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Steel gear vacuum carburizing and quenching

1 Steel gear technical requirements
For gears, compared with other heat treatment methods, vacuum carburizing and quenching has the best comprehensive mechanical properties, so the carburizing process is most widely used in the production of high-parameter gears. 20Cr2Ni4A steel is a high-strength alloy carburizing steel with good hardenability, high core hardness and good toughness. It is often used for heavy-duty gears and shock loads. Used in chemical industry, metallurgy, mining machinery, power station, ship, aviation and other fields. However, due to the high alloy content of this material, there are many retained austenites on the surface after carburizing and quenching, and the hardness is low. In order to improve the surface hardness, cold treatment is usually used to reduce the retained austenite content and increase the hardness. However, there are some controversies about the cold treatment process. Some people think that although cold treatment can effectively reduce the retained austenite content and increase the hardness of the gear, it will promote the formation of martensite micro-cracks and increase the brittleness of the material. Therefore, in practice, some users do not allow cold treatment, and hope to increase the hardness through non-cold treatment; in addition, due to the lack of large-scale cold treatment equipment, for large workpieces, it is sometimes inconvenient to increase the hardness by cold treatment in actual production.
For large gears, due to the large workpiece, heavy mass, low quenching hardness, and in order to prevent grinding teeth cracks, the low-temperature tempering time after quenching is longer, generally 5~20 h, and the final gear hardness is lower. Cold treatment is difficult to meet the technical requirements. The tooth top hardness of the gear treated according to the original process is only 56 ~ 57 HRC, the hardness of the surface of the furnace sample is 58 HRC, and the hardness of the subsurface (0.15 ~ 0.6 mm from the surface, equivalent to the surface after grinding) is 56 ~ 57 HRC. The technical requirements of the drawings generally stipulate that the hardness of the tooth surface is 58-62 HRC, and the gears processed according to the original process cannot meet the technical requirements.

2 Optimization of vacuum carburizing and quenching process for steel gears
There are two common processes for carburizing and quenching of 20Cr2Ni4A steel, one is cooling directly after carburizing, quenching at low temperature and tempering, and the other is carburizing-slow cooling out of the furnace-high temperature tempering-reheating quenching-low temperature tempering. The former has high efficiency, low cost, and small distortion, while the latter has less retained austenite, finer grains, and good strength and toughness. For medium and large high-speed and heavy-duty gears, the performance requirements are high, so the latter is often used for processing.
The low hardness of 20Cr2Ni4A steel after carburizing and quenching is generally considered to be caused by excessive retained austenite. In the case of no cold treatment, measures such as slow cooling after infiltration, multiple high-temperature tempering, lower quenching temperature, and lower low-temperature tempering temperature are generally adopted.
In order to ensure that the subsurface layer has a high carbon content and sufficient hardness after carburizing and quenching, the carbon potential of the carburizing atmosphere should be increased as much as possible while ensuring that the carbide level does not exceed the standard and the carbon black is not seriously produced in the furnace. In order to avoid carbide precipitation due to high carbon potential during carburizing, cooling, heat preservation and quenching, the carbon content on the surface should not exceed the carbon content corresponding to its Acm point.

3 Process optimization conclusion
1) For large and medium-sized gears made of 20Cr2Ni4A steel, it was difficult to reach a hardness of 58 HRC without cold treatment in the past. Through process optimization, the hardness increased by more than 2 HRC to 58-61 HRC without cold treatment, and the microstructure was good, which achieved the expected purpose.
2) When 2OCr2Ni4A steel adopts the technical scheme of reheating, quenching, quenching, and low-temperature tempering after carburizing, in order to ensure the hardness of the surface and subsurface, especially the hardness of the subsurface, the carburizing time should be increased as much as possible while avoiding excessive carbides. Diffusion carbon potential, cooling and heat preservation carbon potential and quenching carbon potential.
3) When tempering at high temperature after carburizing, not only attention should be paid to insufficient tempering, but also over-tempering should be avoided. High-temperature tempering after carburizing will also lead to a decrease in the carbon content in the carburized layer at a considerable depth, too many times of high-temperature tempering and too long time, and the hardness will not increase but decrease after quenching and low-temperature tempering.