What are the principles and advantages of TIG welding? Difference from ordinary welding

GB/T 19867.1-2005 Welding Procedure Specifications for Arc Welding GB/T 19867.4-2008 Welding Procedure Specifications for Laser Welding GB/T 19867.2-2008 Welding Procedure Specifications for Gas Welding GB/T 19867.3-2008 Welding Procedure Specifications for Electron Beam Welding GB/T 19867.5-2008 Welding Procedure Specifications for Resistance Welding

Introduction to TIG Welding TIG welding is a welding technique that builds on the principles of conventional arc welding. It utilizes argon gas to protect the metal weld material; high current is used to melt the weld material into a liquid state on the base material being welded, thereby achieving a metallurgical bond between the metal to be welded and the weld material. Since argon gas is continuously supplied during the high-temperature melting process, the weld material does not come into contact with oxygen in the air, which prevents oxidation of the weld material. As a result, this technique can be used to weld non-ferrous metals such as copper, aluminum, and alloy steel. Classification of TIG welding TIG welding is divided into two types based on the electrode used: gas tungsten arc welding with a consumable electrode and gas tungsten arc welding with a non-consumable electrode. 1. Tungsten Inert Gas Welding Working principle and characteristics: In tungsten inert gas welding, the arc burns between a non-melting electrode (usually a tungsten electrode) and the workpiece. An inert gas that does not react chemically with metals (argon is commonly used) flows around the welding arc, creating a protective gas shield. This shield prevents the tungsten electrode tip, the arc, the molten pool, and the metal at high temperatures from coming into contact with air, thereby preventing oxidation and the absorption of harmful gases. Thus, a dense weld joint is formed, which has excellent mechanical properties. 2. GMAW Working principle and characteristics: The welding wire is fed in through a wire feed wheel, while the electrode tip serves to conduct electricity. An arc is generated between the base material and the welding wire, causing both to melt. An inert gas, argon, is used to protect the arc and the molten metal during the welding process. The difference between it and TIG welding is that in the former, the welding wire serves as the electrode, which is continuously melted and fed into the weld pool; after cooling, it forms the weld seam ； Another approach is the use of shielding gases. With the advancement of gas tungsten arc welding technology, shielding gases have evolved from pure argon to a wide range of mixed gases, such as an argon-rich shielding gas composed of 80% Ar and 20% CO2. Usually, the former is called MIG, and the latter is called MAG. In terms of their operation methods, the most widely used ones at present are semi-automatic GMAW and welding with an argon-enriched gas shield, followed by automatic GMAW. Features of TIG welding: 1. Characteristics of gas tungsten arc welding compared to metal inert gas arc welding (1) High efficiency – due to its high current density, heat is concentrated, resulting in a high deposition rate and fast welding speed. Additionally, it is prone to arcing. For TIG welding (2), enhanced protection is necessary due to the intense arc light and large amount of fumes, so it is important to take proper protective measures. 2. Protective gas 　　(1) The most commonly used inert gas is argon. It is a colorless and odorless gas, with an abundance of 0.935% in air by volume. Argon’s boiling point is –186°C, which lies between the boiling points of oxygen and helium. Argon is a by-product of oxygen production in oxygen plants, which is obtained through the distillation of liquid air. 　　Bottled argon is used for welding in our country, with a filling pressure of 15 MPa at room temperature. The gas cylinder is painted gray and marked with the word “Argon”. The chemical composition requirement for pure argon is: Ar≥99.99% ； He≤0.01％ ； O2≤0.0015％ ； H2≤0.0005％ ； Total carbon content ≤ 0.001% ； Moisture content ≤ 30 mg/m3. 　　Argon is an ideal shielding gas; it is 25% denser than air, which helps to protect the welding arc during flat welding and reduces the consumption of shielding gas. Argon is a gas with extremely low chemical reactivity; it does not react chemically with metals even at high temperatures, thereby avoiding the oxidation and degradation of alloying elements as well as the various problems that arise from this. Argon is also insoluble in liquid metal, so it does not cause porosity. Argon is a monatomic gas that exists in atomic form; at high temperatures, there is no molecular decomposition or atomic heat absorption. Argon has a low specific heat capacity and thermal conductivity, which means it absorbs little heat itself and also transfers little heat outward. As a result, the heat in the arc is not easily dissipated, allowing the welding arc to burn steadily with heat concentrated, which facilitates the welding process. 　　The disadvantage of argon is its high ionization potential. When the arc space is filled with argon, it is difficult to ignite the arc, but once ignited, the arc remains very stable. 3. Disadvantages of TIG welding （1）Due to the large heat-affected zone, TIG welding often results in defects such as deformation of the workpiece after repair, reduced hardness, sand holes, localized annealing, cracking, pinholes, wear, scratches, undercutting, as well as insufficient adhesion and internal stress damage. This is especially evident in the process of repairing minor defects in precision castings, where protrusions appear 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 shortcomings 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 higher current density and emits stronger 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. 　　Applications of TIG welding: TIG welding is suitable for welding oxidizable non-ferrous metals and alloy steels (currently it is mainly used for welding Al, Mg, Ti and their alloys, as well as stainless steel) ； Suitable for single-sided welding with double-sided formation, such as root welding and pipe welding ； Tungsten inert gas welding is also suitable for welding thin sheets. 　　 Tungsten inert gas welding: The diagram on the right shows a schematic of a TIG welding setup. 1—Filling rod; 2—Nozzle; 3—Electrode tip; 4—Welding torch; 5—Tungsten electrode; 6—Welding torch handle; 7—Argon gas flow; 8—Welding arc; 9—Metal melt pool; 10—Wire reel; 11—Wire feeding mechanism; 12—Welding wire. Safety regulations for tungsten inert gas welding: 1) The welding area must be equipped with fire prevention equipment such as sandboxes, fire extinguishers, fire hydrants, and buckets. Flammable materials must be kept at a distance of no less than 5 meters from the welding area. If the required distance cannot be maintained, it should be properly covered with asbestos sheets, asbestos cloth, etc., to prevent sparks from falling on flammable materials. Explosive materials must be kept at a distance of no less than 10 m from the welding area. The TIG welding area should have good natural ventilation as well as fixed mechanical ventilation systems to reduce the hazards posed by harmful gases and metal dust generated during TIG welding. 　　2) Manual TIG welding machines should be placed in a dry and well-ventilated area, and operated strictly in accordance with the instruction manual. A thorough inspection of the welding machine should be carried out before use. Ensure there are no hidden hazards before turning on the power. Welding can be carried out only after the machine is operating properly under no-load conditions. To ensure the welder is connected correctly, it must be properly and firmly grounded to guarantee safety. The on/off operation of the welder’s power supply is controlled by the switch on the power board; it is strictly prohibited to operate the switch under load to prevent damage to the contacts. 　　3) The cooling water system of the TIG welding torch should be checked regularly; any blockages or leaks should be addressed immediately to prevent damage to the torch and to avoid impacts on welding quality. 　　4) The power supply must be turned off when the welder leaves the work area or when the welding machine is not in use. If the welding machine malfunctions, it should be repaired by professionals, and safety measures such as protection against electric shock must be taken during maintenance. The welding machine should be dust-cleaned at least once a year. 　　5) The high-frequency electromagnetic field generated by the high-frequency oscillator of the tungsten inert gas welding machine can cause dizziness and fatigue in people. Therefore, during welding, the duration of exposure to high-frequency electromagnetic fields should be minimized; the high-frequency power supply should be turned off immediately after the arc is ignited. The welding torch and welding cable should be shielded with flexible metal braided wire (one end of the hose is connected to the welding torch and the other end is grounded, with no insulation covering it). If possible, crystal pulse arc starting should be used instead of high-frequency arc starting. 　　6) During TIG welding, the intensity of ultraviolet rays is very high, which can easily cause photophotic keratitis and arc burns; at the same time, ozone and nitrogen oxides are generated, irritating the respiratory tract. Therefore, welders should wear white canvas work clothes when operating, as well as masks, face shields, protective gloves, and foot covers. To prevent electric shock, insulating rubber should be laid on the floor near the workbench, and workers should wear insulated rubber shoes. TIG welding for root pass: Using the TIG welding process for the root pass enables the creation of high-quality weld joints. The TIG welding root pass process is used in the welding of boiler water walls, superheaters, economizers, etc., yielding weld joints of excellent quality; 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, under-welding, and depressions that commonly occur in conventional shielded metal arc welding are not present. 　　(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. The speed increases even more since there is no need to remove slag or repair the weld bead. 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 beneficial for 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) 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 so are the residual stresses. 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 welded 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: Use a WST315 manual tungsten inert gas welding machine; the machine is equipped with a high-frequency arc-starting device, allowing for high-frequency arc starting to be used. 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 last. 　　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. Tungsten inert gas welding is a welding technique that builds on the principles of conventional arc welding; it uses argon gas to protect the metal weld material. A high current is applied to melt the weld material into a liquid state on the base material being welded, thereby creating a molten pool that allows for metallurgical bonding between the metal to be welded and the weld material. By continuously supplying argon gas during the high-temperature melting process, the weld material is kept away from oxygen in the air, which prevents oxidation of the weld material. As a result, this technique can be used to weld non-ferrous metals such as copper, aluminum, and alloy steel. 1. Characteristics of gas metal arc welding compared to tungsten inert gas welding (1) High efficiency: Due to its high current density, heat is concentrated, resulting in a high deposition rate and fast welding speed. Additionally, it is prone to arcing. For TIG welding (2), enhanced protection is necessary due to the intense arc light and large amount of fumes, so it is important to take proper protective measures. 2. Protective gas 　　(1) The most commonly used inert gas is argon. It is a colorless and odorless gas, with an abundance of 0.935% in air by volume. Argon’s boiling point is –186°C, which lies between the boiling points of oxygen and helium. Argon is a by-product of oxygen production in oxygen plants, which is obtained through the distillation of liquid air. 　　Bottled argon is used for welding in our country, with a filling pressure of 15 MPa at room temperature. The gas cylinder is painted gray and marked with the word “Argon”. The chemical composition requirement for pure argon is: Ar≥99.99% ； He≤0.01％ ； O2≤0.0015％ ； H2≤0.0005％ ； Total carbon content ≤ 0.001% ； Moisture content ≤ 30 mg/m3. 　　Argon is an ideal shielding gas; it is 25% denser than air, which helps to protect the welding arc during flat welding and reduces the consumption of shielding gas. Argon is a gas with extremely low chemical reactivity; it does not react chemically with metals even at high temperatures, thereby avoiding the oxidation and degradation of alloying elements as well as the various problems that arise from this. Argon is also insoluble in liquid metal, so it does not cause porosity. Argon is a monatomic gas that exists in atomic form; at high temperatures, there is no molecular decomposition or atomic heat absorption. Argon has a low specific heat capacity and thermal conductivity, which means it absorbs little heat itself and also transfers little heat outward. As a result, the heat in the arc is not easily dissipated, allowing the welding arc to burn steadily with heat concentrated, which facilitates the welding process. 　　The disadvantage of argon is its high ionization potential. When the arc space is filled with argon, it is difficult to ignite the arc, but once ignited, the arc remains very stable.

GB50235 recommends that for pipes that require radiographic or ultrasonic testing, especially those with high requirements regarding the qualification rate of testing and quality standards, a welding process using TIG welding for the root pass should be employed, as this makes it easier to ensure the quality of the weld radiographs.