Microstructural evaluation and interpretation of the mechanically and thermally affected zone under railway wheel flats

During a wheel-rail slide, the temperature is high enough to austenitise the material close to the contact surface. The material is rapidly cooled by heat conduction into the rest of the wheel when the wheelset starts rolling again and thus martensite can form in the surface layer. In the present investigation, the mechanically and thermally affected zones (TAZ) under railway wheel flats have been studied by optical microscopy, scanning electron microscopy and hardness testing. The wheel flat samples were generated in full-scale experiments on the last axle of a train consisting of a locomotive pulling three carriages. The axle load, train speed and locking time of the last axle were varied to simulate several different situations that arise in revenue traffic. The entire population of samples has earlier been studied, but the present investigation goes more into detail on a limited number of samples. Several discoveries are made and suggestions of mechanisms are presented. Among the most important findings are stretched MnS inclusions that act as crack nucleation sites, limited grain growth indicating low austenitisation temperatures and reformation of ferrite and pearlite in the deformed surface layers. (C) 1999 Elsevier Science S.A. All rights reserved.