Microstructural Analysis of Welded Joints in Low Carbon Steel Used for Pipeline Construction in Rivers State

This study presents a microstructural analysis of welded joints in low carbon steel pipes used in pipeline construction operations in Rivers State, South South Nigeria. The integrity of welded joints is critical to pipeline safety in Nigeria's oil and gas sector, where weld defects have been implicated in numerous pipeline failures causing hydrocarbon spillage and environmental damage. Weld samples are obtained from shielded metal arc welding and gas metal arc welding operations performed on API 5L Grade B low carbon steel pipe specimens. Cross-sectional specimens are prepared by standard metallographic preparation and examined by optical microscopy to characterise the weld zone, heat-affected zone (HAZ), and base metal microstructures. Vickers microhardness traverses are performed across the weld cross-section. Tensile and bend tests are conducted on transverse weld specimens in accordance with API 1104 requirements. Findings reveal a ferritic-pearlitic base metal microstructure transforming to a coarser-grained HAZ structure with evidence of grain boundary ferrite and Widmanstatten ferrite formation, which are associated with reduced toughness. SMAW joints exhibit more pronounced HAZ grain coarsening than GMAW joints, attributed to the higher heat input of the SMAW process. All specimens meet minimum tensile strength requirements, but microhardness peaks in the HAZ exceed API 1104 hardness limits for sour service in two SMAW specimens. Keywords: welded joint, low carbon steel, pipeline, microstructure, Rivers State.