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Low Pressure Vacuum Carburizing Heat Treatment Process for Transmission Gears

The automobile transmission is mainly composed of a series of gear transmission devices. These gears need to withstand a lot of friction and collision during use, so they need to improve their overall strength through heat treatment and carburizing process. In order to achieve low cost and high performance, the driven gear of the main reducer is heat treated with environment-friendly low-pressure vacuum carburizing and high-pressure gas quenching technologies.

The vacuum heat treatment process has the following advantages over conventional controlled atmosphere carburizing heat treatment:

1) High temperature technology can be used without producing harmful surface substances.

2) Carburizing under low pressure and vacuum has high surface activity and carburizing efficiency, and the process cycle can be reduced by 20%~50% compared with the conventional controlled atmosphere carburizing heat treatment.

3) When the part is in vacuum state, there will be no internal oxidation, surface non equestrian structure, surface black structure, etc., and there will be no surface alloying element depletion and surface hardenability reduction of the part. The surface hardness and residual compressive stress level of the parts are significantly improved, thereby significantly reducing the early surface failure of the parts and improving the service life of the parts.

4) The combination of low pressure vacuum carburizing and high pressure gas quenching technology can reduce heat treatment distortion and improve product accuracy.

Process parameters:

The material grade is QS1927S0. The technical requirements for heat treatment are: surface hardness 680 ～ 790HV10, core hardness 360 ～ 460HV10, effective hardened layer depth. The finished product is required to be 0.45 ～ 0.75mm (600HV1). Considering the grinding allowance, it is required to be 0.6 ～ 0.85mm (600HV1) after heating. The process flow of heat treatment is feeding → cleaning → degreasing+preoxidation → low pressure vacuum carburizing+high pressure gas quenching → low temperature tempering → air cooling → blanking.

Determination of carburizing and quenching process parameters:

The principle of low pressure vacuum carburizing heat treatment is actually to achieve carburization by means of multiple strong infiltration (through carburizing medium)+diffusion (through protective gas, such as nitrogen) and a centralized diffusion process in a low pressure (generally ≤ 2kPa) vacuum state using pulse mode. The control method of this process is “saturation value adjustment method”. By adjusting the ratio of carburizing and diffusion time, the surface carbon concentration and carburizing depth can be controlled.

In order to ensure the contact fatigue strength of the tooth surface, the carbon concentration of the tooth surface is generally controlled at 0.65%~0.95%. However, in the process of vacuum carburizing, too high carbon concentration will lead to too much retained austenite in the tooth angle, which will affect the service life of the parts.

The setting of carburizing flow is based on the surface area of the parts to be processed. Therefore, in order to accurately set and control the flow of carburizing medium, it is better to use a mass flowmeter. The main reduction gear is carburized with acetylene, and the gear surface area is 86,763.982mm ²， The furnace loading is 64 pieces/furnace, and the acetylene flow is 2000NL/h according to the empirical formula.

Quenching process

The quenching process is mainly divided into three processes of “fast, slow and fast”:

(1) Quickly keep the workpiece surface away from the nose of TTT curve, without intermediate transformation structure (bainite or pearlite).

(2) Slowly reduce the temperature difference between the surface and the center of the workpiece as much as possible. While the surface has completed martensite transformation, control the amount of martensite in the center (as much as possible into bainite) to reduce quenching distortion.

(3) Let the workpiece cool to room temperature as soon as possible to complete the whole quenching process.

According to our own experience, the method of changing the quenching pressure and time is adopted to realize the step quenching, and the cooling speed is controlled by the size of fan stirring, so as to realize the above quenching process.

Fixture of gear

H-shaped bar is selected for the main reduction gear, and the heat treatment is carried out in series (see figure), which is conducive to the uniformity of heating and quenching. The width and groove depth of the H-shaped bar are the key dimensions of the rack design. According to the principle of minimizing the center of gravity of deformation control during heat treatment and the size of parts, when the width of the H-shaped bar is 50 mm (wall thickness 8 mm), the center of gravity of the main reduction gear is the lowest and the trend of thermal deformation is the smallest.

The groove depth of H-shaped bar is 13mm. If the groove is too deep, the swing of the main reduction gear caused by the impact of high pressure gas during the gas quenching process will be restricted, thus affecting the flatness; On the contrary, the groove is too shallow to fix the workpiece, and the workpiece is prone to bruise during transportation, leading to an increase in scrap rate.

Low pressure vacuum carburizing technology is applied to the driven gear of the main reducer, and the strong carburizing and diffusion rhythm are reasonably optimized to obtain the ideal heat treatment hardened layer depth and structure. Grading quenching can realize the transformation of surface/core structure, and obtain ideal surface hardness and core hardness values. Symmetrical and compact part structure and reasonable material rack design can obtain ideal flatness of main reduction gear after heat treatment directly through high-pressure gas quenching.

In addition to mastering reasonable carburizing heat treatment process, the selection of heat treatment equipment will also affect the final product quality.

The RVC series vacuum carburizing+heat treatment integrated furnace produced by SIMUWU is a high-quality product for vacuum heat treatment process of tools and dies. Good temperature control accuracy and uniformity ensure the effective implementation of vacuum carburizing process. SIMUWU focuses on the manufacturing 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 gas quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.