关键词: M50NiL钢, 渗碳, 等离子渗氮, 脉状组织

Abstract: Effect of plasma nitriding temperature on the formation of vein-like structures in carburized M50NiL gear steel was investigated. Metallographic microscopes, scanning electron microscopes (SEM) and automatic microhardness tester were used to analyze the carbonitrides, element distribution and microhardness of the combined carburizing-nitriding layer. The results show that as the plasma nitriding temperature decreases, the thickness of the surface white layer gradually reduces, which is approximately 10, 3.6 and 2.7 μm at the plasma nitriding temperature of 520, 495 and 470 ℃, respectively. The carburized layer thickness of the carburized specimens is close to 1900 μm, with a surface hardness of 740 HV0.2. After plasma nitriding, the surface hardness of the specimens reaches nearly 1200 HV0.2, by an increase of 62% compared to the carburized specimens. After plasma nitriding at 520 ℃, a large number of vein-like precipitates appear in the combined layer of the M50NiL steel, which is primarily carbonitrides and mainly accumulate at grain boundaries. When the plasma nitriding temperature is reduced from 520 ℃ to 470 ℃, the morphology of carbonitride precipitates is further improved to exhibit a circular or elliptical shape, with a small amount in a short rod-like form, and the vein-like structures are basically eliminated. This combined carburizing-nitriding layer structure possesses ultra-high hardness, which further increases the surface residual compressive stress of the material and can prevent the rapid initiation of cracks at grain boundaries where vein-like structures tend to accumulate during service, thereby contributing to the improvement of the contact fatigue strength and bending fatigue strength of components.

Key words: M50NiL steel, carburizing, plasma nitriding, vein-like structure