Failure Analysis and Prevention of Fatigue Fracture in an Automotive Connecting Rod with Surface Decarburization

This paper presents a failure analysis of an automotive connecting rod removed from a three-cylinder, 998 cm 3 engine after an abrupt interruption of engine operation during service. Three connecting rods from the same engine were investigated: one fractured, one plastically deformed, and one apparently intact. The investigation included visual inspection, optical emission spectrometry, Rockwell hardness testing, optical microscopy, and scanning electron microscopy. The chemical analysis revealed approximately 0.72 wt.% carbon, low concentrations of alloying elements and a sulfur content of approximately 0.073 wt.%. The absence of the nickel–chromium–molybdenum alloying system characteristic of SAE 4340 demonstrated that the material was not chemically compatible with the specification provided by the manufacturer on the quality control certificate. The measured carbon content was close to that of SAE 1070; however, the sulfur content exceeded the usual nominal limit for this grade. The average hardness ranged from 28 to 30 HRC. Optical microscopy revealed a predominantly pearlitic microstructure with ferritic regions and a ferrite-rich surface region interpreted as decarburization. The decarburization was identified only by optical microscopy; no microhardness profile or carbon-concentration gradient was obtained. Scanning electron microscopy clearly revealed fatigue striations in the crack-propagation region. The crack initiated near a geometric stress concentration associated with the connecting rod shank and lubrication channel and propagated progressively until final overload fracture. The failure was attributed to the combined effects of an improper material used instead of the expected SAE 4340, lower hardenability, surface decarburization, geometric stress concentration, and cyclic engine loading. The material was inferred to have relatively low hardenability, and the locally decarburized surface layer was considered capable of reducing the local resistance to cyclic plastic deformation and increasing the susceptibility to fatigue-crack initiation. A reported hydraulic-lock event may have created to the observed deformation and final overload.