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my country's gear steel basically meets the national needs and the requirements of localization in the process of introducing technology, and the steel for the transmission gear of heavy vehicles and the rear axle gear of medium and heavy vehicles has yet to be developed and produced. According to the analysis of the status quo of domestic heavy-duty vehicles, the two problems of overloading and poor road conditions are more serious and cannot be overcome in a short period of time. This makes the gears often bear greater overload and impact loads. Overload impact load lies between fatigue and fracture stress. It has a great influence on the service life of gears and often causes early gear failures. From this point of view, Cr-Ni or Cr-Ni-Mo series steels should be selected for large-modulus and heavy-duty automobile gears. For example, German 17CrNiM06 steel is the best, and domestically made 20CrNi3H and 20CrNiMoH steels. The advent of high-power engines promoted the development and application of new Cr-Ni-Mo series gear steels. Such as new gear steel 20CrNi2Mo, 17CrNiM06. An automobile modification company of the FAW Group has developed a new type of truck axle, which is characterized by high power matching the engine. In order to ensure the service life of the gear, higher requirements are placed on the material and quality of the gear. The active bevel gear of the rear axle made of 22CrMoH steel fails early during use, and even breaks the teeth in severe cases. In terms of heat treatment, because the heat treatment process of gear materials is sometimes not stable enough, the effective hardening layer of some gears is not enough, and the hardness of the gear core and surface is low. These are the main reasons for the early failure of the gear. Moreover, Cr easily forms intergranular network carbides, which impairs the mechanical properties of the infiltrated layer. The analysis found that when the hardness of the tooth core of the gear wheel is low, the plastic deformation of the transition layer will cause excessive stress in the carburized layer, which will lead to the formation of cracks in the carburized layer, and finally break the entire gear tooth. For this reason, according to the good driving effect of using 20CrNi3H steel for the active bevel gear of the rear axle of the "Steyr" automobile axle, it is necessary to ensure that the effective hardened layer depth of the gear is 1.8-2.2mm, and the hardness of the gear wheel tooth center is 38-45HRC. The surface hardness of the gear is 60-64HRC, the carbide is between 1 and 3, and the martensite and retained austenite are between 1 and 4. This can increase the service life of the gear by 30 percent to 40 percent .