Why is TIG welding used as a backing process in some welding operations?

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There is no significant difference in terms of process between full TIG welding and TIG welding as a backing process; full TIG welding is suitable for thin-walled pipes with small diameters (usually DN60 and below, with a wall thickness of 4 mm), aiming to ensure good quality and proper shape of the weld root. When the pipe diameter is large and the wall thickness is considerable, TIG welding should be used for root welding, followed by manual welding for the cover pass. The purpose of using manual welding for the cover pass is that for large-diameter pipes, manual welding ensures better appearance quality; it also offers higher work efficiency and lower costs compared to TIG welding. The TIG welding root pass process is used in the welding of boiler water walls, superheaters, economizers, etc., yielding high-quality joints; radiographic inspection shows that the weld grades are all above grade II. Advantages of TIG welding for root pass welding: (1) Good quality – By selecting the appropriate welding wire, welding process parameters, and proper gas shielding, it is possible to achieve good penetration at the root area; the penetration is uniform, and the surface is smooth and even. Defects such as weld beads, lack of penetration, depressions, pores, and slag inclusions, which are common in conventional shielded metal arc welding, do not occur. (2) High efficiency: In the first layer of welding for pipes, manual TIG welding is used as a continuous-welding method. Since shielded metal arc welding is an arc-interrupted welding process, manual TIG welding can increase efficiency by 2 to 4 times. Since TIG welding produces no slag, there is no need to remove slag or repair the weld bead; thus, the speed is increased even further. When performing the surfacing weld in the second layer, a smooth and even TIG welding root pass greatly facilitates the surfacing weld, ensuring good interlayer fusion; this is especially true in the welding of small-diameter pipes, where the efficiency improvement is more significant. (3) Easy to master: The welding of the root weld in shielded metal arc welding must be performed by experienced welders with a relatively high level of technical skill. Manual TIG welding is used for the root pass, and welders who are generally involved in welding work can master it after a short period of training. (4) Minimal deformation: When TIG welding is used for the root pass, the heat-affected zone is much smaller; as a result, the deformation of the welded joint is low, and so are the residual stresses.