A, not commonly used process

1) After carburizing, cooling to quenching temperature, and direct quenching after heat preservation. This method is easy to make the material grain coarse, brittle, workpiece structure stress is large, can only bear the strength of small modulus gear.

2) After carburizing, direct air cooling or slow cooling pit cooling to room temperature, and then re-enter the furnace for heating and quenching. The characteristics of 20CrMnMo steel should have strict requirements on the cooling speed after the workpiece carburizing, otherwise the transformation of the surface structure and the subsurface structure of the workpiece is not synchronized during cooling, resulting in a large tensile stress on the surface of the workpiece, and the workpiece is easy to crack.

3) For deep carburizing parts with high performance requirements, in order to ensure the quality index of the surface carburizing layer and the core organization at the same time, two quenching can also be used, the second heating to the core AC3+(30-50)℃ quenching, the second heating to the surface AC1+(30-50)℃ quenching, but this heat treatment produces large stress and high energy consumption.

2. Common processes

After carburizing, the furnace is cooled to 550℃, and then the furnace is re-heated and quenched. This method is a super common process in the production of 20CrMnMo steel parts at present. There are also problems: the heat treatment time is long, on the one hand, because the furnace needs to be cooled to a certain temperature after carburizing, the lower the temperature of the furnace, the more favorable it is to reduce oxidation and decarburization on the surface of the workpiece. On the other hand, because the workpiece is heated by quenching, it needs to go through a period of time to establish the carbon potential of the furnace gas to ensure the surface quality of the workpiece after quenching.