Research On Robotic Tandem MAG Welding Technology Of The Thick High-strength Steel Plate

With the gradual development of deep sea offshore drilling platforms,the strength and the thickness of the steel used in platforms is becominghigher and higher. The welding of drilling platform legs, as one of the mostcritical step in building a platform, requires higher welding efficiency andbetter welding quality. Because the drilling platform construction in Chinastarts relatively late, the technical level in China is quite backward. Inorder to change this situation, this paper focuses on the research of robotictandem MAG welding procedure of the jack-up drilling platform leg withlarge thickness Q690E HSLA steel.Firstly, the tandem MAG welding temperature field and thermalcycling curves of welded joints with different wire spacing were simulatedwith the ABAQUS finite element analysis software. Through comparisonwith single-wire welding, the best spacing of the two wires was determined.And the characteristics of robotic tandem MAG welding thermal cycle curves were explored to provide theoretical guidance for the follow-upwelding experiments. Secondly, through the experiments of single-passwelding and analysis of weld cross section, the effects of different weldingpower sources (pulse mode or CV (Constant Voltage) mode) and weldingspeeds on the formation of robotic tandem MAG weld were studied.Finally, combining with the above test results, the robotic tandem MAGwelding experiments were carried out with50mm thickness Q690E steel asthe base metal. Through the mechanical properties testing andmicrostructure observation of welded joints and the impact fractureanalysis of SEM photos, the effects of welding heat input, weldingspecifications (high current high speed welding and low current low speedwelding) and shielding gas on the mechanical properties andmicrostructure of Q690E robotic tandem MAG welding joint wereresearched.The final study results show that：(1) When the wire spacing is longer than20mm, there is neckingdown on the center of the weld pool and there are two temperature peaks inthe weld thermal cycle curves. When the wire spacing is25mm, the T8/3ofthe weld zone increases to71.6s and the cooling rate reduces.(2) When the other welding conditions are the same, the weld penetration in both of the power modes are4.7mm. But the weld width andthe surplus height of the seam in the pulsed power mode are16.0mm and3.5mm respectively, which are higher than the results in CV mode. Withthe increase of the welding speed, the weld penetration of robotic tandempulsed MAG first increases and then decreases. When the welding speed is9.2mm/s, the maximum weld penetration is5.9mm.(3) For robotic tandem MAG welding of Q690E HSLA steel, whenthe welding power source is a pulse mode, the welding heat input is1.57KJ/mm and the shielding gas is a mixed gas with80%Ar and20%CO2, the weld joints can obtain excellent comprehensive mechanicalproperties. The tensile strength is more than770MPa and the-40℃low-temperature impact energy is higher than69J.