Protective gas used to prevent or reduce atmospheric contamination of a weld, especially by oxygen and nitrogen. See PROTECTIVE ATMOSPHERE.
The arc is said to be shielded when the metal from the electrode, as it passes through the arc, is protected from contact with the oxygen and nitrogen of the air. With a shielded metal arc electrode, the shielding is usually accomplished by using a heavily coated electrode. The coating, in some instances, produces carbon monoxide and hydrogen as it burns, and also forms a crucible, or hollow shell, extending beyond the end of the electrode. A slag is also formed over the molten metal, protecting it from the air and slowing down the rate of cooling. By this means, varying in detail with different electrodes, the air surrounding the arc is deoxidized and the metal is protected, or shielded, from the oxygen and nitrogen which would otherwise be present. The result is greater tensile strength and ductility of the weld metal.
The primary function of a shielding gas is to exclude the atmosphere from contact with the molten weld metal. This is necessary because most metals, when heated to their melting point in air, exhibit a strong tendency to form oxides, and to a lesser extent, nitrides. Oxygen will also react with carbon in molten steel to form carbon monoxide and carbon dioxide. The various products of these reactions may result in weld deficiencies, such as trapped slag, porosity, and weld metal embrittlement. Reaction products are easily formed by exposure to the atmosphere unless precautions are taken to exclude nitrogen and oxygen.
In addition to providing a protective environment, the shield gas and flow rate also have a pronounced effect on the following:
(1) Arc characteristics
(2) Mode of metal transfer
(3) Penetration and weld bead profile
(4) Speed of welding
(5)Undercutting tendency
(6) Cleaning action
(7) Weld metal mechanical properties
Shielded Metal Arc Welding (SMAW)- In SMAW, gas shielding is achieved by using covered electrodes with certain organic products in the electrode coating material. This material decomposes at arc temperature to produce an atmosphere consisting mainly of carbon dioxide (CO,) and carbon monoxide (CO), with or without small amounts of hydrogen. These gases, while primarily shielding, also contribute to the stabilization of the arc and the general improvement of the arc characteristics.
Generally, the shielding gases developed by electrode coatings are most effective when welding ferrous materials, but they are also useful for some of the hard-to-weld materials, such as aluminum- and copper-base alloys. Specific fluxes are used with these electrodes to form fusible metal oxide slags which do not interfere with the welding operation.
Oxyfuel Gas Welding- Shielding gases in one form or another have always been utilized as a means of preventing contact of the surrounding air with the molten weld metal during a welding operation. In oxyhydrogen or oxyacetylene welding, the shielding gas is inherent and usually consists of a mixture of several gaseous products of combustion, such as hydrogen, water vapor, carbon monoxide, and carbon dioxide. Though these gases are chemically active at welding temperatures, the overall effect of the shielding gas mixture can be oxidizing, neutralizing, or reducing, as needed, by adjusting the oxyfuel-gas ratio. This makes it is possible to weld a variety of materials with the oxyfuel gas flame.
Historical Background
The fact that argon and helium would make ideal shielding gases for all types of welding operations had been known for many years. However, the problem of introducing these gases into the welding area, as well as the problem of high cost, precluded their use. In the early development of gas tungsten arc welding (GTAW), argon or helium, or a mixture of the two, were used. Not only did these inert gases provide protective atmospheres for all materials, but they also provided protection for the nonconsumable tungsten electrode.
With the development of the gas metal arc welding (GMAW) process, it became evident that the composition of the inert shielding gases could be tailored to specific applications by adding small amounts of an active gas, such as oxygen or carbon dioxide, to argon or helium. Later refinements, particularly in the area of welding steel, made it possible to use carbon dioxide or carbon dioxide-argon mixtures for effective shielding. Formerly called Mig welding, the term gas metal arc welding evolved because it is a more accurate description of the gases used in the process. See ARGON, HELIUM, and CARBON DIOXIDE.