What are the advantages of using TIG welding for the root pass?

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Preface: There is no significant difference in terms of process between full TIG welding and TIG welding for root pass; 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 finish pass. The purpose of using manual welding for the finish pass is that, in cases of large pipe 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 high-quality joints; radiographic inspection shows that the weld grades are all at level II or above. 1. 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 pipelines, manual TIG welding is used as a continuous-welding method. Since shielded metal arc welding is an intermittent arc 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 clean the slag or repair the weld bead, resulting in an even faster speed. 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 particularly advantageous in the welding of 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) Low 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 the residual stresses are also low. 2. 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 prepared with a 30° bevel, 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 site 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 extinguishing 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. 3. The main advantages of the widespread use of TIG welding: 1. Argon shielding prevents oxygen, nitrogen, hydrogen, and other gases in the air from having an adverse effect on the arc and the molten pool, thereby reducing the loss of alloying elements and enabling the formation of dense, splash-free weld joints of high quality ； 2. TIG welding features stable arc combustion, concentrated heat, high arc column temperature, high welding productivity, a narrow heat-affected zone, and reduced tendency for stress, deformation, and cracks in the welded parts ； 3. TIG welding is a process that uses an exposed arc, making it easy to operate and observe ； 4. The electrode wears down little, and the arc length is easy to maintain. There is no flux or coating layer during welding, which facilitates mechanization and automation ； 5. TIG welding can be used to weld almost all metals, especially those that are difficult to weld or prone to oxidation, such as magnesium, titanium, molybdenum, zirconium, aluminum, and their alloys ； 6. It is not restricted by the position of the welded parts, allowing welding in all positions. Main disadvantages: 1. TIG welding, due to its large heat-affected zone, often results in defects in the workpiece after repair, such as deformation, excessive hardness, porosity, local annealing, cracking, pinholes, wear, scratches, undercutting, as well as insufficient adhesion and internal stress damage. This is particularly evident in the process of repairing small defects in precision castings, where it appears on the surface. In the field of repairing defects in precision castings, a cold welding machine can be used as a substitute for TIG welding. Due to its low heat generation, the cold welding machine overcomes the disadvantages of TIG welding and addresses the challenges associated with repairing precision castings. 2. Compared to shielded metal arc welding, TIG welding causes greater harm to the human body. TIG welding has a high current density and emits intense light; the ultraviolet radiation generated by its arc is about 5 to 30 times that of conventional shielded metal arc welding, while the infrared radiation is about 1 to 1.5 times that of shielded metal arc welding. The level of ozone produced during welding is also high. Therefore, it is advisable to carry out welding in areas with good air circulation, otherwise it can cause serious harm to the body. 3. For metals with low melting points and high volatility (such as lead, tin). Zinc), making welding more difficult.