Laminar plasma jet surface hardening of the U75V rail steel: Insight into the hardening mechanism and control scheme

Abstract

Insight into the mechanism of laminar plasma jet (LPJ) surface hardening is critical for the controllable preparation of ideal hardened layer to improve the wear and fatigue resistance of U75V rail steel. To reveal the surface hardening mechanism, a series of surface hardening experiments on the U75V rail steel surface were performed under different working parameters using a self-designed laminar plasma generator. Results showed that the geometrical dimension, microstructure and hardness distribution of the hardened layer can be adjusted by changing the arc current, the scanning velocity and the anode nozzle diameter. Further characterization and numerical simulation suggested that the geometrical dimension of the hardened layer is determined by the heat affected zone over the critical austenitizing temperature (730 °C), while its microstructure and hardness distribution depend on the local cooling rate below the critical austenitizing temperature. By changing the working parameters, the heat flux density applied on the workpiece surface and the heating time can be controlled to obtain the desired temperature field within the heat affected zone. Not only a full hardened layer but also a transition layer consisting of martensite, pearlite, ferrite and carbides can be achieved. This configuration of LPJ surface hardening process for U75V rail steel indicates that the LPJ surface hardening with the ability of producing hardness gradient is promising for reducing the risk of crack generation on the rail.