Abstract: The wrench broke during use. The reasons for the fracture of the wrench were analyzed using macroscopic observation, chemical composition analysis, metallographic examination, scanning electron microscopy analysis, hardness testing. The results show that improper control of the heat treatment process for the wrench resulted in high material hardness, martensite formation in the microstructure, the hydrogen embrittlement sensitivity of the material increased, and ultimately led to hydrogen induced brittle fracture of the wrench.