X-ray investigations of the natural and artificial White Etching Layer

We analyzed X-Ray diffraction data on the White Etching Layer (WEL) excised from the surface of a railway track and WEL-like structures obtained by high pressure torsion of the same steel as the rail was made. Data analysis was performed by the whole-pattern fitting method. The railway WEL shows a clear asymmetry of the diffraction lines, which can be attributed to either tetragonal martensite structure or to the formation of a ferrite nanophase with a lattice parameters larger than that of the raw material. X-Ray patterns of the natural WEL were fitted with the two models. The quality of the fit clearly favors the model with two cubic phases over the model with a single tetragonal phase. Artificial WEL samples could be fitted with a single cubic phase model. Several alternative methods were simultaneously used to extract information on crystallite size and internal stress. The analysis of the natural WEL shows that the secondary cubic nanophase is formed of the crystallites of the average size of 8 nm essentially free of any strain. The lattice parameter 0.2881nm corresponds to the 0.26 wt.% concentration of the dissolved carbon. In the artificial WEL samples the crystallite size decreases down to 9 nm at the maximum shear stress of 430, while the internal strain gradually increases. There is however no change of the lattice parameter. This means that no carbon from cementite dissolves in the lattice of ferrite. It must therefore be located in the grain boundaries and dislocation cores. We conclude that the high pressure torsion produces a WEL with a different distribution of carbon than in the natural process of the WEL formation. It involves an immediate severe deformation the sample while natural process takes numerous subsequent cycles of modest deformation with simultaneous local heating of the deformation area. Such a process may lead to the gradual diffusion of the carbon atoms from the grain boundaries into the crystal lattice of ferrite.