Vacuum low pressure carburizing heat treatment for heavy duty gears

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Heavy-duty gears are generally subjected to large torques during use. In order to ensure efficient and stable transmission of power when the parts are in service for a long time, they have high requirements for their fatigue performance. Such parts generally use carburizing and quenching methods to improve Hardness, improve the internal organization of the material to improve the comprehensive performance of the parts. The traditional atmosphere carburizing heat treatment technology is relatively mature and has a wide range of applications in the field of heavy-duty gear carburizing heat treatment. The improvement of traditional atmosphere carburizing heat treatment technology has gradually been unable to meet the design needs of future products.

Vacuum carburizing heat treatment technology has the characteristics of high production efficiency, no oxidation, and cleanliness, which can make up for the shortcomings of atmospheric carburizing heat treatment.

The processing part is a certain type of synchronizer gear hub for heavy-duty commercial vehicle gearboxes. The synchronizer can realize shifting synchronization in manual gearboxes. The gear hub is one of the core components of the synchronizer. The specific size of the parts is designed according to different box types. The typical structure is shown in the figure, the diameter of the outer circle is about φ150 mm, and the effective thickness is between 20 and 30 mm. It is subjected to periodic loads during use, and its tooth contact fatigue strength and surface wear resistance are relatively high. requirements. The material of the parts is 20CrMnTiH alloy steel.

Process comparison

The vacuum test was completed in a vacuum carburizing heat treatment furnace. The test process was vacuum carburizing + oil quenching (high pressure gas quenching) + tempering, the quenching temperature was 960 °C, and the total carburizing process time was 305 min. The specific carburizing and quenching process curve As shown in the figure, the saturation value adjustment method is used to circulate acetylene and protective nitrogen for surface carburization and internal diffusion; the traditional atmosphere carburizing and quenching test is completed in a box atmosphere furnace, and the test process is atmosphere carburizing + quenching + tempering, using Acetone, methanol and nitrogen were used as the carburizing atmosphere, the quenching temperature was 830 °C, and the total time of the carburizing process was 205 min. The process curve is shown in the figure. Because the traditional atmosphere carburizing temperature is lower than the vacuum carburizing temperature, the time required to reach the temperature during heating and cooling is shorter, and the overall process time is also shorter.

The differences in microstructure, hardness and heat treatment distortion of 20CrMnTiH alloy steel synchronizer gear hubs after heat treatment in vacuum carburizing + oil quenching, vacuum carburizing + high pressure gas quenching, and atmosphere carburizing + oil quenching are compared. A few conclusions:

1) Vacuum low pressure carburizing technology has great advantages over traditional atmosphere carburizing in intergranular oxidation and non-martensitic structure control. There is almost no intergranular oxidation and non-martensitic structure after heat treatment, which is conducive to improving carburizing heat treatment. The subsurface organization state of the rear parts improves the performance of the product.

2) Affected by the structure of the parts, for vacuum carburizing and atmosphere carburizing, under the premise of oil quenching, the hardness of the tooth root at the same depth is lower than that of the tooth surface after vacuum carburizing within a distance of 0.4mm. The hardness can be maintained at a relatively high level, and the hardness will continue to decrease from the surface to the core after atmospheric carburizing.

3) The quenching process has an important influence on the distortion of the heat treatment of the parts. The parts are cooled slowly during the high-pressure gas quenching process, resulting in a small temperature difference between the inside and outside, and the distortion of each part is more uniform. Both radial and circumferential directions show good deformation coordination. Compared with oil quenching, it has certain advantages in heat treatment distortion control.

Equipment selection: The VOGQ series vacuum heat treatment furnace produced by SIMUWU is a high-quality product for the vacuum heat treatment process of tooling and molds. Good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum heat treatment process. SIMUWU specializes in the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum air quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.