As a major country in automobile manufacturing and sales, China's automobile manufacturing industry has become an indispensable pillar industry of its economy.
Automotive gears, as key components in automobiles, are mainly used to transmit power and motion, and to change the speed ratio of the engine crankshaft and main shaft gears through them. Due to the random variation of car driving conditions with road conditions, the working condition of car gears is very complex, which requires good internal quality of car gears.
Heat treatment process, characteristics and functions of automotive gears
The internal quality of automotive gears mainly refers to the fact that the microstructure, mechanical properties, and other indicators of the gears meet technical requirements, while other defects must be controlled within the specified technical range.
The inherent quality of automotive gears is the key to determining their quality, which depends entirely on the quality of heat treatment. It is a key factor in achieving low noise, high precision, and long service life of gears.
The heat treatment (process) of automotive gears includes: first, ordinary heat treatment, such as annealing, normalizing, quenching, tempering, and tempering; The second is surface heat treatment, which includes surface quenching (such as induction quenching, laser quenching, etc.) and chemical heat treatment (such as carburizing, carbonitriding, nitriding, nitrocarburizing, etc.).
Tempering
Quenching and tempering is the operation of quenching gears and other parts and tempering them at high temperatures (500~650 ℃). Quenching and tempering treatment is commonly used for gears made of high-quality carbon steel or alloy steel with a carbon content of 0.3%~0.5% (mass fraction).
Quenching and tempering can refine grain size and obtain a uniform, somewhat dispersed, and excellent mechanical properties tempered sorbite structure. Generally, after quenching and tempering treatment, the gear hardness can reach 220~285HBW. The comprehensive performance of quenched and tempered gears is superior to that of normalized gears.
Quenching and tempering are commonly used for preliminary heat treatment of gears (such as nitriding, quenching and tempering before induction hardening) and final heat treatment.

Surface quenching
The quenching hardness of the gear tooth surface is generally 45~55HRC. Surface hardened gears have high load-bearing capacity and can withstand impact loads. Usually, the blank of surface quenched gears is subjected to normalizing or quenching and tempering treatment to give the gear center a certain strength and toughness.
Surface quenching mainly includes induction quenching, laser quenching, and flame quenching. Compared with carburizing quenching, surface quenching has smaller deformation, lower cost, and higher efficiency.
The surface quenching of automotive gears mainly adopts induction quenching technology. Due to the fast induction heating speed, almost no oxidation or decarburization, minimal gear deformation, and easy implementation of local heating and automated production, the cost of heat treatment is low. Therefore, it has been widely used in the modern automotive industry.
Carburization and Carbonitriding
Carburizing quenching
Carburization quenching is the process of first placing gears and other parts into a carburizing medium, heating and holding them at 880-950 ℃ to carburize the surface of the gears, and then quenching them.
Gas carburization process is commonly used for automotive gears. The surface hardness of gears after carburizing, quenching, and tempering is generally between 58~63HRC. At present, carburization and quenching have become the dominant heat treatment process for important automotive gears, such as differential gears, driving axle driven spiral bevel gears, transmission gears, etc.
Carbonitriding
In recent years, the actual temperature of the tooth surface of the AIT carburized gears used in automotive automatic transmissions during operation has reached about 300 ℃, which is much higher than the normal tempering temperature (150-200 ℃). The temperature of this surface will lead to a decrease in hardness and the occurrence of pitting corrosion. Shot peening hardening after carbon nitrogen co infiltration can improve fatigue strength. During carbonitriding, as the nitrogen content increases Δ The HV (hardness decrease) is improved, and the anti tempering performance is improved, with the anti tempering temperature reaching 300 ℃.