Abstract: In view of the insufficient hardness of large gears made of 20Cr2Ni4A steel after carburizing and quenching but not allowing refrigerating treatment, the hardness of the gear is increased by more than 2 HRC to 58-61 HRC free of refrigerating treatment, through the optimization of carburizing and quenching carbon potential, intermediate high temperature tempering process and quenching temperature, meantime, the good microstructure is obtained. It is found that in order to improve the surface and subsurface hardness, the carbon potential should be increased as high as possible under the premise that the carbides do not exceed the standard, when the steel is hardened by reheating after carburizing and high temperature tempering. Repeated tests show that the intermediate high temperature tempering can also reduce the carbon content in a certain depth of the carburized layer, which can affect the carburizing and quenching hardness. Therefore, both insufficient tempering and over tempering should be prevented, especially the latter. If the tempering times are too many and the each time is too long, the hardness will be lower after quenching.

Key words: 20Cr2Ni4A steel, carburizing and quenching, hardness, retained austenite, gear