Research progress of gear annealing and quenching and tempering

For a long time, the forging blanks of car gears are mostly normalized. In recent years, with the diversification of gear materials and the high standard requirements for gear quality, normalizing process is often difficult to meet the actual needs of production, especially the advent of high-power engine promotes the development and application of new Cr Ni Mo series gear steel, which makes the research on post forging heat treatment technology more important and urgent.

During normalizing, the microstructure transformation of steel is carried out in the process of continuous cooling, i.e. within a certain temperature range, so the obtained microstructure is uneven. Moreover, during batch normalizing cooling, forging billets are often cooled in air or air blast in piles, and their cooling speed is easily affected by their position in the pile and surrounding environment, resulting in large fluctuations in the hardness of parts in the same batch, At the same time, it will increase the deformation during carburizing and quenching. For steel with high hardenability, bainite structure will even be produced. In addition, the existence of banded structure of Cr Ni Mo steel gear forging billet will reduce its cutting performance, affect the uniformity of carburized layer structure and hardness, and increase the degree of quenching deformation.

For 20CrNiMo steel, isothermal annealing is a very effective preliminary heat treatment method to reduce hardness. After austenitizing, the required equilibrium microstructure, i.e. ferrite + pearlite, can be obtained by rapid cooling to isothermal temperature in the range of 630 ~ 670 ℃. The optimum isothermal temperature is 640 ~ 660 ℃. Controlling the cooling rate between austenitizing temperature and isothermal temperature is the key to isothermal annealing process. Appropriate water cooling can control the banded structure within the qualified range (≤ grade 2).

With the progress of cold extrusion technology and the development of equipment, the mechanical industry began to use cold extrusion process to manufacture gears. Compared with the traditional forging hot forming process, cold extrusion does not need heating, so there is no oxidation decarburization of the blank. Moreover, when the gear with high dimensional accuracy, small machining allowance and small extrusion modulus can directly extrude the tooth shape, which reduces the traditional gear milling process, ensures the continuity of metal fiber and improves the strength and toughness of the gear. 20CrNiMo steel is a new type of carburized gear steel with high alloy element content. After hot rolling, the structure is bainite with hardness of about 240hb and that after conventional annealing is about 185hb, which is not suitable for cold extrusion. The implementation of cold extrusion process requires the steel to have lower hardness and higher plasticity. Spheroidizing annealing is an effective means to improve the cold formability of steel. The results show that the original structure, austenitizing degree and cooling rate are important parameters affecting the spheroidizing quality; The effect of original microstructure on spheroidizing results is more significant in partial austenitizing than in complete austenitizing; The original structure is equilibrium structure, and the cementite is easier to spheroidize under the condition of partial uneven austenitization.

As we all know, the quality of gears is influenced not only by the steel itself, but also by the heat treatment process to a large extent. The heat treatment process mentioned here not only refers to the carburization and quenching process of gears, but also includes the heat treatment process of gear forging stock. Generally speaking, the heat treatment process of gear forging stock has two purposes: one is to ensure good cutting performance, This requires that the ferrite pearlite structure with uniform equiaxed grain be obtained after heat treatment; The second is to ensure that the gear blank has a uniform structure, which is beneficial to reduce the inevitable irregular deformation during carburizing and quenching, and is of great significance to ensure that the gear has a good contact surface and reduce noise.

At present, foreign automobile manufacturers generally adopt isothermal annealing treatment for gear forging blank, and different isothermal annealing processes are specified for different materials. The production practice shows that the gear after isothermal annealing not only greatly improves the machining performance, but also significantly reduces the deformation after carburizing and quenching. Therefore, the isothermal annealing process of gear forging blank is a new process that should be vigorously promoted.

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