Effect of shot peening and plasma nitriding processes on the surface properties and wear behavior of R260 rail steel

The increasingly demanding loading conditions in railway networks necessitate the use of rail steels with high wear resistance and superior fatigue performance. Rails, as one of the fundamental components of the railway superstructure, are subjected to rolling, impact, and sliding stresses during service. In particular, impact-sliding stresses occur during the transition of the wheel from the rail wing to the crossing nose; this transition generates extremely high contact forces at the crossing nose, which can lead to severe damage in both the crossing nose and the wing rail. The present study applied shot peening to R260 rail steel under three different conditions (16A + 200%, 24A + 200%, and 24A + 1000%). Subsequently, plasma nitriding was performed on both untreated and shot-peened rail steels. Plasma nitriding was conducted for 12 hours at temperatures of 450 °C, 500 °C, and 540 °C using gas mixtures of 20% N2+%80 H2 and 80% N2+%20 H2. To the authors' knowledge, no prior studies have applied shot peening to rail steels before plasma nitriding. Surface characterization of the treated steels was carried out using optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), and microhardness testing. Dry sliding wear tests were conducted under atmospheric conditions on a flat surface using a ball-on-plate configuration with four different normal loads (15, 30, 45, and 60 N) in a reciprocating motion. In addition, a newly developed impact-sliding wear method was employed to investigate the combined effects of forces on untreated and surface-treated rail steels. These tests were performed using a 10 mm diameter hardened steel ball for 4297 cycles (60 m). Increasing both the Almen intensity and the surface coverage ratio resulted in the formation of a gradient structure in the surface layer of R260 rail steel, characterized by high compressive residual stress and pronounced surface hardening. The improvements in grain refinement, compressive residual stress, and surface hardness achieved by increasing the coverage ratio were observed to be more pronounced than those obtained by merely increasing the Almen intensity. The steel treated under the 24A + 1000% shot peening condition exhibited approximately 1.24 GPa compressive residual stress and a surface hardness of approximately 410 HV0.01. However, the shot peening process also increased the surface roughness. The dry sliding and impact-sliding wear resistances of all shot-peened steels were lower than those of untreated R260 rail steel. Only the steel treated under the 24A + 1000% condition showed an approximately 6.5% increase in wear resistance in the impact region compared to the untreated steel. In plasma nitriding treatments, for steels nitrided in a gas mixture of 20% N2+%80 H2, increasing the temperature led to an increase in white layer thickness and γˈ-Fe4N phase content, while the ε-Fe2-3N phase content decreased. Conversely, in steels nitrided in 80% N2+%20 H2, higher temperatures resulted in increased white layer thickness and ε-Fe2-3N phase content. It was also observed that increasing the nitrogen content in the gas mixture led to higher white layer thickness and surface roughness. Dry sliding and impact-sliding wear tests conducted on plasma-nitrided steels under 30 N and 60 N loads indicated that their wear resistances were higher than those of untreated R260 rail steel. Steels with a two-phase (γˈ-Fe4N + ε-Fe2-3N) white layer exhibited better wear resistance than those with a single-phase (γˈ-Fe4N) white layer. The pre-treatment shot peening generally reduced the white layer thickness while increasing the diffusion layer thickness. Moreover, shot peening promoted the formation of the α-Fe phase in steels nitrided in 20% N2+%80 H2 and the γˈ-Fe4N phase in steels nitrided in 80% N2+%20 H2. Nevertheless, the wear volumes of shot-peened and subsequently nitrided steels increased due to the elevated surface roughness.