Selection of FCAW-G protective gas

【Summary Description】 FCAW-G shielding gas selection. When welding flux-cored wire, whether to choose CO2 gas protection or Ar/CO2 mixed gas protection needs to consider the following three aspects. 1) The cost of protection. In general, 80% of welding assembly belongs to labor and management costs, 20% belongs to material costs, of which the cost of shielding gas accounts for about 1/4, or 5% of the material cost. Assuming that the cost of the shielding gas is the only determining factor, replacing the Ar/CO2 mixed gas with CO2 shielding gas can greatly reduce the welding cost. However, often other costs will also affect the welding assembly cost, which will be discussed later. CO2 is cheaper than Ar/CO2 because it is available at a lower cost. The world's carbon dioxide resources are extensive and rich. CO2 is often available as a by-product of other processes. For the welding industry, on the one hand, CO2 can be obtained through the processing or separation of natural gas, on the other hand, CO2 can also be obtained through air. Since the content of Ar in the atmosphere is less than 1%, a large amount of air needs to be treated and processed to extract a certain amount of Ar, and special air separation equipment is required to process the air. The air separation equipment consumes a lot of power and needs to be placed in a dedicated area. 2) Effect of welder preference and productivity. When welding with the same type and size of welding wire, the arc obtained by Ar/CO2 shielding gas welding is more stable, weaker and less spatter than that obtained by CO2 gas shielding gas welding, so it is deeply loved by welders. In the process of CO2 gas shielded arc welding, it is easy to produce large droplet transition (the droplet is usually larger than the diameter of the welding wire), resulting in unstable arc, discontinuous arc and large spatter. The Ar/CO2 mixed gas protects the small droplets from splashing (the droplets are usually smaller than the wire diameter), resulting in a more stable and continuous arc, and the splash is small, as shown in Figure 3. Another feature of Ar/CO2 mixed gas protection also increases the welder's love for it. Compared with CO2 gas welding, its heat transfer capacity is low, so it can maintain the heat and liquid degree of the molten pool. This makes the reaction of the molten pool more thorough and the weld toe portion more easily completely melted. When welding in special positions (such as uphill or inverted welding), the use of Ar/CO2 is more attractive because less skilled welders can also control the arc and increase welding productivity. When using Ar/CO2 mixed gas shielded welding, due to the high content of Ar, more heat is emitted to the welding machine than CO2 shielded gas welding. This means that the welder feels warm while welding. In addition, the torch will be hotter (the duty cycle of the torch under Ar/CO2 shielding gas is lower than that of CO2 shielding gas), which requires the use of a larger torch or more frequent replacement of the same type of torch and its vulnerable parts. 3) Welding quality. As mentioned earlier, the welding using Ar/CO2 shielding gas mixture can maintain the heat and liquid degree of the molten pool, make the molten pool react more thoroughly, and the weld toe part is easier to melt completely, compared with the welding using carbon dioxide shielding gas. Therefore, the weld forming ability and weld quality are greatly improved. In addition, the Ar/CO2 mixed gas splash is small, the weld quality is greatly improved, and the time and cost of post-weld cleaning are reduced. The lower amount of spatter also increases the cost of ultrasonic weld inspection, because if there is too much spatter, the spatter must be cleaned in advance to ensure the accuracy of ultrasonic inspection. Another quality issue that affects the appearance of the weld is the sensitivity of the shielding gas to the gas mark. Air marks, similar to earthworm marks or chicken claw marks, are small grooves, sometimes distributed on the surface of the weld. They are caused by dissolved gases in the weld metal that move out before the bath solidifies, but remain under the solidified slag. Ar/CO2 mixed gas protection has higher gas trace sensitivity than CO2 gas protection alone. The splash transition characteristic of Ar/CO2 shielding gas leads to the generation of a large number of fine droplets, which increases the surface area of the droplets, resulting in the dissolution of a large amount of gas in the weld metal. In addition to the type of shielding gas, there are other factors that affect the sensitivity of gas traces, but they are beyond the scope of this article. Common shielding gases for some major applications. Over the years, FCAW-G shielding gases have gradually become the standard in some major applications. For example, in high deposition welding applications such as flat welding and cross welding, CO2 gas shielding is usually used, because in these welding positions, Ar/CO2 mixed gas shielding does not have much advantage. The shipbuilding industry is also generally inclined to adopt CO2 gas protection, because the arc characteristics of the CO2 gas shield allow better burning of the underlying metal primer. In the North American marine industry, downward welding T, Y, K connection groove welds require smooth weld shape and small welding spatter, so Ar/CO2 mixed gas protection is more appropriate. If the construction shop uses more than one GMAW and FCAW-G process, the shielding gas for both processes is usually standardized. Sometimes, in order to obtain better spatter rate and pulse arc transition, many manufacturers also choose argon/CO2 mixed gas shielded GMAW welding.

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