Influence of gas nitriding on the surface layer of M50 NiL steel for aerospace-bearing applications

Abstract Currently, there is an increased demand for M50NiL steel from the aerospace, automotive, and nuclear industries to improve the characteristics of components, such as wear resistance, fatigue resistance, and surface hardness, while maintaining its corrosion resistance. Previous research has demonstrated that the nitride parameters are duration, temperature, and flow rate. The current investigation examines the efficacy of gas nitride on M50NiL steel from 500 to 550°C over 08–90 h according to the specific requirements of the specimens maintaining a gas ratio of 75:25 (NH 3 :NH 3 diss), a typical operational pressure within the range of 1–2 mbar, and an NH 3 flow rate of 0.4–0.5 l min −1 . Using a hydrogen chloride mixture as an activator for the steel surface, gas nitriding (GN) treatment is carried out in a partially dissociated ammonia atmosphere, resulting in the formation of the layers that are seen. In order to examine the phase transitions within the phase architecture and the chemical constituents of the gas-nitrided layer, methodologies such as energy-dispersive spectroscopy (EDS) and X-ray diffraction (XRD) were employed. The M50 NiL steel specimen, subjected to a thermal treatment at 550°C for a duration of 12 h, was found to possess an anomalously elevated concentration of nitrogen, according to the EDS evaluation. The results obtained from the XRD analysis of the gas nitrided layer indicated the existence of iron nitride phases, predominantly comprising α′-Fe, γ′-Fe4N, and α′N (nitrogen-enriched martensite). By micro-hardness assessment, it is apparent that at a temperature of 500°C over 24 h, the highest hardness value of 1,070 HV was achieved in the nitrided sample, which is approximately fourfold greater than that of the untreated specimen. In the case of case depth, it was observed that at 550°C for a duration of 24 h, the GN treatment resulted in a maximum case depth of 134 μm. The M50NiL specimen, designated GN 500, which underwent nitriding for 24 h, demonstrated the greatest wear resistance among the nitrided samples. Nitrided specimens are subjected to characterization along with electrochemical potentiodynamic corrosion assessment in an aerated 3.5% NaCl solution. The electrochemical evaluations indicated a substantial reduction in the corrosion current density of the specimen’s post-nitriding, accompanied by a shift in the corrosion potential toward the noble direction with the extension of nitriding duration. The results of M50 NiL steel surface characteristics are enhanced after applying gas nitride coating. The findings indicate that controlled pressure GN effectively inhibits the precipitation of chromium nitrides, which is advantageous for the attainment of a thicker nitrided layer.