Microstructure alterations at the surface of a heavily corrugated rail with strong ripple formation

Abstract

The topography, morphology, structure and residual stress state at the surface of a rail with severe longitudinal ripples was characterized using a combination of spectrometry, microscopy, scattering and diffraction methods. The investigations reveal that white etching layers develop at the ripple hills, while the ripple valleys contain a severely plastic deformed layer. While the structure in the ripple valleys remains pearlite, the structure of the white etching layers on top of the ripple hills is nanocrystalline martensite containing cementite particles of a few nanometer size. The microhardness of the white etching layers on top of the ripple hills is about three times higher than the microhardness in the ripple valleys. The microhardness throughout the white etching layer is not homogeneous due to the non-uniform amount of carbon in solute solution and also due to differences in the amount and the size of the fine carbide particles. The whole running surface is under biaxial compressive residual stresses. In the ripple valleys the direction of the principal residual stresses nearly coincides with the longitudinal and the transverse direction on the rail. On the ripple hills the compressive residual stresses due to the martensite formation are significantly higher and the stress state is nearly homogeneous.