Machinery's Handbook, 31st Edition
1576 WELDING SMAW use fluxes to shield the arc and FCAW uses fluxes and gases to protect the weld from oxygen and nitrogen. GMAW and GTAW use mixtures of gases to protect the weld. There are two groups of weld types, groove and fillet, which are self-explanatory. Each type of weld may be made with the work at any angle from horizontal (flat) to inverted (overhead). In a vertical orientation, the electrode tip may move down the groove or fillet (vertical down), or up (vertical up). In any weld other than flat, skill is needed to prevent the molten metal falling from the weld area. Because of the many variables, such as material to be welded and its thickness, equip ment, fluxes, gases, electrodes, degree of skill, and strength requirements for the finished welds, it is not practicable to set up a complete list of welding recommendations that would have general validity. Instead, examples embracing a wide range of typical applications, and assuming common practices, are presented here for the most-used welding processes. The recommendations given are intended as a guide to finding the best approach to any welding job, and are to be varied by the user to fit the conditions encountered in the specific welding situation. Heat Input and Weld Distortion.— When metals are joined in a welding process, a great amount of heat is generated. As this heat melts, coalesces, and joins two or more members at the point of connection, it often changes the position and dimensions of the welded structure to some extent. When a weld joint is fit-up for welding, the gap between the parts to be welded is filled in with filler metal and/or base metal. As the metal in the gap cools, shrinkage occurs, drawing adjacent surfaces together and strengthening the connection, but also causing movement. In jobs where part alignment and dimensional tolerances must be maintained, the effect of such weld distortion should be taken into consideration. Tactics to limit or compensate for these factors include heat input control, using welding fixtures, post-weld stress relief, and finish machining and processing operations. Gas Metal Arc Welding (GMAW) The two most cost-effective manual arc welding processes are GMAW and FCAW. These two welding processes are used with more than 50 percent of the arc welding con sumable electrodes purchased. Gas metal arc welding modes extend from short-circuit welding, where the consumable electrode wire is melted into the molten pool in a rapid succession of short circuits during which the arc is extinguished, to pulsed and regular spray transfer, where a stream of fine drops and vaporized weld metal is propelled across the continuous arc gap by electromagnetic forces in the arc. GMAW is the most-used welding process and the two most common GMAW low- carbon steel electrodes used for production welding in North America are the E70S-3 and E70S-6 from the ANSI/AWS Standard A5 series of specifications for arc welding. The E70S-3 contains manganese and silicon as deoxidants and is mainly used for welding low-carbon steels, using argon mixtures as shielding gases. The wire used in E70S-6 electrodes has more silicon than wire used for E70S-3 electrodes, and is preferred where straight CO 2 or argon mixes are used as the shielding gas or if the metal to be welded is contaminated. The deoxidizing properties of the E70S-6 electrode also may be beneficial for high-current, deep-penetration welds, and welds in which higher than normal impact- strength properties are required. E80S-D2 wire contains more manganese and silicon, plus 0.5 percent molybdenum for welding such steels as AISI 4130, and steels for high-temperature service. The argon + CO 2 mixture is preferred to exert the influence of argon’s inertness over the oxidizing action of CO 2 . E70S-2 electrodes contain aluminum, titanium, and zirconium to provide greater deoxidation action and are valuable for welding contaminated steel plate. When the GMAW welding process is used for galvanized steels, minute welding cracks may be caused by the reaction of the zinc coating on the work with silicon in the electrode. Galvanized steel should be welded with an electrode having the lowest possible silicon
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