Abstract: A 5CrNiMo ring die cracking was found during the machining process after heat treatment.The macroscopic and microscopic fracture analysis, microstructure analysis, microregion component analysis and hardness test of the failure die were carried out.The causes of die failure were analyzed.The results show that the material is folded in the forging process, and the local oxide scale is involved, resulting in forging cracks.The chemical composition of the material is uneven, and the ferrite and pearlite with low hardness are formed.Due to the uneven microstructure, it is easy to produce unbalanced internal stress, and the interface between the two phases becomes the crack source.A large number of holes are distributed along both sides of the crack, and it is suspected that the die material is ingot head and tail material.