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Vacuum Heat Treatment of Vacuum Carburized Alloy Steel Gear

With the development of hard-toothed gear transmission and the expansion of its application fields, the requirements for the performance of gear materials and the heat treatment technology adopted are getting higher and higher. In order to ensure the normal operation of the gear, the gear needs to meet the following main performance requirements:

1) Have sufficient contact fatigue strength and bending fatigue strength;

2) The tooth surface should have high hardness and wear resistance to prevent adhesive wear and abrasive wear;

3) The core of the gear should have sufficient toughness and strength to increase the load-carrying capacity

4) The gear material has good machinability and vacuum heat treatment process performance.

17Cr2Ni2Mo, 18Cr2Ni4W, 20CrMnTi are commonly used materials in gear production.

Discussion on Process Characteristics of Vacuum Heat Treatment of Three Materials

1) 17Cr2Ni2Mo material. Due to the high content of Cr, Ni and Mo in 17Cr2Ni2Mo steel, normalizing + high temperature vacuum tempering is adopted in the process of blank test pieces. In addition, the affinity between Cr and Ni alloy elements and carbon is strong, and the vacuum carburizing speed is fast, and the carbon concentration in the carburizing layer is easy to cause excessively high carbon concentration to form large block and network carbides, so the second stage is used in carburizing. In the carburizing process, the strong infiltration period is correspondingly shortened, the diffusion period is twice the time of the strong infiltration time, and the diffusion temperature is 20°C higher than the carburizing temperature, which enhances the diffusion capacity of carbon. 17CrzNizMo steel forms alloy carbides due to Cr, Ni elements and carbon. After vacuum carburizing, it undergoes high-temperature vacuum tempering, and the alloy carbides are dispersed on the substrate. Therefore, incomplete quenching + low-temperature tempering is adopted. The structure obtained after quenching is Cryptocrystalline martensite and uniformly distributed spheroidal carbides are conducive to exerting the characteristics of high strength, high hardness, high wear resistance and good toughness of the material.

According to the performance and technical requirements of the gear, the production process of the gear is: forging → normalizing → rough machining → quenching and tempering → semi-finishing → tooth profile processing → vacuum carburizing, high temperature tempering → (semi-finishing) → quenching and low temperature Tempering → Grinding. Among them, normalizing after forging and quenching and tempering after rough machining are pre-heat treatment. Normalizing after forging can eliminate the stress generated by the forging process. The quenching and tempering treatment can make the steel obtain a good metallographic structure before carburizing, improve the performance of the gear, and reduce the degree of gear deformation during the carburizing and quenching process.

2) 18Cr2Ni4W material. The production process of gears is: forging→normalizing→rough machining→quenching→semi-finishing→tooth profile processing→vacuum carburizing, high temperature tempering→(semi-finishing)→vacuum quenching, low temperature tempering→gear grinding. 18Cr2Ni4W steel has high content of alloy elements such as Cr and Ni, and it is easy to form banded structure and segregation in the smelting process, so a higher forging ratio is required. The final forging temperature after repeated forging is usually greater than 850°C. Due to the good hardenability of this steel, martensitic structure can be formed under air cooling conditions after forging, and the generation of structural stress may cause cracking of the workpiece. Therefore, in order to avoid cracking, the forged steel can be used in actual operation The waste heat is used to temper the workpiece at high temperature to transform the structure and reduce the stress.

The vacuum carburizing and quenching process is the most critical technology for gears. In the case that the equipment has been determined, it involves selecting the appropriate parameters such as penetrating agent, temperature, time, etc, and formulating the process reasonably. When a well type carburizing furnace is used, the segmented control method is generally used for carburizing, and kerosene is used as the carburizing medium. The whole carburizing process is divided into four stages: exhaust, strong infiltration period, diffusion and cooling. , 650 ℃ vacuum tempering, and then heated to 780 ℃ vacuum quenching. For equipment and gear materials, increasing the carburizing temperature and carbon potential within a certain range can speed up the speed of vacuum carburizing. However, the vacuum quenching temperature should not be too high, otherwise it will cause large deformation. After vacuum carburizing, two-stage slow cooling and then air cooling can reduce the temperature difference between the inside and outside of the workpiece during cooling, thereby reducing deformation. The final metallographic examination results show that the martensite structure of the infiltration layer after this process is fine, with a small amount of carbide and retained austenite.

3) The carbon content on the surface of 20CrMnTi parts increases after vacuum carburizing, and it is a hypereutectoid component, but the carbon content in the center does not change, and it is still a hypoeutectoid component. Due to the different carbon content, the surface and core of the workpiece have different requirements on the quenching temperature. The ideal quenching temperature selected according to the surface composition is lower than the ideal quenching temperature selected according to the core composition. During pre-cooling and quenching, as the selected quenching temperature gradually decreases from the carburizing temperature (about 930°C), it will be closer to the ideal quenching temperature of the surface composition. The reduction of the quenching temperature within a certain range is beneficial to the improvement of the surface hardness. But the effect on the performance of the heart tissue is much more complicated, which may reduce the strength of the heart. To study the effect of quenching temperature on the surface and core properties of 20CrMnTi steel, in order to correctly determine the quenching temperature of this steel after carburizing, so that the gear surface and core can obtain the best performance combination, which is very important for the formulation of the process and the product. Production has very important guiding significance.

Vacuum furnace selection: The RVC vacuum carburizing furnace provided by SIMUWU is an excellent product for this type of process, with the characteristics of good temperature uniformity and high temperature control accuracy. SIMUWU provides a professional team of engineers who can solve various problems encountered in the production process and is committed to giving customers the most convenient and efficient experience.