With the introduction of foreign advanced models, the localization of various gear steels has brought my country's gear steel levels to a new level. Cr-Mn steel from Germany, Cr-Mo series steel from Japan, and SAE86 steel from the United States meet the requirements for gear steels with medium and small modulus. Some domestic truck gears use American grade SAE8822H steel, such as 8t and 10t bridge bevel gears using SAE8822H, the main chemical composition of the steel (mass fraction, %) is 0.19 ~ 0.25C, 0.70 ~ 1.05Mn, 0.15 ~ 0.35 Si, 0.35～0.75Ni, 0.35～0.65Cr, 0.30～0.40Mo. The literature believes that controlling the hardenability is the key to solving the problem of gear distortion. In order to reduce distortion, H steel with Jominy hardenability bandwidth below 4HRC should be selected. The precision (contact area) of gears using H steel after heat treatment is 70% to 80% higher than that of ordinary steel, and the service life is prolonged. Therefore, industrialized countries have successively stipulated the hardenability zone of carburized alloy structural steel. The hardenability band is limited to a very narrow range (4-5HRC) as required. 1) When ordering in Germany, the hardenability of the steel can be required to be within the given range, or the steel with narrow hardenability can be required. 17CrNiM06 is very suitable for the manufacture of large-modulus and heavy-duty automotive gears. The main chemical components (mass fraction, %) of this steel are 0.15-0.20C, 0.40-0.60Mn, 1.50-1.80Cr, 0.25-0.35Mo, 1.40-1.70Ni. This steel has been produced and used in my country. The literature believes that in the carburizing process of 17CrNiM06 steel gears, the cooling rate after carburization is accelerated while the carbon potential in the later stage of carburization is appropriately reduced, and the air cooling is changed to air cooling to prevent the formation of large carbides, and then the high temperature is carried out at 630cC. Tempering is used to precipitate some alloy carbides, in order to reduce the amount of retained austenite during secondary heating and quenching at 820 °C, and finally obtain a better metallographic structure. 2) The Austrian "Styer" heavy-duty automobile factory requires a hardenability bandwidth of 7HRC. 3) Japanese medium and heavy-duty trucks, such as "Hino" brand KB222 9t truck and "Nissan" brand CKL20DD 8t truck, widely use Cr-Mo series steel, such as SCM420H and SCM822H steel, for transmission gears and rear axle gears. 20CrMnMoH and 22CrMoH steels are domestically produced in my country.
This type of steel has high hardenability. Within a certain range, the bending fatigue life of gears increases with the increase of hardenability. The literature pointed out that Changchun FAW started to use 20CrMnTiH steel in the production of "Jiefang" brand 9t truck rear axle gears. The hardness is only 22-24HRC, which does not meet the requirements of the gear technical conditions. When the car is in use, the active and driven bevel gears of the rear axle are damaged early. Therefore, Ct-Mo steel with higher hardenability has to be selected. Its main components refer to Japan's SCM822H gear steel. The main chemical components (mass fraction, %) of this steel are: 0.19~0.25C, 0.55~0.90Mn, 0.15 ～0.35Si, 0.85～1.25Cr, 0.35～0.45Mo. After negotiating with the steel mill, a domestically produced new steel grade 22CrMoH steel was produced. Its hardenability index is J9=36～42HRC, which better meets the requirements of automobile gears. However, the process performance of this steel is poor, and the gear forging blank can be cut only after isothermal annealing treatment. The hardness is 156-207HB, and the metallographic structure is pro-eutectoid ferrite + pseudo-eutectoid pearlite. This steel has high hardenability, and ordinary normalizing is prone to produce granular bainite. The appearance of granular bainite is extremely unfavorable for cutting, which not only greatly reduces the service life of the tool, but also always accompanies the appearance of abnormal microstructure. Due to the inhomogeneity of the metallographic structure, the heat treatment distortion of the gear is finally increased. 4) American automobile manufacturers are trying to reduce production costs, and at the same time, improve the reliability and durability of parts, which requires a high degree of consistency in product geometry and mechanical properties. To improve the consistency of product performance for heat-treated parts, the degree of dispersion of hardness after quenching must be reduced, which is directly related to the width of the hardenability band of steel. The consistency of the hardness of the gear core will reduce the distortion of the heat treatment, thereby improving the precision of the gear and making the residual compressive stress distribution on the surface of the gear teeth more uniform. Some American truck transmission gears and rear axle drive bevel gears are made of SAE8620 steel and SAFA820 steel. American SAE8620H, SAE8822H and other grades of steel have also been produced and used in my country (such as Baosteel Group Shanggang No. 5 Plant, etc.) and used for medium-duty truck transmission gears and rear axle bevel gears respectively.
