Machinery's Handbook, 31st Edition
1578 Gas Metal Arc Welding metal gages is the 0.035-in. (1-mm) diameter electrode. The short-circuit current require- ments for these operations are typically 50 to 200 amps with voltages in the range of 14 to 22 volts. The optimum short-circuit voltage for the majority of applications is 16 to 18 volts. Shielding Gases for Welding Carbon and Low-Alloy Steels.— With more than 40 GMAW gas mixtures available for welding carbon steels, low-alloy steels, and stainless steels, selection is often confusing. Reactive oxygen and carbon dioxide (CO 2 ) are added to argon to stabilize the arc and add energy to the weld. CO 2 can provide more energy to the weld than oxygen. As the CO 2 content in a shielding gas mixture is increased to certain levels, the voltage requirements are increased. Argon + oxygen mixtures will require lower voltages than mixtures containing argon with 10 to 25 percent CO 2 . Helium may also be added to argon if increased weld energy is required. Shielding Gases for Short-Circuit Welding of Carbon Steels.— GMAW short-circuit transfer (SCT) is used mainly for welding thin metals of less than 10 gage, and gaps. With the SCT mode of weld metal transfer, the arc short circuits many times each second. The numerous short circuits switch the arc energy on and off. The short circuits and low cur- rent cause the transferred weld to freeze rapidly. Short-circuit transfer on carbon steel gage metals thicker than 1 ∕ 16 in. (1.6 mm) requires a shielding gas that will provide substan- tial weld energy. For these applications, argon with 15–25 percent CO 2 is recommended. If short-circuit transfer is used on metals thinner than 18 gage (0.047 in., 1.2 mm), melt- through and distortion often occur. Melt-through and distortion can be reduced on very thin-gage carbon and low-alloy steels by using a shielding gas that provides less weld energy than argon + 15 to 25 percent CO 2 mixes. Argon + oxygen mixtures can utilize lower voltages to sustain the arc. Argon mixed with 2 to 5 percent oxygen is a practical mixture for thin carbon steel of less than 16 gage, where there is sensitivity to heat. Shielding Gases for Spray Transfer Welding of Carbon Steels.— With GMAW spray transfer, all traditional argon gas mixtures will provide spatter-free spray weld transfer, depending on the electrode diameter and welding parameters used. The electrode diam- eter and the electrode current density influence the formation of the weld metal to be transferred. For example, with a 0.035-inch (0.9 mm) diameter electrode using a mixture containing argon 75 + CO 2 25 percent, a small globular weld droplet is formed on the end of the electrode tip in the conventional spray transfer parameter range. With the same gas mixture, a 0.045-inch (1.14-mm) diameter electrode, and current above 330 amps, the globular formation disappears and the metal transfers in the spray mode. Spatter potential stemming from shielding gas, with 0.035 inch (0.90 mm) and smaller diameter electrodes can be controlled by reducing the CO 2 content in the argon mixture to less than 21 percent. Each different shielding gas will primarily influence the open arc spray transfer mode by variations in the weld energy provided through the welding voltage requirements. Gas selection in spray transfer must be given careful consideration. In welding of clean cold-rolled carbon steel or low-alloy steel less than 3 ∕ 8 inch (9.5 mm) thick, the energy potential of the arc is less important than it is for welding of steels thicker than 1 ∕ 2 inch (13 mm) or steels with mill scale. The energy level of the arc is also a key factor in welding steels for which higher than normal impact properties are specified. A simple, practical multipurpose gas mixture for carbon and low-alloy steels is argon + 15 to 20 percent CO 2 , and a mixture of argon + 17 percent CO 2 would be ideal. This two- part argon/CO 2 mixture provides higher weld energy than two-component argon + CO 2 mixtures having less than 10 percent CO 2 , argon + oxygen mixtures, or argon + CO 2 + oxygen tri-component mixtures. The argon + 17 percent CO 2 mixture will provide an arc slightly less sensitive to mill scale than the other mixtures mentioned. The argon + 17 percent CO 2 mixture also has practical benefits in that it provides suffi cient weld energy for all GMAW short-circuit and spray transfer applications with cylin der or bulk gases. The argon + 17 percent CO 2 mixture may also be used for all-position FCAW electrodes in welding carbon steels, low-alloy steels, and stainless steels.
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