Microstructure and crystallographic orientation evolutions below the superficial white layer of a used pearlitic rail

Abstract

Although several studies have been conducted on the mechanism of the formation ofwhite layer by cyclic large shear deformation, and its effect on rolling contact fatigue,limited research has been carried out to find a correlation between sub-superficial layerof running contact surface and the crystallographic orientation, microstructural changes,and distribution of shear deformation and dislocation density. In order to understandthe microstructural evolution (i.e., microstructure and crystallographic orientation) a usedpearlitic rail sample removed from the heavy-haul railroad was investigated in the currentwork. A very thin superficial white layer was observed, approximately 15 m below the run-ning contact surface. X-ray diffraction analysis confirmed the formation of a supersaturatedferrite phase with carbon content of approximately 3.78 C wt%. It could be attributed to thecementite dissolution due to severe shear and compressive stresses, during intense shearplastic deformation, from the rail-wheel interaction. The dominance of {110} ferrite grainsparallel to the rail direction was characterised, in the transition layer between white layerand non-deformed pearlite structure, by X-ray diffraction and electron backscattered diffrac-tion techniques. Formation of these grains, corresponding to the closed-pack plane of theferrite matrix at the transition region, leads to great ductility and retarded crack formation.