Abstract: The lower mandrel broken frequently during the forging process of railway wheels. The fracture reason of the lower mandrel was analyzed by means of macroscopic observation, chemical composition analysis, scanning electron microscope analysis, metallographic examination and hardness test. The results show that the size of the R angle of the lower mandrel was too small. When the lower mandrel was working, the R angle broken due to stress concentration. By optimizing the size of the chamfer, the fracture of the lower mandrel could be avoided.