Vacuum annealing process of automobile half shaft

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1.Half shaft

The half shaft is also called the drive axle. The half shaft is a solid shaft that transmits torque between the differential and the drive wheel.

The half-shafts commonly used in modern automobiles have two types, full-floating and semi-floating, according to their different bearing types.

The full-floating axle shaft only transmits torque and does not bear any reaction force and bending moment, so it is widely used in various types of vehicles. The full-floating axle shaft is easy to disassemble and assemble, just pull down the bolt on the flange of the axle shaft to pull out the axle shaft. And the wheel writing axle housing can still support the automobile army, thus bringing convenience to the maintenance of the car. Semi-floating axle shafts both transmit torque and withstand all reaction forces and bending moments. Its supporting structure is simple and the cost is low, so it is widely used in various types of cars with small reaction bending moment. However, this kind of half-shaft support is troublesome to remove, and if the half-shaft is broken while the car is running in a straight line, it is easy to cause the danger of the wheels flying off.

2.Working conditions and performance requirements of the half shaft

1)Work force

The axle shaft is the part that transmits the torque of the engine and drives the wheels to turn. Take the rear axle half shaft of a truck as an example. One end is connected to the rear axle transmission mechanism, and the other end is connected to the wheel side planetary differential transmission mechanism.

When the axle shaft is working, it is subjected to the torsional load of the engine and the impact force caused by bumps. In addition, the half shaft is subject to a large aspect ratio and is also subject to bending stress.

2) Form of failure

Torsional Fatigue Fracture Excessive Wear Excessive Deformation

3) Performance requirements

According to the working conditions and failure modes, the half shaft should have good comprehensive mechanical properties.

Hardness, the core is 26~32HRC; the surface is 50~60HRC, and the depth of the hardened layer of the surface intermediate frequency quenching is 6~8mm

Metallographic structure surface tempered sorbite + part of trosteite, the core is allowed to have about 8% ferrite.

3.The choice of steel

According to performance requirements and technical requirements, half shafts are generally made of quenched and tempered steel. Commonly used steel grades are: 40Cr, 40MnB, 35CrMo, 40CrNiMo, 42CrMo, etc. According to the size and hardness requirements of the half shaft, the above steels all meet the requirements, but from the comprehensive consideration of manufacturability and material management, it is more appropriate to choose 42CrMo steel.

4.Half shaft vacuum heat treatment

1)Processing route

Half-shaft processing route: blanking→forging→vacuum annealing→copying→quenching and tempering→milling splines→intermediate frequency surface quenching, low temperature tempering→straightening→inspection.

2) Vacuum annealing

2-1 Purpose of Annealing

Homogenize the chemical composition and structure of steel, refine grains, adjust hardness, eliminate internal stress and work hardening, improve the forming and machinability of steel, and prepare structure for quenching.

2-2Heating temperature

The complete annealing temperature of alloy steel is generally: Acs + (50~70). Refer to the heat treatment manual. Considering the critical temperature of 42CrMo at 760~790 °C, the final choice of heating temperature is: 840 °C.

2-3 Heating speed

Heating rate: low alloy steel 100 ℃/h, using to warm into the furnace or preheat heating.

2-4Holding time

The annealing time should consider factors such as chemical composition, original structure, and furnace loading. For low alloy steel, when the furnace load is not large, the empirical formula can be used: t=aD=90min

t= (1.5~2)×46=69~92min

The cooling rate of low alloy steel annealing is generally controlled at: 50~100℃/h.

Note: (1) Faster cooling rate will lead to smaller spacing between Р pieces and S, resulting in high hardness, which is not conducive to cutting.

The slower cooling rate will not only lead to an increase in the spacing between P-pieces, but also large blocks of F will appear, which will reduce the strength and hardness, “stick the knife” during cutting, and cause soft spots during quenching.

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