Abstract
<jats:p>In present study, Metal Inert Gas (MIG) welding with ER309L austenitic filler wire was used to weld API X70 and SS 401 steel. This particular joint is mainly used in oil and gas plants where stainless steel need to be connected to the high strength low alloy carbon steel pipelines. MIG welding process is cost effective in these industrial fabrication and maintenance applications as it has high deposition rates and productivity. The investigation focuses on the influence of two distinct heat input levels-HI-1 (0.814 kJ/mm) and HI-2 (0.573 kJ/mm) on the weldment’s microstructure, mechanical performance, and corrosion resistance in simulated seawater and sodium thiosulphate environments. Microstructural examination revealed that the lower heat input (HI-2) produced coarser grains and reduced martensite-austenite (MA) constituent formation in the fusion zone, resulting in improved toughness. Conversely, the higher heat input (HI-1) led to finer grains but increased brittleness. In the heat-affected zone (HAZ), API X70 exhibited grain coarsening-induced softening, while SS 401 demonstrated sensitization and MA phase development, negatively affecting corrosion resistance. Corrosion assessments indicated superior performance of SS 401 over API X70, with minimal weight loss (0.75 mm/year) in seawater due to its protective chromium-rich passive film. However, the dissimilar welded joints, particularly those produced with HI-1-showed significant corrosion degradation (2.1 mm/year in thiosulphate solution), primarily due to microstructural inhomogeneity and galvanic coupling. Mechanical testing results showed that HI-2 welds offered enhanced impact toughness (196 J in the fusion zone) compared to HI-1 (154 J), although the API X70 HAZ under HI-2 exhibited decreased toughness (120 J) likely due to thermal softening. Hardness measurements revealed maximum values near the fusion boundary (~380 HV in HI-1 samples), with noticeable softening in the HAZ regions. These findings suggest that optimizing heat input is critical for balancing mechanical integrity and corrosion resistance in dissimilar metal joints.</jats:p>