The goal of this work is to increase understanding of plastic deformation of the pearlitic microstructure in the wheel/rail contact. The tridimensional gradient of microstructure below the running band of a worn R260 rail is investigated using multi-scale approach based on microstructural observations by optical and Scanning Electron Microscopy (SEM-FEG), microindentation and Electron BackScatter Diffraction (EBSD) investigations. Due to severe plastic deformation, work hardening is progressively experienced by the rail. In the middle of the running band, the pearlitic colonies are fragmented by accumulation of severe shear strain up to 3 mm in depth. At the rail surface, the resulting lamellar structure is elongated and aligned in the shear direction. At a transition depth of 3–4 mm, both fragmented and unaffected pearlitic colonies are observed. In these fragmented colonies, cementite lamellae are heavily bent and partly broken. Correspondingly, a strong increase of large angle grain boundaries (LAGB) is measured. The interlamellar spacing progressively decreases from this transition depth to the near surface. This quantitative analysis of the tridimensional gradient of microstructure will contribute to improve modeling of rail plasticity and crack propagation by RCF by including anisotropy of the running band and effect of in-depth microstructure evolution.

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.

The aim of this study is to increase understanding of the relationship between RCF cracking and plastic deformation of the pearlitic microstructure. Following a multi-technical and multi-scale characterization of the worn rail surface, the multidirectional gradient of mechanical properties below the running band is studied with a microstructural point of view. In order to achieve the characterization of the running band and to quantify the influence of the microstructural/mechanical gradient on the global fatigue behavior, this experimental methodology has been based on the combination of several microstructural investigations by optical and scanning electron microscopy (FEG-SEM) in both rolling and transverse directions, 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, material experiences progressively work hardening and the pearlitic microstructure is modified. In the middle of the running band where the contact patch envelope is more frequent, the pearlitic colonies below the worn rail surface are fractured by accumulation of cyclic and severe shear strain. The resulting lamellar structure of ferrite and cementite is elongated and aligned in the direction of shear strain as the interlamellar spacing decreases. On the contrary, at both sides of the running band where the contact with wheel is rare, the surface is rougher, the pearlitic colonies remain unaffected and the lamellae are not oriented preferentially. The examination of the running band and the microstructure gradient by quantitative analysis will contribute to improve the modeling of rail failure by RCF by enabling to include microstructural data.