Abstract: Numerous delayed fracture failures have been observed in engine flywheel bolts of SCM435 produced by a manufacturing factory, which significantly degrades the reliability of the engine assembly. In this study, the failure causes leading to flywheel bolt fracture are systematically investigated through a series of analytical techniques, including fracture surface microscopic morphology observation, metallographic inspection, chemical composition analysis, tensile property testing and substrate hardness testing. Experimental results reveal that the fracture surface of the bolts displays typical characteristics associated with hydrogen embrittlement, accompanied by distinct precipitates distributed along grain boundaries. Surface corrosion induced during the pickling and phosphating process facilitates hydrogen penetration into the bolt matrix, and the resultant hydrogen embrittlement is identified as the primary contributor to fracture. Furthermore, chemical analysis results indicate that the cumulative content of low-melting-point elements of Pb and Sn in the flywheel bolt material exceeds the specified standard limits. The excessive low-melting-point elements tend to segregate at grain boundaries during the cooling stage following quenching and tempering, thereby weakening grain boundary cohesion and triggering intergranular brittle fracture. The synergistic interaction of these two factors ultimately gives rise to the delayed intergranular brittle fracture of the flywheel bolts. Nevertheless, the test results demonstrate that both the mechanical properties and matrix hardness of the bolts conform to the requirements of relevant national standards, and no abnormalities are detected in the quenched and tempered metallographic microstructure.