Why is TIG welding used for the root pass during the welding of thick plates and steel structures?

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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 (typically DN60 and below, with a wall thickness of 4 mm), with the aim of ensuring good quality and proper shape of the weld root. When the pipe diameter is large and the wall thickness is thick, TIG welding should be used for the root pass while manual welding is used for the cover pass. The purpose of using manual welding for the cover pass is that, in cases of large diameters, manual welding ensures better surface quality; it also offers higher productivity compared to TIG welding, with lower costs as well. The TIG welding root pass process is used in the welding of boiler components such as water walls, superheaters, and economizers, yielding weld joints of excellent quality; radiographic inspections show that the weld grades are all at level II or above. Advantages of TIG welding for root pass welding: (1) Good quality – By selecting the appropriate welding wire, 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 does not produce slag, there is no need to remove slag or repair the weld bead, resulting in an even faster speed. When performing the overlay welding in the second layer, a smooth and even TIG welding root pass greatly facilitates the overlay welding, ensuring good fusion between layers; this is especially true when welding small-diameter pipes, where the efficiency improvement is more significant. (3) Easy to master. Welding the root weld in manual arc welding must be carried out by welders with extensive experience and high technical skills. 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 root pass welding, the heat-affected zone is much smaller; as a result, the deformation of the welded joint is low, and so are the residual stresses. Process Overview (1) Welding Examples: The materials used for the economizer, the tube bundles in the evaporation section, the water wall, and the low-temperature superheater are grade 20 steel, while the tubes of the high-temperature superheater are made of 12Cr1MoV steel. (2) Pre-welding preparation: Before welding, the pipe ends should be notched at a 30° angle, and the area within 15 mm inside and outside the pipe ends should be polished to reveal the natural color of the metal. The gap between the pipe ends should be 1~3 mm. When the actual alignment gap is too large, a transition layer must first be weld-on one side of the pipe groove. Temporary wind-shielding facilities should be set up, and the wind speed at the welding area must be strictly controlled, as wind speeds above a certain level can easily lead to the formation of pores. (3) Operation: A manual tungsten inert gas welding machine is used; the machine is equipped with a high-frequency arc-starting device, allowing for high-frequency arc starting. Arc extinction is different from shielded metal arc welding; if the arc is extinguished too quickly, crater cracks are likely to occur. Therefore, during operation, the molten pool should be guided toward the edge or to areas where the base material is thicker, then the molten pool should be gradually reduced in size to allow the arc to be extinguished slowly, with the shielding gas being turned off at the end. For 20-grade steel pipes with a wall thickness of 3–4 mm, the filler material can be TIGJ50 (for 12Cr1MoV, 08CrMoV can be used); the diameter of the tungsten electrode is 2 mm, the welding current is 75–100 A, the arc voltage is 12–14 V, the flow rate of the shielding gas is 8–10 L/min, and the power supply type is direct current with positive polarity.