Mitigation of subsurface crack propagation in railroad rails by laser surface modification

Abstract

We address the mitigation of subsurface crack propagation in railroad rails via laser surface modification. Microhardness scans and tensile tests, performed on samples of unused and heavily used rail heads, indicate that the severe cyclic plastic deformation that occurs at rail gage corners, during service, leads to cracking. Reducing rail–wheel friction reduces shear forces that contribute to this problem. Laser glazing, the rapid melting and rapid re-solidification of a thin surface layer, is shown here to reduce the friction coefficient of rail steel. These treatments produce a thin (<100 μm) glazed surface layer, intimately bonded to a martensitic heat-affected-zone that is, itself, well bonded to the pearlitic rail steel substrate. The microhardness (Vickers) of the glazed layer ranges from H V655 to H V800, while that of the heat-affected-zone ranges from H V470 to H V1072. The pearlitic steel substrate typically has a hardness of H V300. Static “block-on-ring” friction experiments on standard specimens extracted from laser-treated samples show reductions in the friction coefficient by about 25% relative to untreated surfaces at loads corresponding to prototypic rail service loads. X-ray scans of treated surfaces were inconclusive regarding the nature of the glazed layer. The top surface of a six-foot length of rail was laser glazed on two areas, each ∼10 cm long and ∼2 cm wide. Friction measurements were made on these surfaces on the Association of American Railroad's Cyclic Rolling–Sliding Wear Machine after they were subjected to 20,000 run-in cycles at loads prototypic of service. The laser treatments remained intact after these cycles. Reductions of friction coefficient of ∼40%, relative to untreated surfaces, were observed, which corresponds to a calculated reduction in the crack propagation rate by ∼79%.