Welding design of API 5L X65 pipeline steel: Effects of robotic hybrid laser arc welding versus GMAW on fracture toughness evaluated by SENT tests in air and hydrogen

• X65 was welded by metal active gas (MAG) and hybride laser MAG welding (HLAW). • In HLAW layered oxides and Mn depletion zones near inclusions were formed. • HLAW increased acicular ferrite density, boosting toughness and H sensitivity. • MAG has uniform oxide inclusions but reduced toughness and hydrogen resistance. • HLAW is optimal for pipeline welding with superior properties and efficiency. This study examines the microstructural and mechanical properties of ferritic steel specimens welded using the gas metal arc welding (GMAW) and hybrid laser arc welding (HLAW) processes, focusing on their behaviour in both air and hydrogen environments. Using atom probe tomography, we conduct a detailed atomic-scale analysis to investigate element distribution and oxide inclusion composition. The results show distinct nano chemistry differences, with HLAW specimens exhibiting layered crystalline oxide structures and Mn depletion zones around the inclusions that lead to the formation of higher densities of acicular ferrite and high-angle grain boundaries (HAGBs). These microstructural features are linked to mechanical properties assessed using single-edge notch tension (SENT) testing. The results show that the higher density of HAGBs in HLAW enhances fracture toughness in air and resistance to hydrogen embrittlement. In contrast, the GMAW specimens which show more uniform elemental distribution in amorphous oxide inclusions and lower HAGB density adopt a lower fracture toughness in air and high hydrogen embrittlement sensitivity. This study concludes that the HLAW process is better suited for applications like pipeline welding, where high strength, resistance to hydrogen-induced damage, higher speed, and lower cost are essential.