Material quality is affected by multiple factors. Different origin, smelting methods, quality categories and supply forms of truck bevel gear raw materials affect the internal quality of materials, such as chemical composition, hardenability, grain size, original structure and purity, which in turn affect the whole process of various transformations of heat treatment. Therefore, the deformation trend of materials with different quality is different. In terms of the influence of materials on deformation, the hardenability of materials has the most prominent influence on the distortion of heat treatment, and the hardenability of steel is affected by chemical composition, purity, grain size, inspection factors, sampling position and other factors. Among them, chemical composition plays the largest role, especially C, Mn, Cr and Ti in steel, and the contribution of alloy elements in steel with low oxygen content to hardenability is greater than that with high oxygen content.
Since the chemical composition of the material has a significant impact on the hardenability of the material, and the hardenability of the material is closely related to the heat treatment deformation, and the stability of the distortion of batch parts with the hardenability bandwidth of the material is poor, H steel controlling the hardenability fluctuation range is regarded as the first element to stabilize the distortion of truck bevel gear at home and abroad, At present, the use of H carburized steel with narrow hardenability band matching the required strength of parts has become the consensus of material selection in the whole gear industry. Large foreign companies generally purchase raw materials or forging blanks at fixed points according to their own standards. Some large domestic gear plants have also begun to cooperate with steel mills and gradually purchase steel at fixed points to stabilize the quality of parts. Since 1947, the United States has stipulated the hardenability band of steel in SAE standard and developed “guaranteed hardenability steel” (H steel). By 1977, 74 kinds of H steel have been listed in the standard, of which 1 / 3 are carburized steel; Japan, Britain, Russia, France and Germany also listed H steel in their respective steel standards around 1970. The width of hardenability band (J9) of H steel is generally 12-25HRC units. For gears produced in batch such as truck bevel gear, due to the improvement of accuracy and noise requirements and the progress of metallurgical technology in recent years, such as general motors of the United States and ZF gear plant of Germany, the width of hardenability band is required to be reduced by 50% compared with that specified in SAE and DIN standards, For 16mncrs steel medium modulus gears (M = 3-5) manufactured by ZF gear factory, the hardenability band width has been required to be 7HRC (J10), while Isuzu automobile factory in Japan requires the hardenability band width of carburized steel to be 8 ~ 10HRC (JN), which is now further increased to 4 ~ 8HRC.
China’s Beijing steel plant has provided truck bevel gear steel with hardenability band width of 8-9HRC (J9) to Beijing gear plant. 20crmntiha steel with controlled hardenability band has been adopted for auto parts of our company, which has made great progress in stable deformation compared with that before adoption.
20CrMnTi is a hypoeutectoid alloy structural steel, which is mainly composed of carbon, manganese, silicon, chromium, Qin and other elements, and other elements are residual elements. Carbon is the most effective element affecting the hardenability and hardenability of steel. With the increase of carbon content, the hardenability and hardenability are significantly improved, that is, the values of J9 and j15 are increased. Manganese and chromium can improve the nucleation work and growth rate of pearlite. The dissolution of these two elements and their carbides in austenite will shift the isothermal transformation curve of austenite to the right, increase the stability of undercooled austenite, reduce the critical cooling rate, inhibit the transformation of pearlite and improve the hardenability.