Faulty machining and micro-segregation assisted fatigue failure of cylinder head stud of a diesel generator

A detailed failure analysis of cylinder head stud bolt of a four-stroke diesel generator has been presented. The study included optical emission analysis, tensile testing, hardness testing, optical microscopy, SEM, and EDX analyses. The investigation showed that the main mechanism of failure was corrosion-fatigue. Fatigue initiation was assisted by pitting corrosion induced by faulty machining, moisture, and presence of tempered martensite and non-metallic inclusions at the surface. Propagation of the fatigue crack was encouraged by localized corrosion within the tempered martensite, non-metallic inclusions, and Mn/Cr/Mo/Si micro-segregation. Both excessive C and Mn/Cr/Mo/Si micro-segregation caused the formation of mixed microstructure of tempered martensite and fresh martensite with retained austenite. The non-uniform microstructure led to non-uniform hardness distribution and caused localized deformation during cyclic loading and promoted fatigue crack propagation.