Failure Analysis of a Connecting Rod Bolt in a Compressor Cylinder: A Case Study

The premature failure of a connecting rod bolt from a reciprocating compressor was investigated through a combination of chemical analysis, microstructural characterization, hardness profiling, and macro and microfractographic examinations. The material was identified as 42CrMo4 steel with a sound quenched-and-tempered microstructure and no critical metallurgical anomalies, indicating that material grade and bulk heat treatment were not the primary root causes. Hardness measurements revealed a hardened surface layer on the threads, resulting in increased hardness and reduced local toughness at the thread roots. Scanning electron microscopy showed multiple fatigue crack initiation sites associated with machining-induced surface defects and microcracks at the thread roots, which acted as severe stress concentrators. Fractographic features, including ratchet marks, fatigue facets with striation-like markings, and a final dimpled overload region, demonstrated a failure mechanism dominated by multi-site high-stress fatigue followed by final ductile overload of a severely reduced cross-section. The overall evidence indicates that the failure resulted from the combined effect of severe cyclic tensile–bending loading, inadequate surface and machining quality in the threaded region, and the presence of a high-hardness, low-toughness surface layer precisely at the region of maximum stress concentration. The findings highlight the critical importance of thread root surface integrity and preload control in preventing similar bolt failures in reciprocating machinery.