Wear of conventional pearlitic and improved bainitic rail steels

A major objective in the development of new rail materials is improving their wear performance. Historically, the main method to accomplish this goal was to manufacture rail steels with higher hardness. However, there is a limit to the hardness that can be achieved with conventional pearlitic rail steels. As a result, there has been an increased focus on improving rail steel hardness with alternative microstructures, such as bainitic microstructure, which indeed can achieve higher hardness than conventional pearlitic rail steels. Unexpectedly, the newly developed harder bainitic rails exhibited worst wear performance than conventional pearlitic rails when tested in actual rail track trials. In the first part of this paper, we obtained bainitic and pearlitic rail steel samples that were tested in an actual wheel/rail track. Using these samples, detailed micro-Vickers and Rockwell C hardness measurements at different length scales were conducted to investigate the cause of their wear behavior, namely the fact that softer pearlitic steels exhibited better wear performance than the harder bainitic steels. It was found that the initially softer pearlitic rail steel was work hardened more than the initially harder bainitic rail steel as in-service stresses accumulated on the rails, and thus the better wear performance. The second part of the paper describes pure sliding laboratory experiments that were performed using both pearlitic and bainitic samples. These simpler laboratory experiments, confirmed that indeed pearlitic steel work hardens more with tribological contact testing and exhibits less wear compared to J6 bainitic steel, supporting the rail track findings. It is therefore important to consider the in-service work microhardening behavior of rail steels as the initial hardness cannot reliably predict rail wear and rail life.