Abstract: In response to the problem of fracture of 20CrMoH alloy steel reducer bevel gear shaft during installation and tightening, the macroscopic morphology, microstructure, and hardness of the fracture specimen were tested and analyzed using optical microscopy, microhardness tester, scanning electron microscopy, and other detection methods to identify the cause of the fracture. The results show that the cracks in the 20CrMoH steel bevel gear shaft originate from poor machining of the thread start stop groove, with multiple points of fracture and severe wear in the source area. Subsequently, under the action of bending shear stress, the cracks propagate along the carburized transition layer and fracture, with the fracture morphology showing intergranular features. The chemical composition and hardness of the parts basically meet the requirements, but there is a significant variation in hardness at the core of the parts. Overall, the parts exhibit characteristics of high stress overload fracture, which is caused by stress concentration due to poor machining tool marks and material conditions, leading to the fracture. It is recommended to investigate the stress state during tightening and improve the heat treatment process conditions to reduce the depth of the induction annealing layer and narrow down the heat affected zone.

Key words: 20CrMoH alloy steel, berel gear shaft fracture failure, microstructure, fracture morphology