Microstructure and Impact Toughness of A516 Grade 70 Steel Joints Welded using DF-Modulated P-GTAW Followed by SAW

To enhance the quality and production efficiency of welding medium-thick A516 Grade 70 steel, a novel dual-frequency modulated pulsed gas tungsten arc welding technique was proposed to achieve deep penetration on the large root face without backing. Additionally, submerged arc welding was employed for filler and cover welding, further reducing the welding time and optimizing the microstructure of the welded joints. The impact toughness of the welded joints was evaluated and correlated with the microstructural changes. Results indicated that a fully penetrated root pass weld with a depth of 5 mm was successfully achieved. During one-pass filler welding, the weld metal exhibited superior impact toughness compared to the heat-affected zone (HAZ), which was attributed to reduced grain size and an increased proportion of acicular ferrite. Conversely, in two-pass filler welding, the HAZ exhibited higher impact toughness than the weld metal, reaching 107 J, due to the formation of strip-like bainite that impeded crack extension. This study demonstrated the feasibility of the proposed method for achieving high-quality and efficient welding of A516 Grade 70 steel.