Influence of liquid nitriding on the surface layer of M50 NiL steel for bearing applications

Abstract The synergistic effects of liquid nitriding (LN) duration and temperature on the characteristics of the nitriding region formed on M50NiL steel have been systematically examined. The nitriding procedure is conducted at temperatures 525, 550, 560, and 570°C for durations 3, 6, 8, and 12 h. This nitriding methodology enhances not only the hardness but also other attributes such as wear resistance, fatigue resistance, and surface hardness. The resulting nitrided layers are characterized through optical microscopy, scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction analysis. The findings indicate that the thickness of the nitride layer increases in correlation with the nitriding temperature. Empirical data demonstrated that the nitrides ε-Fe 2–3 (N, C) and γ′-Fe 4 (N,C) present within the compound layer contribute to the enhancement of microhardness. The study emphasizes the effect of LN duration on the tribological and chemical properties of the specimen. When M50NiL bearing steel is subjected to liquid nitriding at 525°C, the resulting nitrided layer primarily consists of a thin ε-Fe 2–3 N layer. The depth of the nitrided layer exhibited significant variation with increasing treatment temperature. The liquid nitriding process effectively enhanced the surface hardness. Energy-dispersive X-ray spectroscopy (EDS) analysis indicated an increased concentration of nitrogen in the nitrided sample treated at 560°C for 12 h. The steel specimen identified as LN525, which was nitride for 8 h, exhibited superior wear resistance compared to the other nitrided samples. Polarization studies reveal that after a 12 h nitriding period at 570°C, the developed expanded austenite began to contract concurrently with a reduction in the thickness, hardness, and corrosion resistance of the nitrided layer. The effects of prolonged nitriding time were attributed to the diminished chemical potential of nitrogen within the salt bath, resulting in the outward diffusion of nitrogen from the sample to the salt bath. Following nitriding at 525°C for 8 h, the nitrided layer of M50NiL steel demonstrated exceptional corrosion resistance.