Residual stresses in martensite after multiple heating events

Paper in proceeding, 2014

The study presented relates to the technical problem concerning repeated friction heating in a railway wheel-rail contact surface, caused by partial or full slip. Wheels and rails are commonly made from medium carbon pearlitic steels. The temperature reached during wheel skidding is sometimes high enough to cause austenite formation in a thin layer, and thereafter rapid self-cooling yields martensite. Volume expansion occurs during transformation to martensite. On repeated heating to temperatures around 100-500°C, the martensite is tempered, and a simultaneous volumetric shrinkage takes place. This causes residual stress fields and discontinuities in strength that, in combination with high stress levels, can lead to initiation of cracks. The FE modelling presented in this paper is done to characterize resulting stress and strain fields due to repeated heating of martensite. The model utilizes temperature and structure dependent material properties, including thermal expansion and flow stress. Specimens were modelled as axisymmetric coins with controlled temperature at the center of the top surface. Much of the results, and parts of the descriptive text, are reproduced from a published article with a wider scope [1] with permission from Elsevier. The main conclusion from the modelling work in this study is that there is a large difference in residual stress gradients depending on starting structure. Exposed to a local heating pulse, as-quenched martensitic coins develop a deep tensile residual stress field, with a low gradient. Tempered martensitic coins, on the other hand, can develop high tensile stresses in the surface, but rapidly declining with depth. The explanation lies in the different mechanisms of residual stress formation.