Machinery's Handbook, 31st Edition
1612 Pipe Welding At the bottom of Fig. 1 are pipe joints in three positions, the first of which, Fig. 1e, corre sponds to the 2G horizontal (non-rotational) position in the upper row. The remaining two are respectively 5G, Fig. 1f , representing pipe with the weld in a fixed vertical (non-rotational) position; and 6G, Fig. 1g , that typifies pipe to be welded at an angle and not rotated during welding. For satisfactory pipe welding, consideration must be given to the chemical com position and thickness of the metal to be welded; selection of a suitable electrode material composition and size; determination of the current, voltage and wire feed rate to be used; preparation of the joint or edges of the pipes; and ways of holding the pipes in the positions needed while welding is carried out. High-quality tack welds, each about 1.5 inches (38 mm) long, and projecting about 1 ∕ 16 inch (1.6 mm) beyond the inner wall of the pipe, are usually made to hold the parts of the assembly in position during welding. SMAW (stick) welding was used almost exclusively for pipe welding until the advent of MIG welding with its potential for much greater rates of deposition. It cannot be emphasized too strongly that practices suitable for SMAW cannot be transferred to MIG welding, for which greater expertise is required if satisfactory welds are to be produced. MIG short-circuit, globular, and spray transfer, and pulsed MIG, with flux or metal-cored consumables (electrodes) can now all be used for pipe welding. Use of all-position, flux- cored, MIG consumables in particular, can reduce skill requirements, improve weld quality, and hold down costs in pipe welding. Among the important items involved in the change to the MIG process is the automatic wire feeder. With today’s wire feeding equipment, an increase of one increment on the dial, say from the 9 to the 10 o’clock position, can increase the wire feed rate by 70 in/min (1.8 m/min). As an example, such an increase could raise the weld current from 110 to 145 amps and the weld voltage from 16 to 17, resulting in an increase of 40 percent in the energy supplied to the weld. Another vital parameter is the amount that the wire sticks out from the contact tip. In low-parameter, short-circuit welding, a small change in the wire stick-out can alter the energy supplied to the weld by 20 to 30 percent. Root passes: Whatever welding process is selected, the most important step in pipe weld ing, as in other types of welding, is the root pass, which helps to determine the degree of penetration of the weld metal, and affects the amount of lack of fusion in the finished weld. During the root pass, the action of the arc in the weld area should reshape the gap between the adjacent sides of the joint into a pear-shaped opening, often called a “keyhole.” As the work proceeds, this keyhole opening is continuously being filled, on the trailing side of the weld, by the metal being deposited from the electrode. The keyhole travels along with the weld so that the root pass produces a weld that penetrates slightly through the inner wall of the pipe. MIG short-circuit root welding of carbon steel pipe requires a gap of 5 ∕ 32 ± 1 ∕ 32 inch (4 ± 0.8 mm), between the ends of the pipe, and the width of the root faces (at the base of the bevels) should be 1 ∕ 16 to 3 ∕ 32 inch (1.6 to 2.4 mm). The recommended bevel angle for MIG pipe welding is 40 ° (80 ° included angle) and the maximum root gap is 3 ∕ 16 inch (4.8 mm). The root pass in 1G welds should be made in the vertical-down direction with the electrode held between the 2 and 3 o’clock positions. When an 0.035-inch (0.9-mm) diameter E70S-3 MIG wire is used with the above root dimensions, weaving is not needed for the root pass except when welding over tack welds. Fill Passes: In welding carbon steel pipe in the 1G position with an 0.035-inch diameter electrode wire, MIG short circuit fill passes should use a minimum of 135 amps and be done in the vertical-up position. Fill passes should deposit a maximum thickness of no more than 1 ∕ 8 inch (3.2 mm). Inclusion of CO 2 gas in the mixture will improve weld
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online