Abstract: The root of a driving gear fractured during tightening. The fracture reasons of the driving gear were analyzed by means of macroscopic observation, chemical composition analysis, hardness testing, metallographic examination and scanning electron microscope analysis. The results show that the raw material had overburning structure, leading to partial grain boundary melting and remelting, there was segregation of Cr element in the remelted region. A large amount of ferrite precipitated from the grain boundary due to overheating. Overburning and overheating of the structure could cause the loss of grain boundary adhesion, reduced its plasticity, ultimately led to the fracture of the driving gear.