扩散时间对20MnCrS5齿轮钢真空渗碳的影响

采用真空低压渗碳高压气淬工艺对20MnCrS5齿轮钢进行表面真空渗碳处理,分析扩散时间对硬度梯度、渗层深度、显微组织以及碳含量分布的影响,并优化真空渗碳工艺。结果表明,随着扩散时间的延长,C原子由表层向基体发生扩散,当扩散时间超过100 min后,C原子的扩散速度减缓;当C含量超过1.0%后,淬火后容易形成尺寸较大的残留奥氏体,随着C含量的降低,显微组织由孪晶马氏体向位错马氏体转变,硬度下降;在本试验条件下,20MnCrS5钢合适的真空低压渗碳高压气淬工艺为930 ℃强渗42 min,扩散140 min,0.6 MPa高压气淬至室温,并在160 ℃低温回火2 h。经该工艺处理后,组织中碳化物等级为1级,残留奥氏体等级为2级,马氏体等级为3级,表层无内氧化,渗碳层厚度约为0.91 mm,符合技术要求。

Effect of diffusion time on vacuum carburizing of 20MnCrS5 gear steel

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.