The 18crmnti material is used for bevel gear of car. After carburizing and quenching, it is tempered at low temperature. The hardness of carburized layer is hrc56 ~ 58. The service time is very short, most of the teeth break from the root during a sudden start, as shown in Figure 19. The macro fracture surface is flat, and the rolling streamline of the material is parallel to the tooth surface, so it shows great brittleness, and no fatigue trace is found on the fracture surface.
Metallographic examination: carburizing layer thickness 0.5 ~ 0.8mm (uneven). The microstructure of the carburized layer is still normal except for the network cementite.
The fracture morphology of bevel gear. Eight of the 12 teeth were fractured from the root, one from the root, and only three were in good condition. The macroscopic fracture surface is relatively flat, and there are more flash points in crystalline form, and no fatigue symbol is found.