Rolling Contact Fatigue Damage in Welded Rail Steel Joints

Welded rail steel joints show a local decrease in material hardness due to both the welding process and post welding heat treatment. Severe plastic deformation leads to changes in the rail surface profile and formation of an extended depression of different geometry in the joint area. The rail surface layer in the depression region is often subject to rolling contact fatigue failure. Here we model the process of rolling contact fatigue damage accumulation in the surface layers of the rail head in the zone of a flash butt welded joint. Modeling is performed using experimental data on the mechanical properties and structural changes of rail steel during operation. The material in the weld region is examined by scanning electron microscopy. It is found that each zone of the welded joint has specific structural features, which correlate well with local changes in the rail metal hardness. The welding process is accompanied by local decarburization. The surface layer is deformed and shows signs of different failure stages, including surface fatigue crack initiation. Mathematical modeling of rolling contact fatigue damage accumulation is carried out using contact and fracture mechanics approaches, taking into account the inhomogeneous strength properties of the rail metal in the weld region, the shape change of the rail head due to plastic deformation and wear, and initial damage in the surface layers. Modeling results indicate that the shape change (flattening) of the rail head and initial damage together lead to the fact that the maximum damage and fatigue crack initiation occur at the surface.