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Optimal Vacuum Heat Treatment for Bevel Gears

Bevel gears have the advantages of high coincidence, stable transmission, and large load-carrying capacity, and are widely used in various mechanical equipment. The strength and hardness requirements of bevel gears are relatively high, and vacuum carburizing and quenching are often used for strengthening treatment in manufacturing. However, the vacuum carburizing and quenching process is complicated, and it is easy to generate thermal stress and tissue stress, which will cause large deformation and affect the transmission quality of the gear.

Bevel gear and vacuum carburizing and quenching process

The bevel gear material is SNC815 carburized steel. Considering the calculation efficiency and economy, the workpiece is simplified during the analysis, and one tooth of the bevel gear is taken for simulation, and the contact surface between this tooth and the rest of the teeth is regarded as the symmetry plane. When dividing the tetrahedral mesh for the single-tooth model, the number of meshes is set to 60,000. The mesh division effect of the bevel gear single-tooth model is shown in Fig. 1.

The vacuum carburizing process of bevel gears is: preheating at 550°C, strong infiltration in a vacuum carburizing furnace for 2 hours, and a carbon atmosphere of 1.1%; diffusion in a vacuum carburizing furnace for 2 hours, and a carbon atmosphere of 0.78%. Obviously, the time ratio of the diffusion period to the hyperosmotic period is 1:1.

The vacuum quenching process of bevel gears is as follows: the initial design vacuum quenching temperature is 840°C; the oil temperature is 60°C when using rapid quenching oil for quenching.

Vacuum carburizing and martensitic transformation of bevel gears

The deformation of the bevel gear during the quenching process is mainly the result of the following two stresses: one is thermal stress, that is, the cooling rate of different parts of the bevel gear is inconsistent during quenching, and the temperature changes are different, resulting in inconsistent cold shrinkage of different parts The other is structural stress, that is, the bevel gear will undergo austenite to martensite transformation during quenching. Due to the different cooling rates between the core and the surface of the bevel gear, austenite to martensite will occur. The order and transformation amount of the body transformation are also inconsistent, and the martensitic transformation will lead to volume expansion, so the inconsistency of the transformation process of the different parts of the bevel gear will generate tissue stress. For vacuum carburized bevel gears, due to the existence of a certain vacuum carburized layer, the carbon content of the gears decreases continuously from the surface to the inside, resulting in a gradual increase in the martensitic transformation temperature from the surface to the core, so its microstructure The role of stress is more complex. Vacuum carburized bevel gears are simultaneously subjected to thermal stress and tissue stress during quenching, resulting in deformation of bevel gears. Therefore, for the finite element analysis of vacuum carburized bevel gear deformation, it is very important to understand the vacuum carburization and martensitic transformation. the

Through the DEFORM finite element analysis, the carbon content analysis results of the bevel gear vacuum carburizing process can be obtained (Fig. 2)

From the analysis of Figure 2, it can be seen that in the process of vacuum carburizing, the maximum carbon content of the bevel gear reaches 1.080% after intensive carburizing, and the carbon content is relatively high, but the vacuum carburizing layer is too thin, so it is necessary to continue vacuum carburizing during the diffusion period; The maximum carbon content reached 0.793% after diffusion, and the vacuum carburized layer became thicker.

During the vacuum quenching process, the cooling speed of the bevel gear surface nodes is faster than that of the core nodes. The analysis shows that the temperature field changes in the surface node area and the core node area of the bevel gear vacuum quenching process are different, and the cooling shrinkage process is inconsistent, which leads to the generation of thermal stress. the

When the vacuum quenching temperature is 840°C, the content of martensite on the surface of the bevel gear is 92.9% to 94.8%, and the content of martensite in the core is the highest, which is 96.8%. This shows that during the vacuum quenching process of the bevel gear, the core undergoes martensitic transformation before the surface. The analysis shows that because the carbon content in the core of the bevel gear is low, the temperature of the martensitic transformation is high, and the vacuum quenching process reaches the martensitic transformation temperature before the surface. Therefore, even if the cooling speed of the core is slow, its The densitic phase transformation also occurs earlier than the surface.

Optimization results of vacuum heat treatment process

In order to solve the deformation problem of bevel gears, it is necessary to adjust the process parameters of vacuum carburizing and quenching. The higher the vacuum quenching temperature, the greater the temperature difference between the oil and the workpiece during oil quenching, the greater the shrinkage deformation, the longer the cooling time, and the greater the structural stress during martensitic transformation, which will increase the bevel gear deformation. The higher the temperature of the vacuum quenching medium (referring to the oil temperature), the smaller the temperature difference between the bevel gear and the cooling medium, the smaller the deformation of cooling shrinkage, the smaller the thermal stress generated during vacuum quenching, and the martensite content during vacuum quenching The smaller is, the corresponding volume expansion is weakened, and the deformation of the bevel gear is also reduced. In this paper, the vacuum quenching temperature of the bevel gear is set to 820°C, and the quenching oil temperature is set to 40°C, 60°C, 80°C, and 100°C, and the deformation analysis is carried out respectively.

The analysis found that under the condition of quenching temperature of 820℃ and oil temperature of 100℃, the deformation of the big end and the small end of the bevel gear are the smallest. In order to optimize the vacuum carburizing and quenching process of bevel gears, the vacuum quenching temperature can be adjusted to 820°C, and the oil temperature can be set to 100°C.

The deformation of the bevel gear after process optimization still does not meet the technical requirements, and the press quenching process can be used to further reduce the deformation of the bevel gear. In this paper, according to the deformation characteristics of bevel gears, 1800N, 3000N and 5000N forces are applied to the deformed areas of the crown of the large end and the inner side of the small end to constrain the deformation of the bevel gear. After the constraint force of 5000N is applied, the deformation of the bevel gear is small, which can meet the processing technical requirements of the bevel gear.