There are two main aspects to the hardenability of steel hardenability steel. (1) Ability of hardening of steel Carburized gear steel meets the contact fatigue strength and bending fatigue strength according to the core hardenability classification required for specific applications. (2) Hardenability of steel The addition of alloying elements on the basis of carbon steel is mainly to improve the mechanical properties of the core. Different alloy series or the same series can change the carbon content to meet the requirements of core hardenability. According to the use of the rear axle gear of the car and the design requirements to select the matching hardenability material, not only can meet the performance, but also necessary to reduce the heat treatment deformation.
Reasons for the deformation of the gear caused by the influencing factors The design parameters are unreasonable, the geometric shape symmetry, the gear stiffness difference, the fluctuation of the quality element composition of the steel, the impurity-containing elements, the purity of the steel, the uneven grain size, and the serious band structure. Poor hardenability, ZF standard is insufficient for internal quality defects of forgings, quenching and cracking, uneven cooling of forgings, poor processing performance, heating temperature is too high or too low during normalizing heat treatment, uneven cooling, resulting in poor hardness stability The final heat treatment method and heat treatment process parameters of the excessively large amount of gears are improperly selected. The final heat treatment of the gears of various gears is unreasonable in design, the quenching oil (coolant) and the quenching method are improperly selected, and the quality of the layer is uneven.
When the surface hardness of the rear axle gear is about 600 HV, the bending fatigue strength of the gear can be improved. High hardness also improves the wear resistance of the gear. However, the surface hardness is not as high as possible. For example, a surface hardness of 800 HV or more will reduce the pitting resistance of the gear. The bending impact strength of the gear above 700 HV will have a certain influence, so the surface strength of the gear is mostly controlled at about 650 HV. In addition, the microstructure of the gear also has a certain impact on the strength properties of the gear. The retained austenite content has an effect on improving the contact properties of the teeth and the wear resistance of the carbides to the gears. Of course, the defects of the microstructure can cause great damage to the life of the gear. The influence of non-martensitic surface and non-metallic inclusions on the bending fatigue strength and pitting resistance of the gear is sometimes immeasurable, which will greatly reduce the service life of the gear.
3. Heat treatment deformation of carburized gear steel for rear axle After the carburizing gear steel is used for carburizing, the surface carbon concentration increases. In the high temperature region of quenching and cooling, the core and surface layer are subjected to two corresponding stresses, and the surface layer is high. Carbon austenite is a region where heat treatment shrinkage deformation is obvious from the quenching temperature to the Ms temperature range. At this time, the core is transformed from low-carbon austenite to ferrite or low-carbon bainite and martensite, and the volume is increased, and a large internal stress is generated between the two.
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