Abstract: During the assembly process of the driving gear shaft of the rear axle bevel gear, a thread fracture occurred. The fracture was located on the surface near R, with a flat and metallic luster. The fracture crack started on one side of the surface and propagated radially. There was a black area on the surface at the crack source. The fracture process was clearly divided into two stages. Through comprehensive analysis of the fracture morphology, metallography, hardness analysis, energy spectrum analysis, etc., it was determined that the thread fracture was mainly due to the unqualified metallographic structure of the thread surface during the heat treatment process, which was needle like martensite. The hardness of the thread surface was uneven, some of which were too high, and did not meet the technical requirements. In addition, there are defects with poor roughness during the processing, which can easily lead to surface cracks and fractures at the R angle due to excessive and uneven stress. By changing the heat treatment process of the thread, from the original overall carburizing quenching+high-frequency annealing, to the process of first applying anti-seepage agent during heat treatment and then high-frequency annealing the thread, it is ensured that there is no stress concentration at the thread after heat treatment, avoiding thread fracture failure. Ensure that there is no stress concentration at the thread after heat treatment to avoid thread fracture failure.