The selection of gear material is very important for qualified and high-quality spiral bevel gear, which is mainly based on the load borne by the gear, the accuracy requirements of spiral bevel gear and the speed. The load is measured by the unit compressive stress on the tooth surface of spiral bevel gear. According to its size, it is divided into light load, medium load, heavy load and overweight load. The manufacturing accuracy of spiral bevel gear has an important influence on the accuracy, tolerance and meshing of tooth profile. The accuracy of spiral bevel gear for automobile (Gleason system) is generally 6 ~ 8 levels, and the accuracy of gear for machine tool is generally 6 ~ 8 levels (medium and low speed) and 8 ~ 12 levels (high speed). The higher the speed of the spiral bevel gear at work, the more times of alternating stress on the tooth root and tooth surface of the spiral bevel gear, and the more serious the wear of the tooth surface. Therefore, the circumferential speed of spiral bevel gear at work can be taken as the scale for materials to bear wear and fatigue. It is generally divided into three types: low-speed gear (1 ~ 9m / s), medium-speed gear (6 ~ 10m / s) and high-speed gear (10 ~ 15m / s).
Low alloy carburized or carbonitriding steels (20CrMnTi, 20CrNi3, 18Cr2Ni4WA, 40Cr, 30CrMnTi and other steels) are suitable for gears with heavy load, high speed or medium speed and subject to large impact load. The heat treatment adopts carburizing, quenching and low-temperature tempering. The surface of the spiral bevel gear can obtain a hardness of 58 ~ 63HRC, and the core of the spiral bevel gear has high strength and toughness due to good hardenability. The surface fatigue strength, bending strength of tooth root, impact resistance and wear resistance of the gear after this heat treatment are higher than those of the spiral bevel gear after surface quenching. However, due to the large deformation of heat treatment, the spiral bevel gear with high accuracy requirements generally needs grinding. It is mainly applicable to the gearbox and rear axle gear of automobiles and tractors with poor working conditions. Compared with carburizing, carbonitriding has the advantages of high mechanical properties, small heat treatment deformation and short production cycle. It can also be applied to medium carbon alloy steel or medium carbon steel. Therefore, most spiral bevel gears can choose carbonitriding process.
When the spiral bevel gear in the transmission system of automobile rear axle works, especially the driving wheel is impacted frequently and subjected to large force, so it has high requirements for materials. Practice has proved that 20CrMnTi steel has high mechanical properties. After carburizing, quenching and low-temperature tempering, its surface hardness can reach 58 ~ 62Hrc and core hardness can reach 30 ~ 45hrc. The heat treatment processability and normalizing cutting processability are good. After carburizing, quenching and tempering, the spiral bevel gear can also be shot peened to increase the surface compressive stress, so as to improve the durability of the spiral bevel gear. When forging the bevel gear, the fillet radius of the bevel forging blank should be larger than that of the traditional forging blank, so as to avoid the folding of the bevel forging blank; The size of the forging blank in the height direction shall be greater than the corresponding size of the forging to make the metal flow adopt upsetting forming. In order to reduce the friction resistance during metal flow, so as to reduce the forging deformation force and die wear; Along each flow plane, the cross-sectional area of the forging blank shall be equal to the sum of the cross-sectional area of the forging and the cross-sectional area of the flash.