INFLUENCE OF SHIELDING GAS COMPOSITION ON THE MICROSTRUCTURE AND HARDNESS OF WELDED JOINTS IN API 5L X70 PIPE STEEL

This study investigates the influence of shielding gas composition on the microstructural evolution and mechanical properties of welded joints in API 5L X70 pipe steel. The research was conducted using Metal Active Gas welding with three distinct gas mixtures: pure carbon dioxide, an argon-carbon dioxide blend, and an argon-oxygen mixture. Systematic analysis using optical microscopy and Vickers microhardness testing revealed that the argon-carbon dioxide mixture significantly promotes the formation of acicular ferrite. This microstructural refinement resulted in an optimal balance of hardness and toughness, while simultaneously reducing welding defects such as spatter and porosity. The findings provide a technical basis for optimizing welding protocols in high-pressure gas infrastructure to ensure structural reliability and safety.