Machinery's Handbook, 31st Edition
Flux-Cored Arc Welding 1587 Contact Tip Recess.— The dimension labeled contact tip recess in Fig. 2, and indicated as 1 ∕ 8 in. (3 mm), should be about 1 ∕ 2 in. (13 mm) for a minimum electrode extension of 3 ∕ 4 in. (19 mm), for FCAW welding. This dimension is critical for obtaining high-quality welds with all-position electrodes because they have a fast-freezing slag and operate with low to medium current and voltage. If the recess dimension is less than the optimum, the voltage may be lower than the minimum recommended, and if the settings are less than the minimum, the fast-freezing slag may solidify too rapidly, causing excess porosity or “worm tracks” on the weld surface. The recommended length of electrode extension for all-position FCAW, E71T-1 elec trodes is 3 ∕ 4 to 1 in. (19 to 25 mm). The size of this extension not only affects the minimum required parameters, but a long electrode extension also ensures preheating of the elec trode and allows lower current to be used. Preheating the electrode is further beneficial as it reduces moisture on the electrode surface, and in the electrode flux. When a change is made from the GMAW to the FCAW process, welders must be aware of the influence on weld quality of the electrode extension in the FCAW process. Porosity and Worm Tracks.— As mentioned above, porosity and worm tracks typically result from a combination of incorrect electrode extension, incorrect welding settings, humidity, electrode moisture, refill scale, rust, paint, oils, or poor welding technique. Where humidity levels are high, potential for porosity and worm tracks increases. The FCAW process is less sensitive to mill scale than the GMAW spray transfer mode but mill scale will often cause excess weld porosity. The best way to avoid the effects of mill scale, rust, oil, and surface contaminants is to grind the area to be welded. Another way to reduce porosity is to keep weaving to a minimum. If the correct size flux- cored electrode is used, weaving can be kept to a minimum for most flux-cored applica tions. The forehand technique produces the best weld bead surface on fillet weld beads up to 3 ∕ 4 in. (19 mm) steel thickness in the flat and horizontal weld positions. On larger single- pass fillet welds, the backhand technique is beneficial because the voltage directed at the weld provides additional weld puddle control to the fluid welds. The backhand technique used for flat and horizontal welds produces a more convex weld bead, reduces potential for porosity, and increases penetration. If porosity or worm tracks occur, the prime solution is in weld practices that increase heat at the weld, but the following remedies can also be tried. Grind clean the surface to be welded; use recommended electrode extensions; increase current (wire feed rate) decrease voltage; use the backhand welding technique; slow down travel speed, consider use of a different electrode formulation containing increased deoxidizers, avoid weaving; change from argon + CO 2 mixture to straight CO 2 ; and provide a protective cover to keep the electrode spool clean and dry. Welding with 0.045-in. (1.2-mm) Diameter All-Position Electrodes.— Fig. 5 shows wire feed settings for welding of steel with 0.045-in. (1.2-mm) diameter, E71T-1 all- position electrodes using a mixture of argon + 15 to 25 percent CO 2 as the shielding gas, and an electrode extension of 3 ∕ 4 in. (18 mm). Parameters for vertical up welding, shown at the left in Fig. 5, include setting the wire feed rate at the twelve o’clock position, or about 350 in/min, using 200 to 190 amps, and setting the voltage between 24 and 25 volts. Opti- mum parameters for flat welding, shown at the right in Fig. 5 include setting the wire feed rate at three o’clock position, or 560 in/min (240 mm/s), and 270 amps at 25 to 27 volts. Welding with 0.052-in. (1.3-mm) Diameter All-Position Electrodes.— Settings for ver tical up and flat welding with all-position E71T-1 electrodes of 0.052-in. (1.3-mm) di - ameter are seen at the left in Fig. 6. These electrodes are suited to welding steel having thicknesses of 1 ∕ 4 in. (6 mm) and thicker. For vertical up welding, the wire feed rate is set between the ten and eleven o’clock positions, or 250 in/min (106 mm/s), with about 200 amps at 25 volts. Flat welding with these electrodes is best done with the wire feed rate set between the two and three o’clock positions, or 490 to 560 in/min (207 to 237 mm/s), giving approximately 300 amps at 28 volts.
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