Abstract: 20CrMnTi carburizing steel was subjected to vacuum carburizing treatment by using vacuum low-pressure pulse carburizing technology and quenching and tempering treatment, and the effect of diffusion time on the carbon concentration of the carburized layer, hardness, and microstructure was analyzed. The results show that after vacuum low-pressure pulse carburizing, when the diffusion time is short, e. g., 1125 min, due to the large surface carbon concentration gradient, significant internal stress is generated between the surface and subsurface layers, which can easily lead to internal cracks. When the diffusion time extends to 2125 min, the surface carbon content significantly decreases from 1.30% to 0.80%, the carbon content changes slowly from the surface to the core, and the surface hardness changes smoothly. When the diffusion time is short, after quenching, large-sized carbides and a large amount of retained austenite are observed in the surface layer. When the diffusion time prolongs, the carbide size and retained austenite content in the surface structure are significantly reduced. After 960 ℃ pulse vacuum low-pressure carburizing treatment for 52 min+diffusion for 2125 min, cooling to 840 ℃ for oil quenching, and then tempering at 180 ℃ for 2 h, the carbide in the carburized layer is grade 1, the core microstructure is grade 2, and the effective depth of carburized layer is about 3.12 mm, and the hardness changes smoothly from the surface to core, meeting the requirements of carburizing.

Key words: 20CrMnTi steel, vacuum carburizing, diffusion time, microstructure, hardness gradient