Microstructure, hardness and tribological behaviour of plasma nitrided R260 rail steel with different nitrogen and hydrogen gas mixtures

Railway components are essential parts of transportation infrastructure, withstanding complex and dynamic loads during their operational lifespan. This work aims to improve the wear resistance of railway rails by investigating the correlation between the microstructure, hardness and tribological behaviour of R260 pearlitic rail steel. This will be achieved through a twelve-hour plasma nitriding process at a temperature of 450 °C, using two different N2-H2 gas mixtures (20 vol. % N2 + 80 vol. % H2 and 80 vol. % N2 + 20 vol. % H2). The treated surfaces were characterised using light optical microscopy, scanning electron microscopy, X-ray diffraction and microhardness testing. The unlubricated tribological behaviour of plasma-nitrided steel was investigated in a ball-on-flat tribometer at room temperature. The 20 and 80 vol. % N2 gas mixture led to a biphasic compound layer of ε-Fe2-3N and γˈ-Fe4N. As the amount of nitrogen in the gas mixture increased, the total compound layer, surface roughness and surface residual compressive stress increased. In the tribological tests, the steel treated with a nitrogen-rich gas mixture (80 vol. % N2) yielded better results due to the formation of a thicker compound layer, a deeper case-hardening and higher compressive residual stress within the nitride layer.