Microstructural studies have been made on material from full-scale infield experiments performed with a train consisting of a locomotive and three carriages running on standard rail. One axle of the train was locked to simulate wheel skid that sometimes occurs in regular service. When the wheel slides against the rail, the material close to the surface is heated by friction and heavily sheared. In accordance with experience from revenue traffic, it was found that cracks often originate from skidded surfaces. Two probable mechanisms for initiation of cracks that can later propagate by fatigue are presented. The first type is formed through the action of volume expansion upon martensite formation. The second type forms by shearing of initially equiaxed, 5 nm large MnS inclusions which are stretched to up to 100 μm long flattened needles. The adhesion between these and the base material is very low, and they therefore serve as crack initiation sites.
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