Machinery's Handbook, 31st Edition
ELECTRON-BEAM WELDING 1611 must be less than about 1.5 in. (38 mm). Electron-beam welding at atmospheric pressure requires beam-accelerating voltages above 150 kV, but lower values can be used with a protective gas. Helium is preferred because it is lighter than air and permits greater pene tration. Argon, which is heavier than air and allows less penetration, can also be used to prevent contamination. Required safety precautions, such as radiation shields to guard workers against the effects of X-rays when the electron beam strikes the work, are essential when electron- beam welding is done at atmospheric pressure. Such barriers are usually built into enclo sures that are designed specifically for electron-beam welding in a partial vacuum. Ade quate ventilation is also required to remove ozone and other gases generated when the process is used in the atmosphere. Carbon, low-alloy, and stainless steels; high-temperature and refractory alloys; cop- per and aluminum alloys can be electron-beam welded, and single-pass, reasonably square, butt welds can be made in materials up to 1 in. (25.4 mm) thick at good speeds with nonvacuum equipment rated at 60 kW. Edges of thick material to be electron-beam welded require precision machining to provide good joint alignment and minimize the joint gap. Dissimilar metals usually may be welded without problems. Because of the heat-sink effect, electron-beam welds solidify and cool very rapidly, causing cracking in certain materials such as low-ferrite stainless steel. Although capital costs for electron-beam welding are generally higher than for other methods, welding of large numbers of parts and the high welding travel rates make the process competitive. Pipe Welding Pipe Welding.— Welding of (usually steel) pipe is commonly performed manually, with the pipe joint stationary, or held in a fixture whereby rotation can be used to keep the weld location in a fixed, downhand, position. Alternatively, pipe may need to be welded on site, without rotation, and the welder then has to exert considerable skill to produce a satisfac tory, pressure-tight joint. Before welding stationary pipe, a welder must be proficient in welding in the four basic positions: 1G flat, 2G horizontal, 3G vertical, and 4G overhead, depicted in Fig. 1 at the top in Fig. 1a, Fig. 1b, Fig. 1c, and Fig. 1d. Positioning of Joint Components in Pipe Welding
Fig. 1c. Vertical Position 3G
Fig. 1a. Flat Position 1G
Fig. 1b. Horizontal Position 2G
Fig. 1d. Overhead Position 4G
Fig. 1e. Horizontal Position 2G Pipe Axis Vertical
Fig. 1f. Position 5G Pipe Fixed, Axis Horizontal
Fig. 1g. Position 6G Pipe Fixed, Axis Inclined
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