For a long time, the forging blanks of car gears are mostly treated by normalizing process. In recent years, with the diversification of gear materials and the high standard requirements for gear quality, the normalizing process is often difficult to meet the actual needs of production. In particular, 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 stacked in the air for cooling or blowing cooling, and their cooling speed is easily affected by their position in the stack and the 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 20CrNi2Mo steel, isothermal annealing is a very effective preliminary heat treatment method to reduce hardness. After austenitizing, the sample can be rapidly cooled to 630-670 ℃ and isothermal, and the required equilibrium microstructure, i.e. ferrite + pearlite, can be obtained. 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 extruding the gear with high dimensional accuracy, small machining allowance and small modulus, the tooth shape can be directly extruded, which reduces the traditional gear milling process, ensures the continuity of metal fiber and improves the strength and toughness of the gear. 20CrNi2Mo steel is a new type of carburized gear steel with high content of alloy elements. After hot rolling, the structure is bainite with a hardness of about 240hb. After conventional annealing, the hardness 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 influence of the original structure on the spheroidizing results is more significant when it is partially austenitized than when it is fully austenitized. The original structure is an equilibrium structure. Under the condition of partial uneven austenitization, cementite is easier to spheroidize.
As we all know, in addition to the factors of steel itself, the quality of gear is also affected by the heat treatment process to a great extent. The heat treatment process mentioned here not only refers to the carburization and quenching process of gear, but also includes the heat treatment process of gear forging blank. Generally speaking, the heat treatment process of gear forging stock has two purposes: one is to ensure good cutting performance, which requires the ferrite pearlite structure with uniform equiaxed grain 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. Foreign 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 of stress popularization.