Abstract: The gear steel 20MnCrS5 was subjected to vacuum low pressure carburizing high pressure gas quenching treatment, and the effects of diffusion time on hardness gradient, carburized layer depth, microstructure, and carbon distribution were analyzed, and the vacuum carburizing process was optimized. The results show that carbon atoms diffuse from the surface to substrate with the prolongation of diffusion time. When the diffusion time exceeds 100 min, the diffusion speed of carbon slows down. When the carbon content exceeds 1.0%, it is easy to form large-sized retained austenite after quenching. While with the decrease of carbon content, the microstructure changes from twin martensite to dislocation martensite, and the hardness decreases. Under the experimental conditions, the suitable vacuum carburizing process for the 20MnCrS5 steel is vacuum low pressure carburizing at 930 ℃ for 42 min, diffusing for 140 min, and high pressure gas quenching at 0.6 MPa, then tempering at 160 ℃ for 2 h. After carburizing by this process, the carbide in the microstructure is Grade 1, the retained austenite is Grade 2, and the martensite is Grade 3. There is no internal oxidation on the surface layer, and the thickness of the carburized layer is about 0.91 mm, which meet the technical requirements.

Key words: 20MnCrS5 steel, vacuum carburizing, diffusion time, carbon content, carburized layer depth, hardness gradient, microstructure