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Shielding gases are inert or semi-inert gases that are commonly used in several welding processes, most notably gas metal arc welding and gas tungsten arc welding. Their purpose is to protect the weld area from atmospheric gases, such as oxygen, nitrogen, carbon dioxide, and water vapour. Depending on the materials being welded, these atmospheric gases can reduce the quality of the weld or make the welding process more difficult to use. Other arc welding processes use other methods of protecting the weld from the atmosphere as well – shielded metal arc welding, for example, uses an electrode covered in a flux that produces carbon dioxide when consumed, a semi-inert gas that is an acceptable shielding gas for welding steel.

Common shielding gases

Shielding gases fall into two categories—inert or semi-inert. Only two of the noble gases, helium and argon, are cost effective enough to be used in welding. These inert gases are used in gas tungsten arc welding, and also in gas metal arc welding for the welding of non-ferrous materials. Semi-inert shielding gases, or active shield gases, include carbon dioxide, oxygen, nitrogen, and hydrogen. Most of these gases, in large quantities, would damage the weld, but when used in small, controlled quantities, can improve weld characteristics.

Properties of shielding gases

The important properties of shielding gases are their thermal conductivity and heat transfer properties, their density relative to air, and how easy they undergo ionization. Gases heavier than air (e.g. argon) blanket the weld and require lower flow rates than gases lighter than air (e.g. helium). Heat transfer is important for heating the weld around the arc. Ionizability influences how easy the arc starts, and how high voltage is required. Shielding gases can be used pure, or as a blend of two or three gases. [143638] [143639]

Helium is lighter than air; larger flow rates are required. It is an inert gas, not reacting with the molten metals. Its thermal conductivity is high. It is not easy to ionize, requiring higher voltage to start the arc. Other gases are often added. Blends of helium with addition of 5-10% of argon and 2-5% of carbon dioxide can be used for welding of stainless steel. In comparison with argon, helium provides more energy-rich but less stable arc. Helium and carbon dioxide were the first shielding gases used, since the beginning of World War 2.

Argon is heavier than air; lower flow rates are needed to blanket the weld. It is an inert gas, not reacting with the molten metals. It has low thermal conductivity. It ionizes easily. It is often used as pure when welding aluminium and other nonferrous metals, though other gases can be added. A blend of argon with 25-50% of helium is used for some nonferrous metals, as helium improves heat transfer into the base material and makes the molten metal more fluid. An oxidizing component (oxygen, carbon dioxide) is usually added to stabilize the arc for welding of steels; without it the arc control can be difficult as the arc tends to stray. In industrial gas business it is known as "the big A".

Carbon dioxide has good heat transfer properties; it dissociates in the weld and recombines in contact with the colder metal. Due to the presence of dissociated oxygen, the weld zone has oxidizing properties, producing more slag. Carbon dioxide can be used as pure, or in a mixture with argon as 5 to 25%. Increasing percentage of carbon dioxide increases width and depth of the weld penetration. For welding of stainless steels where carbon content control is required, an argon-helium blend with 1-2% of carbon dioxide can be used. "Trimix" blends of argon-oxygen-carbon dioxide are more common in United Kingdom, while argon-carbon dioxide blends are more common in the USA. In comparison with argon-carbon dioxide mixture, for steel welding, pure carbon dioxide increases spatter and the arc is less stable. [143640] Pure carbon dioxide provides deep weld penetration and is very cheap. [143641]

Oxygen is used in small amounts as an addition to other gases; typically as 2-5% addition to argon. It enhances arc stability and reduces the surface tension of the molten metal, increasing wetting of the solid metal. Its presence increases the amount of slag. Argon-oxygen (Ar-O2) blends are often being replaced with argon-carbon dioxide ones. Argon-carbon dioxide-oxygen blends are also used.

Nitrogen is used for welding of some stainless steels. It increases the weld penetration and enhances arc stability. It however can cause porosity in carbon steels. Argon-carbon dioxide-nitrogen blends can be used. Pure nitrogen is also used, or can be blended with 10% of hydrogen, depending on application. Blends with nitrogen content are used to weld nitrogen-containing alloys (up to 0.5% nitrogen increases mechanical properties and resistance to pitting corrosion) to prevent loss of nitrogen from the metal.

Hydrogen is used for welding of some stainless steels. It improves the molten metal fluidity, and enhances cleanness of the surface. It can however cause hydrogen embrittlement of many alloys and especially carbon steel, so its application is usually limited only to some stainless steels. It is added to argon in amounts typically under 10%. It can be added to argon-carbon dioxide blends to counteract the oxidizing effects of carbon dioxide. Its addition narrows the arc and increases the arc temperature, leading to better weld penetration.

Nitric oxide addition serves to reduce production of ozone. It can also stabilize the arc when welding aluminium and high-alloyed stainless steel.

Other gases can be used for special applications, pure or as blend additives; e.g. sulfur hexafluoride or dichlorodifluoromethane. [143642] Sulfur hexafluoride can be added to shield gas for aluminium welding to bind hydrogen in the weld area to reduce weld porosity. [143643] Dichlorodifluoromethane with argon can be used for protective atmosphere for melting of aluminium-lithium alloys. [143644]

Applications

The applications of shielding gases are limited primarily by the cost of the gas, the cost of the equipment, and by the location of the welding. Some shielding gases, like argon, are expensive, limiting its use. The equipment used for the delivery of the gas is also an added cost, and as a result, processes like shielded metal arc welding, which require less expensive equipment, might be preferred in certain situations. Finally, because atmospheric movements can cause the dispersion of the shielding gas around the weld, welding processes that require shielding gases are often only done indoors, where the environment is stable and atmospheric gases can be effectively prevented from entering the weld area.

See also



References

  • Cary, Howard B. and Scott C. Helzer (2005). Modern Welding Technology. Upper Saddle River, New Jerseymarker: Pearson Education. ISBN 0-13-113029-3.



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