The internal quality of automobile gear mainly means that the microstructures, mechanical properties and other indicators of gear meet the technical requirements, while other defects must be controlled within the specified technical range. The intrinsic quality of automobile gear is the key to determine the quality of gear, which completely depends on the quality of heat treatment, is the key factor to achieve low noise, high precision and long life of gear. Under certain conditions, the internal quality of gear can be effectively improved by selecting proper heat treatment process, improving microstructure, reducing deformation, preventing quenching and improving mechanical properties.

Automotive gear heat treatment (process) includes: one is common heat treatment, such as annealing, normalizing, quenching, tempering, quenching and tempering; The second is surface heat treatment, including surface hardening (such as induction hardening, laser hardening, etc.) and chemical heat treatment (such as carburizing, carburizing, nitriding, nitrogen carburizing, etc.).

1. The conditioning

Quenching and tempering is the operation of tempering gear and other parts at high temperature (500~650℃) after quenching. Quenching and tempering treatment is often used for gear made of high quality carbon steel or alloy steel with carbon content of 0.3%~0.5% (mass fraction).

Tempering and tempering can refine grains and obtain uniform, good mechanical properties and certain degree of dispersion. Generally, after quenching and tempering, the hardness of gear can reach 220~285HBW. The comprehensive performance of tempered and tempered gear is better than that of normalizing gear.

Quenching and tempering is commonly used for gear preparation heat treatment (such as nitriding, quenching and tempering before induction hardening) and final heat treatment.

2. Surface quenching

Surface quenching is a local quenching method in which the surface of gears and other parts is quenched to a certain depth while the core remains unquenched. Surface quenching through rapid heating, so that the surface of the gear quickly reach the quenching temperature, in the heat can not penetrate the core of the gear immediately cooling, local quenching.

After surface hardening layer is obtained by surface heating and quenching, the core of the gear can still keep the original microstructure and properties unchanged, so as to improve the fatigue strength and wear resistance and maintain the toughness of the core of the excellent comprehensive performance, and can save energy, reduce the quenching deformation of the gear.

The hardening hardness of gear tooth surface is generally 45~55HRC. Surface hardened gear has high bearing capacity and can withstand impact loads. Usually the blank of a surface hardened gear is normalized or tempered so that the core of the gear has 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.

Induction hardening process is mainly used for surface hardening of automobile gear. Due to the fast induction heating speed, almost no oxidation, decarbonization, gear deformation is very small, but also easy to realize local heating and automatic production, heat treatment cost is low. Therefore, it is widely used in modern automobile industry.

3. Carburizing and carbonitriding

(1) carburizing quenching carburizing quenching is to put the gear and other parts into the carburizing medium, heating and heat preservation at 880~950℃, so that the surface of the gear carburizing, and then quenching, so that the surface of the martensite and other microstructure. Carburized and hardened gears are usually made of low carbon alloy steel. The purpose of carburizing quenching is to make gear and other parts in the surface of high strength and wear resistance at the same time, the heart still maintain a certain strength and high plasticity and toughness.

Automobile gear commonly used gas carburizing process. After carburizing and tempering, the surface hardness of gear is generally 58~63HRC. At present, carburizing and quenching has become the leading heat treatment process for important automotive gears (such as differential gears, drive axle bevel gears, transmission gears, etc.).

(2) carburizing carburizing is a chemical heat treatment process that makes the surface of gears and other parts penetrate carbon and nitrogen at the same time. General carbonitriding infiltration layer depth control in 0.2~0.8mm, the scope of application is mainly to bear medium and low load wear-resistant parts. Compared with conventional carburizing, its surface wear resistance is greatly improved and its service life is higher. Carbonitriding is often used in automotive transmission gears, etc.

In recent years, the actual temperature of the tooth surface of automatic transmission AIT carburizing gear in the work is about 300℃, far higher than the normal tempering temperature (150~200℃), this surface temperature will lead to the reduction of hardness, resulting in pitting corrosion. Shot peening after carbonitriding can improve fatigue strength. In the carbonitriding process, with the increase of nitrogen content δ HV (hardness drop), the tempering resistance is improved, and the tempering resistance temperature reaches 300℃.

4. Nitriding and nitrocarburizing

(1) nitriding. Nitriding is a chemical heat treatment process in which nitrogen atoms are infiltrated into the surface of gears and other parts to form nitriding layers. Nitriding can improve the surface hardness, wear resistance, fatigue strength and corrosion resistance of gear. Nitriding treatment temperature is low, so gear deformation is small, no grinding or only fine grinding can be.

Nitriding gear due to nitriding layer thin (about 0.15~0.70mm), hardening layer has the tendency to peel off, so the bearing capacity is generally not as high as carburizing gear, not suitable for impact load or strong wear occasions.

Japan adopts nitriding process in heat treatment of automobile transmission gear, and Germany Clocker- Ion company applies ion nitriding to automobile gear, which improves gear accuracy and service life.

(2) nitrocarburizing. Nitrocarburizing is a chemical heat treatment process mainly nitriding and permeating carbon. Nitrocarburizing can significantly improve the wear resistance, adhesion and abrasion resistance, fatigue resistance and corrosion resistance of gears. At present, gas nitrocarburizing is used in car, light bus transmission gear and other parts.