This paper addresses the effect of gear steel on distortion and residual stresses due to phase transformation in carburizing and quenching. In particular, the martensitic and bainitic phase transformation expansion and transformation plasticity properties of two automotive gearbox steels (20CrMnTiH and 20MnCr5) and their physical parameters are measured by experiments of transformation plasticity properties. Numerical simulations of the actual carburizing and quenching process of the gearbox spline helical gears were carried out in combination with the thermal and mechanical properties with temperature variations calculated by the material design software JMAT-Pro. In particular, the phase transformation properties of the two materials and their influence on the distortion and residual stresses after carburizing and quenching were verified by experiments of transformation plasticity and numerical simulations. A reliable basis is provided for predicting the distortion mechanism of gear steels in carburizing and quenching.

Based on the multi-field coupling effect of temperature, diffusion, and phase change, the finite element model of carburizing and quenching was established. The 20CrMnTiH steel helical gear as the research object, prediction accuracy of carburizing, and quenching model of complex helical gear was studied. The material properties database of experimental steel was established by JMatPro, and the material thermophysical parameters needed in the calculation process were obtained. The carburizing and quenching process of transmission helical gear was numerically simulated by thermodynamic three-dimensional coupling analysis method combined with actual heat treatment process. The microstructure morphology, macro hardness, and deformation were characterized. The experimental results show that the microstructure of the hardened surface layer was acicular martensite and a small amount of residual austenite. The highest hardness appears at the surface layer of 778.8 HV, the effective hardened layer depth was 0.9 mm, and the maximum deformation of the gear was 0.055 mm. By comparing the experimental measurement results with the simulation results, they were in good agreement, which verifies the accuracy of the finite element model. This indicates that the model has good prediction ability in carburizing and quenching process.

In this paper, the elliptical deformation mechanism of the bearing ring after quenching has been studied by using the theory of “metal-thermal-mechanical” and the multi-field coupling simulation method. It has been mainly considered that it occurred as an experimental phenomenon in the past that, when the bearing ring had been oil quenched, a vapor film had usually formed on its surface, and the breakage and boiling of the vapor film had been observed to be uneven in a circumferential direction. For this reason, based on the phase transition kinetics, several types of heat transfer boundary conditions on the outer surface of bearing rings during oil quenching have been assumed, and the multi-field coupled simulation of the quenching process of bearing rings has been carried out in this paper. According to the simulation results, it could be concluded that when two different heat transfer boundary conditions have been set on the outer surface of the bearing outer ring in the form of orthogonal symmetry, the quenched bearing outer ring would produce elliptical deformation. The above simulation results have been basically verified by comparing the experimental results.

Bearings are usually used at high-speed and high-load conditions, so the bearing sleeve must be able to withstand large contact stress, which requires heat treatment. However, during the quenching process of the bearing sleeve, a vapor film is formed on the surface. Since the vapor film is not uniform along the axial direction during the process of boiling and breaking, it will cause uneven distortion of the bearing sleeve. This uneven distortion will cause the outer ring of the bearing to have elliptical distortion after heat treatment. However, the mechanism of this problem and how to reduce the elliptical distortion has not been completely solved. In this paper, two points on the inner wall and outer wall of the bearing sleeve in the orthogonal direction are measured by means of experiments, and the mechanism of the ellipticity distortion during the quenching process is revealed. Additionally, through its horizontal placement, oblique placement, and vertical placement, the cooling curve and cooling rate curve changes in the process of entering the quenching agent are studied; based on the cooling curve, the heat transfer coefficient of the bearing sleeve during quenching is obtained by the heat transfer equation. Input the heat transfer coefficient into the CAE heat treatment simulation software COSMAP, according to the Metallo-Thermo-Mechanics theory and the method of multi-field coupling, simulating the quenching process of GCr15 and 16MnCr5 materials. By comparison, the ellipticity value of the bearing placed at a 45° angle is smaller than that of the horizontally and vertically placed bearings, with the 16MnCr5 material having the smallest ellipticity value of 0.00016. The simulation results verified the distortion mechanism of the bearing sleeve during the quenching process and how to reduce the solution of elliptical distortion.