Process characteristics of CO2/MAG welding technology applied in boiler and pressure vessel industry

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1 Advantages, Disadvantages and Misunderstandings of CO2 Gas Shielded Welding The CO2 gas shielded welding method has the characteristics of open arc, slag-free, energy saving, high productivity, low cost, small deformation, strong rust resistance, low hydrogen content in the weld, good crack resistance, and can be welded in all positions. Therefore, this welding method is widely used, and its popularity is increasing year by year. However, it is rarely used in the boiler and pressure vessel industry. Analysis of the reasons mainly includes the following misunderstandings. 1.1 The quality of the welded joints of CO2 welding is lower than that of electrode arc welding. The spatter during the welding process is large and it is not suitable for welding important welding products. 1.2 The arc atmosphere has strong oxidizing properties, the oxygen content of the weld metal is high, and the impact toughness value of the welded joint is low. ； The welding process qualification is unqualified and it is difficult to apply it to welding production. 1.3 CO2 welding seam formation is poor and the welding bead has a narrow bulge (such as a hump welding bead) ； Welds are prone to welding defects such as undercuts and lack of fusion. With the improvement of advanced control technology of CO2 welding power sources, the development of high-quality welding materials and the application of new welding processes, the above-mentioned shortcomings of CO2 welding (large spatter, poor forming, low toughness) can be effectively solved. 2 Improvement of CO2 welding process 2.1 Experts proposed in the 1980s: The CO2 welding process is not suitable for welding boilers and pressure vessels because its plasticity and toughness are unstable. The main reason is that the past CO2 welding wire standards followed the old standards of the former Soviet Union, and the Mn content of the welding wires was relatively high.: (Mn: 1.8~2.1%), high Mn/Si ratio, high weld strength, and low plastic toughness. With the improvement of welding wire quality, European and American welding wire standards are quoted, and the Mn/Si ratio is appropriate (Mn: 1.4~1.85% Si: 0.8~1.15%), the plastic toughness value of CO2 welding seam is slightly higher than that of alkaline low hydrogen electrode. 2.2 Using mixed gas shielded welding (MAG welding), the alloy element transition coefficient is high and the weld has excellent comprehensive mechanical properties. The impact toughness value of welded joints is relatively high. For example, the mechanical properties of the deposited metal of MG-51T solid welding wire produced by Tangshan Kobelco are shown in the table below.: Welding method Yield strength σs (MPa) Tensile strength σb (MPa) Elongation δ (%) Impact toughness Akv (J) CO2 46056032110MAG520600311602.3 The arc of Ar+CO2 mixed gas shielded welding (MAG welding) stabilizes the cathode spots and improves the stability of the arc. Adding 75~95% Ar to the shielding gas increases the thermal power of the arc, reduces the surface tension of the molten pool, and has good wettability of the molten pool metal. The weld bead is flat without convex defects, improves the weld penetration shape and appearance, and reduces the tendency of undercutting. 2.4 MAG welding arc enhances the stability of droplet transfer. There is less spatter during droplet short-circuit transition (10~20% less than CO2 welding). When the welding current exceeds the critical current of jet transition (such as current I >280A during ø1.2 solid wire MAG welding), the molten droplets reach the jet transition state, achieving spatter-free welding. 2.5 When the welding current is lower than the critical current, pulse melting electrode power supply (such as Tangshan Panasonic AG1 series pulse MIG/MAG welding machine) can achieve pulse droplet and jet transition without spatter. 2.6 CO2 welding uses flux-cored wire. Since the arc and molten pool are protected by gas + slag, it has the characteristics of less spatter, less pores, high toughness, large penetration depth, and high deposition speed. It is more suitable for the welding of important pressure components of boiler pressure vessels. 2.7 Controlling the appearance of the weld also relies on the skilled operating skills of the welder. For example, the gun-handling method for vertical welding and all-position welding is shown in Figure 1. It is recommended to use the counter-crescent gun-handling method. The welding seam has a low residual height and no undercut defects. 3 Application of CO2/MAG welding process on welding pressure components of boiler pressure vessels 3.1 Φ≥159 * 5. Boiler pressure vessel barrels and pipes use MAG automatic welding and semi-automatic welding or tungsten argon arc welding (TIG welding) base + MAG semi-automatic filler cover welding, replacing electrode arc welding and submerged arc automatic welding, and the work efficiency is increased by more than 2 to 3 times. 3.2 The MAG welding process is used for the longitudinal and circumferential welds of the pressure vessel barrel, the flange, manholes and reinforcement plate welds, which can reduce the total welding cost by 39.6-58.7% (compared to electrode arc welding), with an average reduction of 48.5%. 3.3 The boiler body and the heat-resistant steel pipe are welded using special flux-cored wire CO2 gas shielded welding. Compared with electrode arc welding, the efficiency can be increased by 2.02-3.88 times. 3.4 For flux cored wire CO2 gas shielded welding of spherical containers, the X-ray flaw detection pass rate reaches 99.04% ； The weld quality is high, the cost is low, and considerable economic benefits are achieved. The cost of CO2 welding measured in a company's oil tank construction welding was 65% lower than that of electrode arc welding. 3.5 The comprehensive advantages of CO2/MAG welding process of high quality, high efficiency and low cost are unmatched by other welding methods. According to relevant information: In a certain industry, the amount of deposited metal in CO2 welding accounts for 8% of the total deposited welding amount to 15%, and an economic benefit of 565 million yuan can be obtained. 4 Summary 4.1 The process of combining Ar+CO2 mixed gas protection and solid welding wire ； Using a process combining CO2 gas shielding + flux cored wire ； After the welding process evaluation, it is completely suitable for the welding of pressure components of boiler pressure vessels. 4.2 Non-pressure components are welded using a process that combines CO2 gas protection + solid welding wire, which can meet the technical requirements of the welding structure. The CO2/MAG welding process is a high-quality, efficient and low-cost welding method. It will be vigorously promoted and applied in the boiler and pressure vessel industries and will achieve good economic benefits.