Fatigue Behaviour of Railway Wheel Steels under Constant and Variable Amplitude Loading

Abstract

In this paper, the influence of the microstructure on the fatigue behaviour of specimens from defined rim positions of original railway wheels R7 (SAE 1050) and tyres B6 (SAE 1065) is characterised under constant and variable amplitude loading. Due to the industrial heat treatment and the component size, the ferrite fraction and the cementite lamellae spacing of the ferriticpearlitic microstructures increase with increasing tread distance. The microstructural gradients influence the fatigue behaviour in a characteristic manner. Mechanical stress-strain hysteresis, temperature and electrical resistance measurements were performed. Temperature and electrical resistance data represent the actual fatigue state in highly stressed volume parts and are not related to a gauge length. Furthermore the electrical resistance is qualified to detect a proceeding fatigue damage in load-free specimens and components. For variable amplitude loading a new testing procedure was applied, which combines any kind of near-service load spectrum with short single step measuring sequences. The plastic strain amplitude, the temperature and the electrical resistance data of each single step sequence are plotted in cyclic ‘deformation’ curves and represent the sum of microstructural changes caused by near-service loading. The substitution of the plastic strain amplitude by the changes of the temperature and the electrical resistance leads to modified Morrow and Manson-Coffin curves. Electron microscopic investigations allow to interpret the measured fatigue data on the basis of microstructural details.