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Causes and preventive measures of air holes in manual tungsten argon arc welding operation

1 Introduction
        Manual tungsten argon arc welding (referred to as TIG welding), because it uses inert gas for protection, and uses bright arc welding, so the welding quality is high, welding deformation is small, weld defects are less, process performance is good, wide application range, operation Convenient and widely used. However, in the application process, if the operation is not correct, some defects will occur. Among the defects, the proportion of pores is more than 50%.
  
2 Status survey
        A unit participated in the construction of a 300MW unit power plant, which has 16228 high-pressure welding ports on the heating surface of the boiler, including: wall-mounted superheater, low-temperature reheater, high-temperature reheater, and economizer, etc. The statistics are shown in the attached table.
        It can be seen from the attached table that the number of pores and strips is 202, and the number of unqualified repairs is 402, accounting for 50.25%. It can be seen that the pores are the main form of deterioration of welding quality. Therefore, by analyzing the causes of stomata production, we should take relative measures to strengthen quality management, thereby improving the overall qualification rate of the project.
  
3 Analysis of the main points of generating pores
        The factors that cause stomata are mainly determined by five aspects: human, machine, material, method and ring. Each aspect contains several small link factors. Now, the influencing factors are listed in the form of fishbone diagram (see photo) for analysis. .
3.1 Human factors
        In the work, some welders have a weak sense of responsibility and a low quality awareness; some welders do not participate in formal welding training or short training period, no license operation; some welders have long working years and rich experience, but because of their older age There will be physical decline, dizzy eyes, operational errors, etc., which are the main factors that cause stomata.
3.2 Material factors
        The purity of argon gas is less than 99.95%, the welding material or base metal is unqualified, and the oil, rust, water and other materials in the range of 10~15mm of the weld bevel and the base material are not cleaned.
3.3 Environmental factors
        Some working environment wind speed is greater than 2m / s, or some of the pipes have a wind in the hall, the wind is greater than the protective force of argon, wind protection measures are not done. In the rainy season, the climate is humid, the surface of the base metal will be wet, and there will be a layer of frost on the surface of the material in winter; due to the dense tube rows, many obstacles and difficult welding positions, it is also a common cause of common pores.
3.4 Tools and equipment
        The accuracy of the argon arc table (pressure reducer and flow meter) used by the welder is inaccurate. Some argon arc meters are damaged by bumping during use, and the scale of the pressure reducer is not returned to zero. The flowmeter has errors and cannot be used. Value, and this phenomenon did not cause the welder to pay attention.
        The argon arc torch used in tungsten argon arc welding is an important tool for tungsten argon arc welding. Whether the argon arc welding torch is in good condition is also an important part to ensure the reduction of the number of air holes in the weld. The main damage points of the argon arc torch that cause the air holes are:
        (1) The outer sleeve of the argon arc welding torch is damaged and leaked; the outlet hole of the nozzle is blocked by the splash; the small splash attached to the nozzle outlet hinders the smooth flow of the gas, thereby forming a turbulent flow; the nozzle is used for a long time, Under the action of high temperature, the outlet is easy to break and produce a gap, so the protection effect is not good; the nozzle diameter is not properly selected. If a small-diameter nozzle is selected, the range of argon gas sprayed is small, and the protection effect is not good.
        (2) The tungsten pole is bent differently. When the tungsten pole is biased to one side, the shielding gas is also biased to one side, resulting in poor protection. The tungsten tip has a large particle spatter that solidifies in contact with the wire or the molten pool, hindering the normal flow of the airflow.
        (3) Some joints are loose, such as conductive nozzles, nozzles, argon arc welding gun wires and air supply hose belts, argon arc table outlets, belts and argon arc bolts, etc., loose joints, so that air intrusion, reducing the purity of argon And protection effects.
        (4) Argon air supply hose is an important channel for conveying argon gas. It is subject to artificial pressure, heavy material extrusion, or belt leakage and air leakage, which can also cause pores, which is an important factor to reduce the welding qualification rate.
        (5) During the use of the sandwich, the tungsten electrode is clamped at the end. When welding for a long time, the end is easily deformed under high temperature, which affects the argon gas output effect or causes the clamped tungsten electrode to occur. Eccentricity, hindering the passage of airflow.
3.5 Process factors
        (1) The oil, rust, water, etc. on the surface of the base metal and welding consumables are not cleaned.
        (2) The welding parameters are not properly selected, the welding current is large, and the welding wire and the welding gun are not coordinated.
        (3) The assembly clearance is large. When the bottom is made, the molten iron water flows to the back part, and the argon gas is not effectively protected.
        (4) When the wire is fed, the angle of the wire is too small or too large, the angle of the argon arc gun is too small, and the protection effect is poor; too large hinders the welder's line of sight and affects the welder's operation.
        (5) The tungsten pole extends long and reduces the protective effect; the tungsten pole protrudes too short, hindering the welder's line of sight.
        (6) The distance between the tungsten electrode and the molten pool is too close, which is easy to cause "tungstening" phenomenon; the distance is too far, which reduces the protection effect of argon gas.
  
4 Solutions
4.1 Strengthen the education of welders' responsibility, improve everyone's quality awareness, strengthen professional training for employees, and resolutely put an end to undocumented employment. For older welders, it is recommended to downgrade or transfer to use.
4.2 According to the process requirements, the oil, rust, water and other materials in the range of 10~15mm on the surface of the base metal should be cleaned before welding, and the purity of argon gas should be checked. Use qualified welding materials.
4.3 When working in the field, it should be equipped with rainproof and windproof shed. When the wind is greater than level 6, the welding should be stopped. For some long pipes and pipes, the lighter should be tested with the lighter flame before welding. In the coastal humid area, the northern part of the winter, The frost and moisture of the solder joint are baked with an oxyacetylene flame before welding. When encountering difficult dense rows or obstacles and difficult welding positions, welders with higher welding techniques should be selected for welding.
4.4 The argon gas meter used should be inspected regularly, and the damaged argon arc table should be replaced in time. Always check the outer sleeve of the welding torch, the nozzle, the contact tip, the sandwich is intact, whether there is any damage, blockage, bulging, etc. Whether the joint parts are loose or leaking, whether the tungsten is bent or not, and the above faults are eliminated in time.
4.5 When welding, select the appropriate welding parameters and assembly clearance. When the arc is applied, the angle between the argon arc welding torch and the workpiece is controlled at 50°~80°, the welding wire and the workpiece are 15°~30°, and the tungsten extension length is 3~. 5mm, tungsten tip is 3~5mm from the molten pool.
5 Conclusion
        Through years of experience in welding construction, the reasons for the generation of pores in TIG welding have been summarized, and measures to control the occurrence of welding defects such as pores have been proposed. The welding quality has been improved and the welding cost has been reduced, which has contributed to the enterprise.

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