Abstract: Means such as optical microscope and scanning electronic microscope observation and hardness testing were used to analyze the fracture reason of a ZG42CrMo chain link of dummy bar. The results show that because of the improper induction quenching, surface hardened layer of the chain link was hard and thin and hardness gradient from the surface to the matrix was steep, which resulted in the formation of little cracks first at the surface hardened layer when receiving pressure stress. At the same time, because the as-cast structure was overheated and quenching temperature was low, there were lots of network ferrite and Widmannstatten structure in the microstructure of the chain link, which resulted in the decrease of hardness and ductility of the matrix material. So the little cracks haven formed on the chain link surface expanded rapidly from the corner stress concentration area to the center hole and led to brittle fracture failure of the chain link of dummy bar.