Failure Mechanism Analysis of 25Cr2MoV Bolt Fracture

Based on a failure case involving in-service 25Cr2MoV bolts in a centrifugal fan of a petrochemical unit, a systematic failure investigation was conducted. Macroscopic examination, metallographic analysis, hardness testing, scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS) were employed to characterize the failure features and investigate the underlying failure mechanism. The results indicate that the fracture originated from the thread root, where significant stress concentration was present. The fracture surface exhibits brittle fracture characteristics, indicating a predominantly brittle fracture mode. The microstructure consists mainly of lath martensite, and the measured hardness is significantly higher than the standard requirement, suggesting insufficient toughness associated with an inadequately tempered condition. In addition, high-temperature oxidation occurred during service, resulting in surface degradation and potentially facilitating crack initiation. Combined with the analysis of on-site assembly conditions, the reported excessive installation torque and concentrated failure distribution suggest abnormal preload conditions, which may have contributed to additional bending/shear-related stress components and promoted crack propagation. Comprehensive analysis suggests that the bolt failure was a brittle fracture associated with abnormal installation conditions, insufficient material toughness, and local combined stress states.