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An 80%Ar-10%CO2-10%He ternary gas mixture was used as the shielding gas during the narrow-gap welding of thick Q690E high-strength steel plates. Complete and defect-free welded joints were obtained, and the microstructure and mechanical properties of the welded joint were investigated. The weld zone consists of a solidification area and interlayer zone, and the heat-affected zone consists of a coarse-grain heat-affected zone (CG-HAZ) and a fine-grain heat-affected zone (FG-HAZ). The microstructures of the weld zone are mainly lath bainite (LB), acicular ferrite (AF), and granular bainite (GB). The microstructure of the CG-HAZ is lath martensite (LM) and the microstructure of FG-HAZ is GB. Methods with different heat inputs were used to study their effects on the mechanical properties of the welded joint. It was found that the microstructure and mechanical properties of the welded joints are better with lower heat input. With tandem wire narrow-gap GMA welding, the tensile strength of the joints declined from 795.3 to 718.3 MPa and the impact toughness at −40 °C resulted in a weak position in the weld zone, which declined from 76~81 J to 55~69 J, when the welding speed reduced from 350 to 250 mm/min. With oscillating-arc narrow-gap GMA welding, the tensile strength achieved 853.4 MPa and the impact toughness at −40 °C was around 69~87 J. The results indicated that, under the appropriate heat input, the tensile strength of the joint exceeds 770 MPa and the low temperature impact toughness at −40 °C exceeds 69 J. A 155 mm-thick Q690E steel welded joint was obtained and the mechanical properties of the welded joint meets the requirements of the offshore drilling platforms.

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