Numerical study about the effect of bainitic traces on plasticity in ferritic-pearlitic railway wheels

Wheel microstructure significantly affects railway wheel performances. Although wheel microstructure traditionally consists of pearlite-ferrite, it is possible to detect some intermediate structures traces, for instance bainite, generated during the thermal treatment process. The difference in mechanical properties between pearlite and bainite causes stress and strain incompatibilities at their interface, and this gives rises to early crack initiation and propagation. This paper aims at investigating the presence of bainitic traces in a ferritic-pearlitic microstructure of a railway wheel through semi-analytical and numerical finite element simulations. The effect of bainitic spot shape, dimension and distribution is studied as well as stress and plastic strain in the proximity of the heterogeneities is analysed. The radius-interaxis ratio parameter, defined as the ratio between the bainitic spots radiuses and the distance between two neighbouring spots centres, is introduced to study the effect of circular-sectioned bainitic spots on the steel plasticity. It is observed that the bainitic traces form regions subjected to overstress and overstrain in the ferritic-pearlitic matrix and that the area of the bainitic spots influences the size of these regions. The bainitic spot distribution is found out to increase further the overstrain in case the radius/interaxis ratio is higher than 0.2, corresponding to about 11% of bainite in the ferritic-pearlitic matrix. Beyond this threshold value, bainite traces can compromise both RCF and wear response.