Abstract： High strength low alloy steel Q690E thick plate of offshore drilling platform was welded by GMAW with different heat inputs. The effect of heat input on the microstructure and toughness of weld joint was investigated. The results indicated that the microstructure of the weld mainly consisted of acicular ferrite, and the best low temperature toughness could be obtained at the heat input 1.56 MJ/m. Under the condition of the same welding heat input, the microstructure of the weld was more uniform and the low temperature toughness was better at smaller welding current and slower welding speed. Besides, under the condition of the same heat input, the microstructure of the weld from robotic singlewire MAG welding was based on acicular ferrite and granular bainite. Moreover, the toughness of the weld metal of robotic single wire MAG welding was better than twinwire MAG welding.
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