Reasonable Choice Of New Energy Gear Shaft Forging And Heat Treatment Process

 

With the continuous development and upgrading of new energy vehicles, gear shafts as key components in the automotive transmission system also put forward higher requirements for their production. The speed of the new energy-toothed shaft is generally 12000 ~ 16000r/min. The noise is not large due to the absence of engine noise, and the gearbox NVH requirements are prominent.

So the precision requirements for the toothed shaft are higher than those of fuel vehicles. The force situation is a bit complex when the gear shaft works in the transmission system. So the gear shaft should have good mechanical properties such as impact resistance, wear resistance, high strength and hardness, and good internal structure properties to ensure that it can work normally in high-load and high-speed movement.

Forging Process Selection

At present, hot die forging is generally used for disc tooth parts. The main reason is that forging is relatively large, and hot die forging is easier to achieve from the process point of view. The forging of shaft parts requires hot die forging, cold forging, or warm forging according to the size and cost of the part.

Hot Die Forging

In the hot die forging process, the metal flow is quite complex, and defects such as folding and insufficient filling may occur during forming. The corresponding mold will have serious wear and mold cracking.

Cold forging

Cold forging process is now popular in the production process of shaft parts. The main advantages are long mold life, good productivity, high product consistency, can effectively reduce machining allowance, shorten machining time, and reduce costs. However, the cold forging process also has its inherent disadvantages.

First of all, before forging, the annealing process is generally helps to improve workpiece plasticity. The spherical pearlite structure is formed by annealing when the subsequent processing process has high-frequency quenching requirements. The hardness of quenching cannot reach the hardness that may be achieved by the flake pearlite structure after normalizing or tempered sostenite structure after tempering. Therefore, it is necessary to increase normalizing or tempering treatment.

Secondly, heat treatment deformation is difficult to master. After normalizing or tempering treatment, the structure of the blank should undergo the recrystallization process which can be difficult to achieve accurately.

Warm forging

After warm forging or hot forging blanks, waste heat normalizing treatment is generally adopted, which has a low cost and saves energy. However, there is a certain gap in the uniformity of the structure compared with the normalizing process. If the cost permits, isothermal is ideal for normalizing, which realizes the effective control of the quality of the tooth blank of transmission gears and shaft parts. This improves the machinability and heat treatment deformation stability.

The key to the formulation of The isothermal normalizing process is to reasonably control the speed and time of fast cooling as well as slow cooling and isothermal treatment in the intermediate cooling stage according to the austenite isothermal transformation curve. Isothermal normalizing can obtain uniform microstructure and hardness compared to ordinary normalizing. Pretreatment of isothermal parts reliably achieves good machinability and stable quenching deformation.

Heat Treatment Process Selection

Carburizing quenching is the main technical method of hardening gear surfaces. However, the deformation problem after carburizing and quenching seriously affects the quality of gear processing. At present, the carburizing heat treatment of toothed shafts of new energy vehicles is generally divided into 3 categories: atmosphere furnace carburizing oil quenching, vacuum carburizing oil quenching, and vacuum carburizing gas quenching.

Carburizing in conventional atmosphere furnaces and carburizing in vacuum furnaces comparison

1. Carburizing in a traditional atmosphere furnace

(1) Working environment. Gas carburizing process under the condition of an ordinary atmosphere.

(2) Flexibility of heat treatment process. The carbon potential concentration requirements on the surface of parts vary greatly. It takes a long time to adjust the carbon potential.

(3) Part quality. The amount of carbon on the surface of the carburized layer and the carburizing depth control cannot be accurate. The carburizing effect is relatively uneven; long operation time cycle; After carburizing, the parts have intergranular oxidation. The surface is gray, and the surface roughness is above Ra<>μm.

(4) Cost: low cost.

(5) Size change and quantitative display. Compared with vacuum carburizing, ordinary carburizing has a large control amplitude, large size change, and scattered distribution.

1. b) Carburizing in a vacuum furnace

(1) Working environment. Gas carburizing process under conditions below ordinary atmosphere.

(2) Flexibility of heat treatment process. No need to adjust the time of carbon potential, control carburizing pressure, and carburizing gas flow.

(3) Part quality. The control of carbon amount and carburizing depth on the surface of the carburized layer is simple and accurate; The carburizing effect is uniform. It can shorten the working time, and the carburizing time is about 3/1 ~ 2/<> of ordinary carburizing

(4) Cost: high cost.

(5) Size change and quantitative display: Compared with ordinary carburizing, vacuum carburizing has a small control amplitude, small size change, and concentrated distribution.

Comparison of vacuum carburized gas quenching and oil quenching

1. a) Gas quenching.

(1) Adaptation surface: the gas quenching furnace has a narrow application surface for steel with a large diameter and poor hardenability.

(2) Part quality. for steel with good hardenability, gas quenching has a better structure than oil quenching. The deformation and surface roughness are also better. So, the gas quenching furnace is suitable for high-end products or special products, with slow cooling and small deformation.

(3) Cost: high cost

(4) Energy saving and environmental protection. After using nitrogen quenching, the gas is easy to recover and recycl, which is energy-saving, clean, and environmentally friendly.

1. Oil quenching.

(1) Adaptation surface. Under the conventional furnace type and the same power, the applicable surface of the oil furnace is wider than that of the gas quenching furnace.

(2) Parts quality. The oil quenching furnace is more applicable, fast cooling and large deformation.

(3) Cost. low cost.

(4) Energy saving and environmental protection. The use of oil quenching produces exhaust gas such as oil smoke, which is polluting to the environment.