Abstract: 4Cr13 steel links of a batch of single-phase circuit breakers fractured during debugging. The reason for fracture was analyzed by optical microscope, scanning electron microscope and Vickers hardness tester, combining with manufacturing process of links. The results show that the brittle over-load fracture of 4Cr13 steel links occurred inside the 90° corner, and the fracture mode was quasi-cleavage fracture. The hardness at inner side of 90° corner of 4Cr13 steel links was relatively high, and the radius of chamfering fillet was small, resulting in the stress concentration inside the 90° corner of links during bending. And then the microcracks formed, leading to the fracture of 4Cr13 steel links due to the force during debugging finally. The reasonable design of link structure and controlling the heating rate of annealing process before bending process could avoid the occurrence of fracture of links.