Effect of carburizing time and diffusion time on the carbon diffusion behavior of 18CrNiMo7-6 gear steel during two-stage gas carburizing

The reasonable coordination of carburizing time and diffusion time in two-stage carburizing is key to balancing carburizing efficiency and microstructural performance. This research establishes a finite element model for the carbon diffusion process during the two-stage gas carburizing of 18CrNiMo7-6 gear steel and conducts experimental validation. Based on this, a quantitative study is conducted on the influence of the selection of strong carburizing time and diffusion time on the carbon concentration gradient distribution of the carburized layer. The results indicate that the diffusion behavior of carbon element at the surface and core of the workpiece shows distinct differences during the carburizing and diffusion stages. During the carburizing stage, the carbon concentration at both the surface and the core gradually increases, but the rate of increase at the surface is significantly higher than that at the core. In the early stage of the diffusion stage, the carbon concentration at the surface rapidly decreases while that at the core rapidly increases. After reaching a certain critical diffusion time, the surface carbon concentration changes little, while the core carbon concentration continues to increase slowly. Considering both the enhancement of carburizing efficiency and the desired carbon concentration distribution, the carburizing time can be extended while ensuring that the diffusion time does not fall below the critical diffusion time.