Abstract: The 40Cr bolt of a floating head heat exchanger broke after service for less than 2 a. The failure cause of the bolt was analyzed by macroscopic observation, chemical component analysis, hardness testing, metallographic examination and fracture analysis. The results show that due to improper quenching and tempering treatment, the microstructure of the bolt was tempered sorbite with the martensite orientation. Ferrite precipitated along the network of the original austenite grain boundaries, leading to higher hardness and insufficient resistance to sulfide stress corrosion. After long-term service in the wet hydrogen sulfide environment and under tensile stress, wet hydrogen sulfide stress corrosion and hydrogen induced cracking took place, resulting in the fracture of the bolt.