Vacuum heat treatment process optimization of reducer gear shaft

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1) The depth of the surface carburized layer is 0.8 mm~1.2 mm, the depth of the surface hardened layer is 1.7 mm~4.0 mm, and the surface hardness reaches above HRC55 to ensure excellent wear resistance and contact fatigue resistance;

2) The hardness of the core is HV350~420, ensuring that the core of the gear shaft has high toughness and high enough strength. If the core toughness is insufficient, the root of the gear shaft at the step is prone to cracks or fractures under impact load or overload. ;

3) It has good vacuum heat treatment process performance. At high carburizing temperature (900 ℃ ~ 950 ℃), austenite grains are not easy to grow and have good hardenability.

In order to meet the above performance characteristics of the gear shaft, the material composition of the gear shaft has the following requirements.

1) Low carbon, the carbon content is generally 0.15%~0.25%, so that the center of the part has sufficient plasticity and toughness;

2) Add alloy elements to improve hardenability, such as Cr, Ni, Mn, B, etc., often added to the material;

3) Add elements that hinder the growth of austenite grains, mainly adding a small amount of elements such as Ti, V, W, Mo, etc. that enhance carbide formation, to form stable alloy carbides.

According to the performance and composition characteristics of the gear shaft, the materials usually used for the gear shaft include 20CrMnMo, 20CrMnV, 20CrMnTi, 40Cr, 42CrMo, etc. Now, 20CrMnMo is used as the single-speed reducer gear shaft specimen material for testing. The chemical composition of the material is as follows: carbon C0.17~0.23, silicon Si0.17~0.37, manganese Mn0.9~1.2, molybdenum Mo0.2~0.3, chromium Cr1.25~1.65, nickel Ni ≤0.030, copper Cu ≤ 0.030, allowing residual sulfur S ≤ 0.03, phosphorus P ≤ 0.03, and the rest is iron Fe. This material is suitable for single-speed reducer intermediate gear shafts and clutch gear shafts that are subject to medium loads, good lubrication conditions and average impact force. It is also suitable for tie rod shafts of automobile oil pumps. Because after vacuum carburizing quenching and vacuum tempering, the surface hardness of the shaft reaches above HRC55, and the surface wear resistance is also good. Its core hardness is between HV350~420. 20CrMnMo steel also has high strength and high toughness. It has good hardenability, good comprehensive mechanical properties after heat treatment, high fatigue life limit and the ability to resist multiple impacts. In addition, 20CrMnMo steel also has the advantages of small vacuum quenching deformation, high strength and durable strength at high temperatures, but the disadvantage is high temper brittleness.

20CrMnMo steel has the above excellent properties, which are related to the alloying elements Cr, Mn, and Mo added to the steel. Cr and Mo can increase the concentration of carburization and improve the hardenability of steel. Cr can promote the secondary hardening of steel and improve the hardness and wear resistance of the carburized layer of steel. Tests have proven that when the Cr content reaches more than 12%, the steel has better high-temperature oxidation resistance and resistance to corrosion in oxidizing media. 20CrMnMo steel has better comprehensive mechanical properties after vacuum quenching and vacuum tempering, and can be used in infiltration. Chromium-containing carbides are formed in carbon steel, thereby improving the wear resistance of the steel surface; Mn easily forms manganese oxides in steel, which can quickly increase the strength and toughness of the steel, and can improve the hardenability of large-section gear shafts And the stability of vacuum tempering allows the gear shaft to be tempered at a higher temperature, which can eliminate the stress of machining and casting and forging, and optimize the plasticity and toughness of the gear shaft. ​

Mechanical processing and heat treatment process flow: The structural diagram of the gear shaft of the single-speed reducer is shown in the figure.

1.Bearing block

2.Gear

3.Bearing block

4.Oil seal block

When using 20CrMnMo steel to make a single-speed reducer gear shaft, the commonly used processing route is as follows: blanking → forging → rough and semi-finishing of the blank → vacuum carburizing → vacuum quenching → high-temperature vacuum tempering → high-frequency quenching of the gear → Grinding. The gear shaft sample is directly out of the furnace for vacuum oil quenching after vacuum carburizing, that is, the quenching temperature is consistent with the carburizing temperature. Vacuum heat treatment processes such as carburizing quenching and vacuum tempering can greatly improve the comprehensive mechanical properties of 20CrMnMo automotive reducer gear shafts. Therefore, optimizing the carburizing quenching and vacuum tempering processes can improve the quality level of the gear shaft, thereby improving the performance of the single-speed reducer assembly and the entire vehicle.

1) Unloading materials. Calculate the shaft diameter based on the function and stress of the shaft to be produced, and select round steel bars of the corresponding size to be cut into short bars on a sawing machine or cutting machine to facilitate forging.

2) Forging. Put the short bar stock into an electric furnace and heat it to 1 180 ℃ ± 10 ℃, then take it out and use die forging to form the required blank. The final forging temperature is 1 130 ℃. ​

3) Rough and semi-finishing of the blank. Install the blank forged in the previous process on the CNC lathe, rough machine the outer circle and end face at one end, and semi-finish the outer circle, inner hole and end face; then turn around and rough machine the outer circle, inner hole and end face at the other end, and change the tool to fine The outer circle, inner hole and end face of the other end of the car are processed; the processing roughness value is Ra3.2, and the grinding amount left on one side of the outer circle is 0.15 mm~0.2 mm; the gear is processed on the gear hobbing machine.

4) Carburizing and quenching. Put the roughly machined gear shaft into the vacuum quenching furnace. The gear shaft is vertically suspended into the furnace. The temperature in the furnace is heated to 925°C, and the content of the carbon atmosphere in the furnace is adjusted. The carburizing time is about 3 hours. , remove, quench with oil and cool to room temperature.

5) High temperature vacuum tempering. Put the quenched gear shaft into a vacuum tempering furnace, heat the gear shaft to be treated to 550 °C, keep it warm for 2 h, then open the furnace and air-cool to room temperature.

6) High frequency quenching of gears. The processed gears are subjected to high-frequency quenching treatment on the high-frequency quenching machine.

7) Grinding. The outer cylindrical surface of the bearing block and oil seal block of the gear shaft is ground on a cylindrical grinding machine.

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