Influence of chemical composition and prior microstructure on diode laser hardening of railroad steels

Diode laser surface treatment of three different railroad steels, namely pearlitic railroad-rail steel (PRS), ferritic-pearlitic railroad-wheel steel (FPRS) and bainitic railroad-rail steel (BRS), has been investigated. Power-ramping experiments, with in-situ surface temperature measurement, were initially carried out to assess surface temperature effects on treated layers. Influence of laser energy on characteristics of treated layers like surface topology, case-depth, microstructure and micro-hardness was comprehensively analyzed. Results revealed that the hardened layer depth is governed by both the process parameters as well as the nature of the substrate steel. With melting, significant reduction in hardness was noted owing to an increase in residual austenite and coarsening of martensitic microstructure. Unlubricated dry-sliding wear tests performed under close to real rail/wheel conditions revealed substantial improvement in wear resistance of hardened/melted rail steels compared to the parent substrates. The observed differences in characteristics of the laser-treated layers and their tribological behavior have been correlated with the chemistry and prior microstructure of the parent substrates.