Multiscale characterization of head check initiation on rails under rolling contact fatigue: Mechanical and microstructure analysis

Head Check is a major rolling contact fatigue (RCF) defect initiated after cyclic wheel/rail contacts by severe plastic deformation of the rail head surface. A field analysis campaign has been conducted on a site affected by Head Checking. The aim of this study is to increase understanding of the relationship between RCF cracking and plastic deformation of the pearlitic microstructure. Investigations are performed on samples taken at several stages of accumulated tonnage in the early stages of formation of the defect. To supplement previous characterizations of Head Checking and studies of damage mechanisms, several investigations have been carried out with a microstructural point of view. Following a multi-technical and multi-scale characterization of the worn rail surface, this experimental methodology has been based on the combination of several microstructural investigations by optical and scanning electron microscopy (FEG-SEM), microindentation performed on the gradient profile and determination of quantitative data on the pearlitic aggregates through the gradient by Electron Backscatter Diffraction (EBSD). Due to the severe plastic deformation near the gauge corner of the worn rail, the microstructure is progressively modified. During the first cycles of wheel/rail contacts, a strong evolution of the decarburised layer is observed. In transverse sections, the ferrite phase is severely shear-strained and the cementite lamellae within the pearlitic colonies begin to bend in the shear direction. Severe misoriented areas (>10°) are observed in the proeutectoid ferrite, suggesting that the proeutectoid ferrite is more strained than the pearlite colonies in the first stages of accumulated tonnage. Near surface hardness increases with the total traffic load and the thickness hardness gradient becomes significant as rail experiences work hardening. At the early stages of development, surface rolling contact fatigue cracks are initiated in the soft ferrite phase. Cracks propagate progressively below the rail along the shear-strained ferrite phase at the boundary with pearlitic colonies. This improvement in rail Head Check defect characterization with a microstructure point of view and the study of its development will contribute to improve the understanding of the entire damage mechanism of Head Checking and the modeling of rail failure by RCF by implementing microstructural data.