Selection of Shielding Gases for Flux Cored Arc Welding: In-depth Analysis and Application Guide

In the field of welding, flux cored arc welding (FCAW-G) plays a significant role and is widely used in heavy manufacturing, construction, shipbuilding, offshore facilities and other industries for the welding of low-carbon steel, low-alloy steel and various alloy materials. The choice of shielding gas is crucial for the welding effect, and the commonly used ones are 100% pure CO? or a mixture of 75% - 80% Ar and 20% - 25% CO?. This article will deeply explore the advantages and disadvantages of these two shielding gases to assist welding practitioners in making informed choices.

Shielding Gas Working Principle: The Invisible Shield in the Welding Area

?The core function of the shielding gas is to block the air and prevent oxygen, nitrogen and water vapor from eroding the welding pool and electrode. During the welding process, the shielding gas is ejected from the welding torch nozzle, creating an isolated environment around the electrode to ensure the stable combustion of the arc and the normal solidification of the molten pool. Both CO? and Ar/CO? mixtures can effectively fulfill this responsibility and also participate in the construction of the arc plasma region, affecting the heat conduction of the arc and the force on the molten pool, although there are differences in their performance in these aspects.

Shielding Gas Characteristics: Differences from a Microscopic Perspective

?1. Ionization Potential and Arc Stability: The ionization potential determines the ease of gas conduction. The ionization potential of CO? is 14.4 eV, lower than that of Ar at 15.7 eV, which gives CO? an advantage in igniting and maintaining the arc and can quickly establish a stable welding arc.

2. Heat Conduction and Droplet Transfer: The high heat conduction ability of CO? makes it different from the Ar/CO? mixture in droplet transfer, arc shape, weld penetration and temperature distribution. The higher heat conductivity promotes the formation of large droplet transfer during droplet transfer, affecting the weld formation and penetration control.

3. Reactivity and Weld Composition: CO? is an inert gas at room temperature, but it decomposes into CO, O? and oxygen atoms under the high temperature of the arc and becomes an active gas, which is prone to oxidation reactions with metals. Ar is an inert gas, and the Ar/CO? mixture has relatively lower reactivity. This difference leads to changes in the content of alloy elements in the weld metal. For example, when using the Ar/CO? mixture, the deposition efficiency of the electrode alloy is higher because some alloy elements react with the oxygen decomposed from CO? to form oxides that enter the slag, increasing the content of deoxidizers such as Mn and Si in the weld, thereby increasing the weld strength but reducing the elongation and impact toughness.

Inert Gases and Mixture Gases: Compatibility in Application

Although inert gases can protect the molten pool, when used alone for the welding of iron-based metals, problems may occur. For example, when using Ar to protect the welding of stainless steel, the arc will be elongated and the outer coating of the electrode will melt prematurely, resulting in poor weld formation. Therefore, Ar/CO? mixture gases are mostly used for the welding of iron-based metals. In North America, 75% Ar + 25% CO? or 80% Ar + 20% CO? mixtures are commonly used for stainless steel FCAW-G welding, and some welding wires require 90% Ar + 10% CO?, and an Ar content lower than 75% will affect the arc performance.

Factors in Shielding Gas Selection: Trade-offs between Cost, Welder and Quality

1. Cost Consideration: Choices behind the Economic Account: In welding costs, labor and management account for 80%, materials account for 20%, and shielding gas accounts for about 1/4 of the material cost. CO? has a wide range of sources and can be obtained at low cost through natural gas processing, air separation and other methods. However, Ar is scarce in the atmosphere, and its extraction requires complex equipment and high energy consumption, resulting in high costs. If only the gas cost is considered, CO? is the first choice, but the actual decision needs to be comprehensively weighed.

2. Welder Preference and Productivity: The Connection between Operating Experience and Efficiency: When using the same welding wire, the Ar/CO? mixture has a more stable arc, less spatter, and stable droplet transfer, which can maintain a good state of the molten pool, is beneficial to welding in special positions and improves productivity. However, its higher Ar content increases the heat radiation received by the welder, and the welding gun is prone to overheating, requiring a higher-power welding gun or more frequent replacement of wearing parts.

3. Welding Quality: The Key Guarantee of Weld Quality: The Ar/CO? mixture performs well in weld formation, reducing spatter and reducing post-weld cleaning costs, which is helpful for ultrasonic testing. However, it is more sensitive to gas marks. Because the fine droplets increase the gas dissolution amount, gas marks may affect the appearance and performance of the weld.

Typical Application Scenarios: Selection Preferences in Industry Practice

In flat and horizontal high deposition welding, CO? is commonly used due to its cost advantage and meeting welding requirements; the shipbuilding industry favors CO? because its arc can effectively burn off the primer on the base metal; in the North American offshore construction industry, when welding specific groove welds, the Ar/CO? mixture is preferred due to the pursuit of weld appearance and low spatter. If multiple gas shielded welding processes are used in a workshop, the shielding gas is often standardized, and some manufacturers also choose the Ar/CO? mixture to optimize the GMAW welding effect.

Conclusion: Comprehensive Consideration and Precise Decision-making

The selection of shielding gas for FCAW-G needs to balance cost, quality and productivity. Different manufacturers have different production requirements and cost considerations, and should be determined based on the impact of the gas on various aspects in actual welding operations. After selecting the shielding gas, it is necessary to select a suitable electrode to ensure the best balance of welding quality and efficiency.

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