Experimental Characterization of the Tridimensional Gradient of Microstructure Induced by RCF in the Rolling Band of Rails

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.