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Gear low pressure vacuum carburizing

M1X transmission is a horizontal compact six-speed manual transmission and AMT transmission. According to the original design requirements, in order to achieve low cost and high performance, the main reduction driven gear adopts environmentally friendly low pressure vacuum carburizing and high pressure gas quenching technology for heat treatment.

Compared with conventional controlled atmosphere carburizing heat treatment, this process has the following advantages:

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

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

3) When the parts are in a vacuum state, there will be no internal oxidation, surface non-horizontal structure and surface black structure, etc. At the same time, the parts will not have problems such as depletion of surface alloy elements and the reduction of surface hardenability. The surface hardness of the parts and the level of residual compressive stress on the surface are significantly improved, thereby significantly reducing the early failure of the surface 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.

The technical requirements for heat treatment of MIX main reduction driven gear are: surface hardness 680~ 790HV10, core hardness 360~460HV10; effective hardened layer depth, the finished product requires 0.45~0.75mm (600HV1), considering the grinding allowance, the heat requires 0.6~ 0.85mm (600HV1).

The vacuum heat treatment process flow is: feeding → cleaning → degreasing + pre-oxidation → 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 in the low-pressure (generally ≤2kPa) vacuum state, using pulse mode, through multiple strong infiltration (passing into carburizing medium) + diffusion (passing protective gas, such as nitrogen) and A concentrated diffusion process to realize the carburization process and meet the requirements of the depth of the infiltrated layer and the metallographic structure of the pattern. The control method of this process is the “saturation value adjustment method”, that is, in the strong infiltration period, the austenite is dissolved in carbon atoms and saturated, and in the diffusion period, the dissolved carbon atoms are diffused to the inside to reach the target required value. The ratio of carbon and diffusion time to achieve the purpose of controlling the surface carbon concentration and the depth of the carburized layer.

The input parameters of the low-pressure vacuum carburizing simulation program include the carburizing temperature, the original carbon concentration of the workpiece to be carburized, and the surface saturated carbon concentration after carburizing. Surface carbon concentration after diffusion, final surface carbon concentration, depth of infiltration layer and enrichment rate of medium on workpiece surface.

Flow setting of carburizing medium in low pressure vacuum carburizing process

The setting is based on the surface area of the processed parts, so in order to accurately set and control the flow rate of the carburizing medium, it is best to use a mass flow meter. The M1X main reduction gear is carburized with acetylene, and the empirical formula for calculating the acetylene flow rate is: acetylene flow rate (NL/h) = 1250 (NL/h) +125 (NLh.mm2) x parts surface area (mm2) x number of parts/106. In general, the actual acetylene flow value can be determined by about 10% in the calculation result according to the actual situation. The surface area of the MIX main reduction gear is 86763.982mm2, and the furnace load is 64 pieces/furnace. According to the empirical formula, the acetylene flow rate is calculated as 1944NL/h, and the actual process is 2000NLh.

Determination of quenching process

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

(1) Fast

Let the workpiece surface avoid the nose tip of the TTT curve, and no intermediate transformation structure (bainite or pearlite) will be generated.

(2) slow

Minimize the temperature difference between the surface and the core of the workpiece. When the surface has completed the martensite transformation, control the amount of martensite at the core (as much as possible to transform into bainite) to reduce quenching distortion.

(3) Fast

Allow the workpiece to cool to room temperature as quickly as possible to complete the entire quenching process.

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