Gear, as a widely used mechanical component in mechanical transmission, has the characteristics of accurate transmission ratio, high bearing capacity and high transmission efficiency. However, in the actual meshing process of gear, under the repeated contact stress, the surface of gear will often be damaged by local metal peeling. Tooth surface fatigue failure is a common failure mode of gear. According to statistics, the ratio of tooth surface contact fatigue failure to all failure modes of gear can account for 20.3%. In order to improve the wear resistance and hardness of the gear tooth surface and the fatigue life of the gear, the gear is often carburized and quenched in the production process.
At present, there are mainly two kinds of gear carburizing and quenching processes in China:
(1) After carburizing and precooling, quenching is carried out directly;
(2) After carburizing and air cooling, quenching is carried out once.
Methods (1) after quenching, the amount of retained austenite in the carburized layer of gear is high, the surface is easy to form network carbide, and the surface hardness is difficult to meet the technical requirements. Method (2) it is still difficult to meet the technical requirements for the key parts with high mechanical properties. In order to overcome the above technical defects, it is usually used to grind the gear after carburizing and quenching, but the grinding of the gear will turn the pressure stress into the tension pressure, which is harmful to the mechanical properties of the gear; or it can be shot peened, but the pressure stress will decay in the high-speed rotating heat generation, which will affect its mechanical properties. In terms of carburizing and quenching of gears and contact fatigue analysis of gears, some scholars in our country have carried out researches on the following aspects:] the carburizing and quenching of gears has been simulated by using special heat treatment simulation software, and the effects of temperature, carbon content and phase transformation have been analyzed; Malu  has carried out many experiments on carburizing process of bevel driven gears, The solutions to the problems of quenching deformation after carburizing and oxidation and decarburization during secondary press quenching are put forward. The stress and strain nephogram of involutemade of 40Cr under different loads are obtained, which provides the basis for fatigue life experiment analysis. The forging and heat treatment process of large gear are studied, The improved process scheme was put forward through the combination of finite element thermal mechanical coupling numerical simulation, experimental verification, microstructure observation and performance analysis. In this paper, the finite element contact mesh model of involute is established based on the discussion of its surface carburizing and quenching process, and the contact fatigue analysis is carried out with awe fatigue module.
In order to solve the shortcomings of the existing gear carburizing and quenching process, a new carburizing and quenching process is proposed in this paper, and its flow chart is shown in Figure 1. The process is carried out in the steps of carburizing, precooling quenching, high temperature tempering, secondary quenching and low temperature tempering. The precooling quenching reduces the temperature of the gear, which can reduce the content of retained austenite on the gear surface, reduce the internal stress of the gear, and refine the grains. At the same time, the undercooled austenite is transformed into martensite or bainite to obtain martensite or lower bainite structure, and then with high temperature tempering, the strength, hardness, wear resistance, fatigue strength and toughness of the gear can be greatly improved, so as to meet the use requirements of the gear. There is still a lot of retained austenite in gear after quenching, and the retained austenite can be transformed by secondary quenching to improve the hardness of steel. Gears tempered at low temperature have high hardness and wear resistance.
The specific process of carburizing and quenching of gear surface is as follows.
(1) Carburizing. Put the gear into the active carburizing medium, heat it to about 920 ℃ and keep it warm, so that the active carbon atoms decomposed from the carburizing medium penetrate into the gear surface, so as to obtain high carbon on the surface.
(2) Precooling quenching. The process includes three steps: precooling, quenching and cleaning. First, the gear is precooled to 850 ℃ and kept warm for 30-45 min; then the gear is immersed in the quenching oil at about 60 ℃ and the quenching oil is stirred; finally, the gear is cleaned with gasoline to prevent impurities in the quenching oil or solid substances attached to the workpiece surface after the quenching oil is heated at high temperature, so as to avoid detergent sticking to the workpiece surface.
(3) High temperature tempering. Conduct high temperature tempering within 4 hours after the completion of step (2), heat the gear to about 600 ℃, and keep it for a proper time.
(4) Secondary quenching. After high temperature tempering, the gear is heated to about 800 ℃ and kept for a proper time, then the gear is immersed in quenching oil for secondary quenching.
(5) Low temperature tempering. First, clean the gear with gasoline, then heat the gear to 150 ~ 180 ℃, and keep it warm for a proper time. In this process, the heat preservation time should be extended as much as possible to eliminate the stress and ensure the performance of the workpiece.
The strength, hardness, wear resistance, fatigue strength and toughness of the gear can be greatly improved by using this process.