems to study fatigue phenomena with special consideration of microstructural details. Mechanical stress-strain hysteresis, thermometrical, and electrical measurements were used for a comprehensive characterization and evaluation of the cyclic deformation behaviour. The measured physical quantities show a strong interrelation with the underlying fatigue process. Light, scanning, and transmission electron microscopy (TEM), together with digital image processing, allow to interpret the measured data on the basis of microstructural details. In defined fatigue states, TEM investigations were performed to correlate the loading conditions with the appearing dislocation structures.