Due to improper treatment and excessive carburization, massive and network carbides will appear on the surface of carburized gear. When in use, the plastic deformation capacity of gear will be reduced, the impact resistance will be weakened, the bending fatigue performance of tooth root will be reduced, the tooth sharp angle will become brittle and easy to crack. After quenching, carburized gear is easy to crack during grinding.
1. Cause analysis
(1) When the gear is carburized in solid medium, the carbon potential in the carburizing box is too high, and the carbon potential cannot be adjusted arbitrarily. Therefore, the higher the carburizing temperature and the longer the time, the greater the degree of surface hypereutectoid. Especially for carburized steel containing strong carbon compound forming elements Cr and Mo, the diffusion of carbon is slow, the surface carbon concentration of gear carburized layer is higher, and the carburized layer with hypereutectoid composition is reached. During cooling, cementite precipitates from austenite grain boundary to form network distribution.
(2) When carburizing in gas medium, if the carbon potential in carburizing furnace is too high and the strong carburizing time is too long, excessive carburization of gear surface will also occur.
2. Preventive measures
(1) During solid carburizing, in order to prevent excessive carburizing caused by too high carbon potential, lower carburizing temperature or weaker carburizing agent can be used.
(2) During gas carburizing, in order to prevent excessive carburizing on the surface, the diffusion stage is arranged in the later stage of carburizing. The time of strong carburizing and diffusion stage can be operated according to the heat treatment process.
(3) For gears with excessive surface carburization, diffusion treatment should be carried out in a low carbon potential carburizing furnace, or quenching should be carried out after carbide spheroidizing annealing.