| Large thick plate structures of high-strength steel are widely used in marineequipment, shipbuilding, pressure vessels and heavy-duty machinery and so on. At present,the welding of those large thick-walled structures generally use manual SMAW orsemi-automatic welding. The conventional process is not only complex, but also theproduction inefficient, long product cycles, unstable quality, high labor intensity and poorworking environment and so on. So a new welding technology for hick plate structures isurgently needed, which is efficient, high-quality, energy-saving, low supplies, low energyconsumption. This paper studies a new kind of welding technology for large thick platestructures, narrow gap MAG welding(NG-MAG), under the background of the rack platebutt welding rig. The new process uses double U-shaped groove, the groove angle is1°~2°, the assembly gap is small, one seam per layer, without carbon arc gouging. Comparedwith traditional manual welding, NG-MAG vertical welding greatly improves productionefficiency, reduces labor intensity and improves the production environment, savingmanufacturing costs.Firstly, the feasibility of the new welding process, the narrow gap MAG verticalwelding for rack plate, is investigated. Analyzes the advantages of new technology Withtraditional manual SMAW comparative, sums up the technical difficulties of the NG-MAGvertical welding new process, such as unstable sidewall fusion，uneasy shape of weld.To research technical difficult problems of NG-MAG vertical welding, simulates thetemperature field of the NG-MAG welding by the large-scale commercial softwareMSC.MARC, study thermal physics characteristics of the new technology, such as weldingthermal cycle and so on. Combined analyzes the weld pool dynamic characteristics, whenthe arc stays in the groove sidewalls, the pool is approximately circular, head thin tail thick ellipsoidal when moving from the center towards the groove sidewalls, and head thick tailthin ellipsoidal when moving from the groove sidewalls towards the center. Unlikeconventional non-swing welding, arc swing welding thermal cycle curve shows"multi-peak" phenomenon in the process of heating and cooling, which caused by theheat source moves gradually away from and successive approximation. The actualtemperature field of NG-MAG vertical welding is measured, the simulation results andexperimental measurement results are identical each other. Through simulation analysisfound that, the residence time sidewall and the swing amplitude are important factors in theimpact of sidewall fusion and weld shape.To further confirm the important factor of influencing the sidewall fusion and weldshape, A series of comparison tests are carried out under different parameters, such asdifferent groove gap, sidewalls residence time, the distance from the arc center to the sidewall and swing frequency, and Then optimize each process parameters. It was found thatwhen the groove gap is18~22mm, the arc can steady burning, good sidewall fusion andthe weld is concave and beautiful; the long sidewalls residence time conducive to goodsidewall fusion, and the simulation results are consistent with test, but too long sidewallsresidence time can make the pool of the two side falling serious; the distance from the arccenter to the side wall is the important factor of arc burning, too far or too near cause thebad sidewall fusion; oscillating frequency is another important factor of the weld shape, theslower swing, the more discontinuous weld, the faster wobble, the more unstable torch,both will ultimately affect the weld shape.These results lays the theoretical foundation for NG-MAG vertical welding in largethick plate structures welding, and provides guidance for the actual welding. It canpromote further research of China’s large thick high-strength steel welding efficiency andautomation technologies. It is of great significance for manufacturing level anddevelopment of marine engineering equipment, vessels, pressure vessels and other areas. |
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