目的 采用试验研究、模拟计算和理论分析相结合的手段,对20MnCrS5齿轮钢真空低压渗碳过程的组织性能演变机理进行研究.方法 引入符合真空渗碳强渗、扩散交替进行的扩散边界条件,并修正硬度计算方程,开发真空低压渗碳高压气淬过程仿真模型.分别建立ϕ15 mm×100 mm圆棒试样二维轴对称和三维实体有限元模型,对20MnCrS5圆棒试样不同工艺参数下真空渗碳过程进行模拟仿真,开展真空渗碳试验与仿真分析化研究.结果 二维轴对称模型和三维实体模型计算精度接近,可以代替三维模型,提高计算效率.不同工艺参数真空渗碳过程得到的模拟和试验结果吻合较好,验证了改进模型和方程的可用性,并对不同工艺条件下碳浓度、组织和性能演变规律进行了研究.而后将模型应用到德国FZG(Forschungsstelle für Zahnräder und Getriebebau)标准齿轮样件,对其真空渗碳过程进行了模拟,结果可较好地反映齿轮不同位置碳浓度分布的特点,进一步验证了模型的准确性.结论 通过本研究,揭示了20MnCrS5齿轮钢真空低压渗碳过程的组织性能演变机理,并为复杂零部件真空渗碳过程工艺开发提供了新的思路.
Multi-field Coupling Simulation Analysis of 20MnCrS5 Vacuum Low Pressure Carburizing and Gas Quenching
Vacuum low-pressure carburizing stands as an advanced surface heat treatment technique,conferring strengthened casing with heightened fatigue resistance to transmission gears. In this study,a comprehensive approach,encompassing experimental research,computational simulations,and theoretical analysis were adopted to scrutinize the microstructural evolution within the vacuum low-pressure carburizing process of 20MnCrS5 gear steel. A multi-field coupling model,considering the synergistic effects of temperature,concentration,phase transformation,and stress,was introduced to simulate the vacuum carburizing process specific to 20MnCrS5 steel. The carbon diffusion model was used to incorporate the growth kinetics of carbide phases and compute concentration-dependent diffusion coefficients. The phase transformation behavior was characterized using the Johnson-Mehl-Avrami equation. Simulating the quenching process involved determining the heat convection coefficient based on an inverse analysis of cooling curves. Complex diffusion boundary conditions were implemented to depict alternating carburization and diffusion during industrial vacuum carburizing. Furthermore,diffusion boundary conditions were established to simulate the inherent alternating diffusion and strong carburization in vacuum carburizing. Corrections were applied to hardness calculation equations,and a simulation model was developed for the vacuum low-pressure carburizing process,followed by high-pressure gas quenching. It was recognized that three-dimensional models often demanded more nodes and elements,necessitating higher computing resources,a pragmatic approach was explored. In certain scenarios,a two-dimensional model was preferred to enhance computational efficiency. Two finite element models were constructed:one in two-dimensional axisymmetric geometry and the other in three-dimensional solid geometry. These models simulated the vacuum carburizing process for cylindrical rod samples (ϕ15 mm×100 mm) under varying process parameters. Results demonstrated that the two-dimensional axisymmetric model and the three-dimensional solid model exhibited comparable computational accuracy,offering practical alternatives while significantly improving computational efficiency. The simulation results,especially the simulated carbon distribution,closely aligned with experimental data under varying process parameters,affirming the utility of the refined model and equations. This study delved into carbon concentration,microstructural evolution,and performance patterns under different process conditions,offering crucial insights. Key findings included the minimal impact of pulse interval on carburization,increased carburized layer depth and a more uniform carbon concentration distribution under longer diffusion times and moderately strong carburization conditions,and heightened non-martensitic phase formation after quenching at elevated carburization temperatures,resulting in a greater hardened layer depth. Optimal vacuum carburizing for 20MnCrS5 alloy,satisfying bending fatigue strength requirements for heavy-duty gears,was determined as 930 ℃ for 42 min and diffusion for 140 min for vacuum carburizing based on experimental optimization. The model was subsequently applied to simulate the vacuum carburizing process of a standard gear specimen following the German FZG (Forschungsstelle für Zahnräder und Getriebebau) standard. Results accurately depicted carbon concentration distribution at various gear positions,providing additional validation of the model's accuracy. This study elucidates the microstructural evolution in the vacuum low-pressure carburizing process of 20MnCrS5 gear steel,offering valuable insights for advancing vacuum carburizing processes for intricate components. Furthermore,it contributes fresh insights into the composition-microstructure-property relationship in the vacuum carburizing process through an integrated theoretical and experimental approach. The model serves as a guide for the customized heat treatment of alloy steel gears,considering vacuum carburizing techniques for optimal mechanical performance.
万方数据
维普
