Analytical Investigation of Slag Inclusions in Nigerian Weld Fabrications Using Electron Microscopy and Their Effect on Fatigue Life

This study presents an analytical investigation of slag inclusions in multi-pass shielded metal arc welds produced at a fabrication yard in Port Harcourt, Rivers State, South South Nigeria, characterising inclusion morphology, composition, and spatial distribution by electron microscopy, and quantifying their effect on the fatigue life of welded joints. Slag inclusions are among the most common weld defects in SMAW joints and their influence on fatigue performance is a critical structural integrity concern for Nigerian oil and gas fabrications. Weld cross-section samples from carbon steel butt and fillet joints are prepared by standard metallographic sectioning. SEM-EDX and EPMA mapping are applied to characterise inclusion size, morphology, elemental composition, and distance from the weld toe, the highest-stress region. Fatigue specimens (R ratio 0.1) are tested at three stress ranges to generate S-N data. Inclusion population metrics are statistically correlated with fatigue life using multivariate regression. The effect of inclusion proximity to the weld toe on fatigue crack initiation site is assessed by post-fracture SEM fractography of fatigue specimens. Findings show that inclusions greater than 300 micrometres in maximum dimension within 0.5 mm of the weld toe reduce mean fatigue life by 35 to 55 percent relative to inclusion-free toe regions. Inadequate inter-pass cleaning is identified as the primary inclusion generation mechanism. Keywords: slag inclusion, weld defect, fatigue life, electron microscopy, Port Harcourt.