Microstructural transformation of a rail surface induced by severe thermoplastic deformation and its non-destructive monitoring via Barkhausen noise

The paper presents a new concept for evaluation of surface damage of rails subjected to long-term cyclic loading. A rail cyclically loaded during 20 years of traffic was examined by combination of several non-destructive techniques with high sensitivity to surface damage, namely positron annihilation spectroscopy, X-ray line profile analysis and magnetic Barkhausen noise. The parameters of magnetic Barkhausen noise were correlated with the parameters describing the rail surface integrity, i.e. thickness, hardness, stress state and microstructure of the damaged layer. Good correlation between these parameters was observed and it has been demonstrated that thickness of the damaged layer can be measured non-destructively by the magnetic Barkhausen noise technique. Cyclic loading of the rail introduced a high density of dislocations into the sub-surface region and also vacancies which agglomerated into small clusters. Phase analysis of the rail surface revealed that repeated severe plastic deformation induced multiple phase transitions.