Abstract: The hook tension spring of a certain lock broke during use. The reasons for the fracture of the tension spring were analyzed using macroscopic observation, chemical composition analysis, scanning electron microscopy analysis, metallographic examination, microhardness testing. The results show that under the action of cyclic stress, some of the coating on the surface of the tension spring peeled off, and improper heat treatment of the tension spring resulted in uneven hardness distribution of the tension spring. The surface hardness of the tension spring was relatively high, and microcracks appeared at the peeled off part of the coating, ultimately led to brittle fracture of the tension spring.