The radial hardness-profile and the microstructure of railroad car axle materials treated by surface rolling, determined by novel examination methods

Surface rolling is a cold-working technique used for hardening the surface of steel and ductile cast iron components. This process increases the surface hardness and improves the fatigue properties of components, so it is commonly used to treat railroad car wheel axles. The present paper examines the influence of this surface strengthening technique on the microstructure of the railroad car wheel axle material by hardness tests, optical microscopy (OM), and other novel examination methods, such as scanning electron microscopy (SEM), X-ray line profile analysis (XLPA), non-destructive magnetic evaluation (NDE) and automated electron backscatter diffraction (EBSD). The results show that surface rolling causes an increase in hardness down to a depth of ~10 mm. It is also shown, that the increase in hardness is not due to grain refinement or change in grain morphology; thus it is likely to be caused by an increase in dislocation density.