Fracture behavior of bainitic and pearlitic rail steel webs

Abstract

The failure of railway rail occurs mainly due to processes such as: detail fracture, shelling, and transverse fissures [1]. Plastic deformation of the railhead due to the heavy rolling loads transmitted through the wheel/rail contact surface is the main reason for the formation of detail fracture [2]. Such a failure process is visible, and catastrophic rail failure can be prevented by regular examination of the top surface of the railhead. The fracture behavior of railheads has been studied for a long time and some significant results have been obtained [3–8]. The study of the failure of rail webs is less reported. In webs, defect accumulation may occur under cyclic service loadings. Once the crack size reaches the sub-critical dimension, piping propagation along the vertical direction of the web can be accelerated, while the cyclic vertical tension loads can cause horizontal fissures of the web. Considering the complexity of the failure modes related to the cyclic loadings of rail tracks, it is necessary to study the fracture behavior of rail webs under a simplified loading profile first. Recently an improved bainitic steel for heavy load application has been studied [9–12]. It has some significant benefits over the existing pearlitic rail steels. Bainitic steels derive their strength from ultra-fine structures with a lot of dislocations which are harmless but confer high strength [13]. In contrast, pearlitic steels obtain their strength from the fine grains of pearlite. However, there is a limit to the production of very fine grains in the manufacturing and post-heat treatment processes. In the present work, the fracture behavior of specimens cut from bainitic and pearlitic steel rail webs was studied. An empirical model of mode I stress intensity factor was used to evaluate the plane strain fracture toughness of the two materials. Rail web specimens from the two materials, with simulated cracks emanating from a trough thickness circular defect, were prepared by electric discharge machining to simulate horizontal fissure of rail webs. Static tensile tests were performed under displacement control to study the overloading behavior and obtain the residual strength for the plane strain fracture toughness calculation based on this geometry. The rails used in the present work were new (have not been in service) and were supplied by the Transporta-