Shielding gas for the MAG welding of carbon and low alloy steel

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    MAG welding is also known as “metal active gas welding”, is a gas shielded arc welding method by adding a certain amount of active gas such as O2 , CO₂, etc., in the inert gas argon. MAG is characterized by its good welding quality, high efficiency, easy automation and other advantages that can be used for welding in various positions, especially for carbon steel, alloy steel and stainless steel and other ferrous metal materials. It can be carried out by short circuit transition, jet transition and pulse jet transition, which can obtain stable welding process performance and good welding joints, The purpose of welding protective gas is to improve the quality of the welding seam, reduce the width of the heating band of welding seam, and avoid material oxidation. The shielding gas for MAG include:

    Pure gases: Pure argon or carbon dioxide(CO₂);

    The binary gas mixture: Argon and oxygen, argon and carbon dioxide, argon and helium, and argon and hydrogen.

    The ternary mixture: Helium argon and carbon dioxide.

    Different welding mixtures should be selected according to different welding materials and its base materials.

    Ar + CO₂

    This is the most commonly used typical mixed gas, suitable for short circuit transition, spray transfer and under the condition of the pulse transition welding of carbon steel, low alloy steel materials, has a stable arc, little splash, easy to obtain axial injection transition, and oxidation resistance, which overcome the surface tension, the liquid argon welding metal viscous, cathode spots easily slips and other issues, can be used for high speed welding of low carbon steel and low alloy steel.

    The commonly used mixing ratio (volume) is 70%Ar+30% CO₂ (C-30) and Ar80% + CO₂20% (C-20), which is suitable for all-position welding under short circuit transition, such as ASTM (America) A335 P11 pipe TIG backing welding +MAG filler cover welding process, and the qualified rate is 100%. During MAG welding, the shielding gas shields the atmosphere while protecting the high-temperature metal. Poor protection will cause pores and pits. In the Ar+CO₂ mixture, obvious pores can be seen in the welding of 2% CO₂, while the pores are greatly reduced in the welding of 10% CO₂. When pure CO₂ is used, pores are almost non-existent.

    Ar + O2 

    The arc stability can be improved by adding trace O2 in shielding gas, and the surface tension of droplet, pool and the defect of undercut can be significantly reduced. When welding high alloy steels such as stainless steel and high strength steels with higher strength grade, the content of O2 should be controlled in 1% ~ 5%, and when welding carbon steels and low alloy structural steels, the content of O2 can reach 20%. The mixture is suitable for jet transition and pulse transition of carbon steel and low alloy steel welding wire, suitable for flat welding and fillet welding, as well as steel melting very narrow gap welding.

    Ar+He

    The shape and color of the arc change with the change of the ratio of the gas in the welding process of Ar-He mixture gas. With the more content of Helium in mixed gases, the arc gradually shrink, arc column and concentration, and gradually turn into orange, this is mainly due to the pure helium line in the orange wavelength range, with the increasing of helium content, the number of helium atom ionization, a compound in arc gradually increased, the relative intensity of the spectral lines is also increasing, the visually changed from white to orange color.

    80%Ar + 15%CO₂+ 5%O2

    The ternary mixed gas concentrates the respective advantages of Ar, CO₂ and O2, and the arc is more stable, the welding depth and width are moderate, and can obtain a good forming. It can weld carbon steel, low alloy steel, stainless steel of various thickness, suitable for various transition forms, can be called the perfect gas mixture.