Influence of Mn addition on cyclic deformation behaviour of bainitic rail steels

The relationship between the Mn content and the cyclic deformation behaviour in bainitic rail steels is not yet clearly understood. The aim of this paper is, therefore, to study the cyclic plastic response of bainitic rail steels with different Mn contents. In order to meet these goals, low-cycle fatigue tests under fully-reversed strain-controlled conditions are performed at strain amplitudes in the range of 0.6 to 1.0%. The microstructure is analysed by TEM, SEM and XRD, and fracture surfaces are examined by SEM. The results show that the cyclic stress-strain response is significantly affected by the Mn addition. The steel without Mn initially strain hardens and then strain softens until final failure, while the steels with Mn content have an initial stage of strain hardening followed by saturation. Concerning the fatigue response, the addition of Mn is detrimental under higher strain amplitudes but, on the contrary, leads to longer lives under lower strain amplitudes. The fatigue response is explained by the different microstructure morphologies associated with the Mn alloying element.