The automotive transmission system has many types of shaft components, with different structural characteristics and production requirements. With the continuous development and upgrading of new energy vehicles, the gear shaft, as a key component in the automotive transmission system, has also put higher production requirements. The speed of the new energy gear shaft is generally 12000-16000r/min, with high noise. Due to the absence of engine noise, the NVH requirements for the gearbox are prominent, so the precision requirements for the gear shaft are higher than those for fuel vehicles. When a gear shaft operates in a transmission system, its stress situation is complex. Therefore, the gear shaft should have good mechanical properties such as impact resistance, wear resistance, high strength and hardness, and good internal organizational performance to ensure its normal operation in high load and high-speed movements.
Gear shaft parts are generally forged by hot die forging, mainly because the forging is relatively large, and hot die forging is easier to achieve from the technology perspective. The forging of shaft parts requires selecting hot die forging, cold forging, or warm forging based on the size and cost of the parts.
Hot die forging
In hot die forging, the metal flow is relatively complex, and defects such as folding and insufficient filling may occur during forming. Corresponding molds may experience severe wear and cracking.
The main advantages of cold forging technology used in the production process of shaft parts are long mold life, good productivity, and high product consistency, which can effectively reduce machining allowance, shorten machining time, and reduce costs. But the cold forging process also has its inherent drawbacks.
- Firstly, an annealing process is commonly used to improve the plasticity of cold forged blanks before forging. However, the spherical pearlite structure formed by annealing cannot achieve the hardness achieved by normalizing the flake pearlite structure or tempering the sorbate structure after quenching and tempering when there are high-frequency quenching requirements in subsequent processing processes. Therefore, additional normalizing or quenching and tempering treatment is needed.
- Secondly, it is difficult to grasp the deformation caused by heat treatment. After normalizing or quenching and tempering treatment, the microstructure of the blank undergoes a recrystallization process. Except for the stress between the macroscopic shapes formed by processing (such as processing rounded corners, commonly referred to as macroscopic stress), there is no intergranular internal stress (commonly referred to as microscopic stress). Therefore, in subsequent heat treatment, the stress release is limited, and the deformation is relatively small; When choosing the cold forging blank process, we mainly value its economy (high forging accuracy). Micro stress is released when subsequent heat treatment directly uses processes such as carburizing and quenching, carbon nitrogen co infiltration, etc., due to grain deformation during forging. Sometimes the heat treatment deformation is relatively large. At the same time, due to grain shape, “swallowing” (this phenomenon is currently not supported by strong theoretical support, and there is no consensus in the academic community) and coarse grain phenomenon are formed, Affecting the mechanical strength of the product.
After warm or hot forging, the blank is generally treated with residual heat normalization, which is cheap and saves energy. However, there is a certain gap in the uniformity of the structure compared to the normalization process. If cost allows, it is recommended to use isothermal normalizing. The use of isothermal normalizing technology has effectively controlled the quality of gear blanks for transmission gears and shaft parts, thereby improving cutting processability and heat treatment deformation stability. The key to formulating the isothermal normalizing process is to reasonably control the speed and time of rapid and slow cooling during the intermediate cooling stage and the temperature and time of isothermal treatment based on the isothermal transformation curve of austenite. Isothermal normalizing can achieve a uniform and consistent microstructure and hardness compared to ordinary normalizing. The parts treated with isothermal treatment can reliably achieve good cutting performance and stable quenching deformation.
Carburization quenching is the main technical method for quenching hardened gear, but the deformation problem after carburization quenching seriously affects the quality of gear processing. The carburizing heat treatment of gear shafts in new energy vehicles is generally divided into three categories: atmosphere furnace carburizing oil quenching, vacuum carburizing oil quenching, and vacuum carburizing gas quenching.
Comparison of Carburizing in Traditional Atmosphere Furnaces and Vacuum Furnaces
(1) Traditional atmosphere furnace carburization.
- 1) Working environment: Gas carburization process carried out under normal atmospheric pressure conditions.
- 2) Flexibility of heat treatment process: The requirements for carbon potential concentration on the surface of parts vary greatly, requiring a long adjustment time for carbon potential.
- 3) Part quality: The surface carbon content and carburization depth of the carburized layer cannot be accurately controlled; The carburizing effect is relatively uneven; Long homework time cycle; After carburization, the parts have intergranular oxidation, with a gray surface and a surface roughness of Ra15 μ above m.
- 4) Cost: Low cost.
- 5) Size change and quantitative display: Compared to vacuum carburization, ordinary carburization has a larger control range, larger size change, and more dispersed distribution.
⑵ Carburizing in a Vacuum furnace.
- 1) Working environment: Gas carburization process carried out under conditions below one atmospheric pressure.
- 2) Flexibility of heat treatment process: No need to adjust the potential carbon time, control the carburizing pressure, and carburizing gas flow rate.
- 3) Part quality: The control of surface carbon content and carburization depth of the carburized layer is simple and accurate; The carburizing effect is uniform; It can shorten the operation time, and the carburizing time is about 1/3 to 1/2 of that of ordinary carburizing; After carburization, the parts remain in a glossy state and will not undergo intergranular oxidation.
- 4) Cost: High cost.
- 5) Size change and quantitative display: Compared to ordinary carburization, vacuum carburization has a smaller control range, smaller size change, and more concentrated distribution.
Comparison between Vacuum Carburization Gas Quenching and Oil Quenching
(1) Gas quenching.
- 1) Adaptability: The gas quenching furnace has narrow applicability to steel with a larger diameter and poor hardenability. There are still technical barriers, and it is difficult to achieve a pressure above 25 bar.
- 2) Part quality: For steel with good hardenability, the microstructure obtained by gas quenching is better than that obtained by oil quenching, and the deformation and surface roughness are also better; Gas quenching furnace is suitable for high-end or special products with slow cooling and small deformation.
- 3) Cost: High cost (nitrogen consumes more than oil, has a longer working cycle, and the material frame and furnace components are more prone to wear and tear under frequent cold and hot exchanges).
- 4) Energy saving and environmental protection: After nitrogen quenching, the gas can be recycled and reused, which is energy-saving, clean, and environmentally friendly.
⑵ Oil quenching.
- 1) Adaptability: Under conventional furnace types and equal power, oil furnaces’ applicability is wider than gas quenching furnaces.
- 2) Part quality: Oil quenching furnace has a wider range of applications, fast cooling, and large deformation.
- 3) Cost: Low cost.
- 4) Energy conservation and environmental protection: Oil quenching produces exhaust gases, such as oil fumes, which cause significant environmental pollution.
(3) Through comprehensive comparison, if the same forging billet is used for processing, according to the accuracy requirements of the tooth deformation of the customer’s gear shaft parts and the stability of variable shape control, the heat treatment selection order is vacuum carburizing gas quenching, vacuum carburizing oil quenching, and atmosphere furnace carburizing oil quenching.