Popularization and Application of TIG Welding Technology in Inspection and Repair of Power Plants
I. Overview
The quality of the pressure components of the unit and the quality of the pipeline are one of the keys to the safe operation of power generation equipment. Most of the pipes in the field are small in diameter and cannot be double-sided welded. How to ensure the quality of the weld is a key issue. Before 2000, the butt welds of thin-walled pipe welds were generally welded by oxyacetylene gas welding, electrode arc welding, root gasketed arc welding and tungsten argon arc welding. At that time, due to the limitations of the welder's technical level, coupled with the welder. Difficult to resist the emotions and the welding position and operating space restrictions and other reasons, the promotion and application of new TIG welding technology encountered resistance, pressure parts and pipeline leakage accidents occurred frequently, seriously threatening the safe and stable operation of the unit.
In 2000, during the minor repair, 64 pairs of butt welds were replaced, and oxyacetylene gas welding was used. The gas welding joints suffered three consecutive accidents during the operation of the unit for one month. The reason was that the heated surface tube was welded by oxyacetylene gas welding. Concentration, the joint heat affected zone is large, the overheating is serious, and the plasticity and toughness are poor. If the welder is not operating properly, the flame is too fast to withdraw from the molten pool, and it is easy to produce shrinkage cavities. Therefore, when the boiler is running, leakage often occurs at the joint of the gas welding joint of the heated surface pipe. Such accidents have occurred in the welding of the previous heated surface tubes. For this reason, the comparison of the four welding methods shows that the TIG welding technology has the greatest superiority. Therefore, it is decided to follow the principle of formulating the welding process from August 2000. TIG full argon arc welding technology is adopted for the inspection and welding of the boiler heating surface. The qualified rate of the non-destructive testing of the welded joint is over 95%, and the application is satisfied. effect. Therefore, in the next four years, it will gradually be applied to the part of the pressure-bearing pipeline that transports steam, water, oil, gas, etc., and the effect is very obvious. In the past six years, the "burst" accident of the heated surface due to welding quality has dropped. 100%, ensuring safe and stable long-term operation of the unit.
In view of this, TIG welding technology, as a new welding technology and new technology, has unique and irreplaceable advantages in welding metal materials, and has important technical and economic value. This welding technology should be caused in the maintenance and installation of power stations. More emphasis and greater development.
Second, the development and current status of pipeline welding technology
In the power system power generation enterprises, when installing and repairing boilers, for such small-diameter thin-walled pipes, oxyacetylene gas welding was generally used from before the founding of the country to the 1970s, and some were subjected to electrode arc welding. For such welding joints, boiler manufacturers use oxyacetylene gas welding and electrode arc welding in addition to contact welding. The oxidized acetylene gas welding is used to weld the boiler heating surface tube, the heat is not concentrated, the joint heat affected zone is large, the overheating is serious, and the plasticity and toughness are poor. If the welder is not operating properly, the flame is too fast to withdraw from the molten pool, and it is easy to produce shrinkage cavities. Therefore, when the boiler is running, leakage often occurs at the joint of the gas welding joint of the heated surface pipe.
Third, the advantages of TIG welding technology
1 TIG welded metal materials are virtually unlimited in variety and quantity. They have many characteristics in shape and form, area and other dimensions, bonding strength and performance. All this determines TIG welding technology in metallic materials. There are many excellent applications in the welding process.
2Ar is one of the most stable inert gases and has a good protection effect. Gas protection replaces slag protection and the weld is clean and free of slag. The inert gas argon forms a closed gas flow around the molten pool and the arc, effectively preventing the intrusion of harmful gases from the outside, thereby obtaining a high quality welded joint.
3 TIG welding arc is compressed and cooled by argon flow. The arc is concentrated to ensure the penetration of the root of the weld. The heat affected zone of the joint is small, and the deformation of the weldment and the tendency of cracking are small. It is especially suitable for welding steel with a large tendency for air quenching.
4 Ar is insoluble in metals and does not react with metals, so alloying elements are generally not burnt. The TIG welds are particularly pure and there is no weld slag in the welded pipe joints. Weld quality is better than oxyacetylene gas welding and electrode arc welding.
5 Bright arc welding, the molten pool is clearly visible, the operation is easy to master, and it is easy to realize automation.
The argon air flow acts on the welding area to flush the joint, which accelerates the cooling speed of the welded joint, can improve the structure and performance of the joint, and can reduce the residual deformation of the weldment after welding.
7Ar is a monoatomic gas with low heat capacity, low thermal conductivity and low heat consumption. It is very beneficial for stable arc combustion. Even under conditions of small current and long arc, the arc is stable, easy to operate, and the weld quality is easy to control. All-position welding of the small-diameter pipe in a difficult-to-weld position.
Due to the good quality and easy operation of TIG welding, in recent years, it has become a common method for all power generation companies to weld heated surfaces when installing and repairing boilers. Under the conditions of on-site inspection, due to the limitation of welding position and operation space, the commonly used process method is TIG welding bottom electrode arc welding cover or TIG full argon arc welding.
Fourth, the welding of pressure pipes
In the past, the welding system of the power system was mainly used to weld the bottom of the electrode and the method of docking the cover, which is easy to produce defects such as insufficient penetration of the root and slag inclusion, which is a great threat to the safe operation of the unit. There have been many occurrences of weld breakage and blasting accidents in the power system, and the cause of the accident has been analyzed, which is related to the serious incomplete penetration of the weld root. In recent years, in foreign power station units in construction welding, all welds are required to be fully penetrated. In China, it has been applied to the main steam, main feed water, reheat section, cold section, header pipe socket and turbine system piping of welding high-pressure units in the power system.
1. TIG welding technology for pipe welds
1 Welding equipment and appliances: welding power supply, welding torch, gas supply system, cooling system and welding control system.
2 Power supply: TIG welding power supply has two kinds of AC and DC. AC power is used to weld aluminum and its alloys, magnesium and its alloys. Welding carbon steel, alloy steel, titanium and its alloys, copper and its alloys all use DC power.
3 Welding torch: TIG welding torch consists of nozzle, tungsten clamp, wire, gas pipe, water pipe, control knob, etc. It plays the role of clamping electrode, conducting current, conveying argon gas and controlling the working system of the whole machine. The welding torch has self-cooling. Both water and cold.
2. Welding materials: Materials used for tungsten argon arc welding include tungsten, argon and filling materials.
1 Tungsten: Commonly used tungsten has tantalum tungsten and tantalum tungsten.
2Ar: Ar contains more or less impurities such as 02, N, CO: and moisture, which has a certain influence on the effect of Ar protection. The more chemistry of the metal material being welded, the greater the impact. Welding different metals requires different purity requirements for Ar.
3 Filling materials: Filling materials for tungsten argon arc welding are available in both wire and fusible rings.
a, argon arc welding wire: Ø2.5 mm welding wire is recommended for pipe welding bottom welding. Ø2.0 mm wire is also used for small diameter pipes with particularly thin pipe walls.
b. The fusible ring has two kinds of mushroom shape and rectangle shape. When the pipe weld is bottomed, it is assembled in the gap of the mouth, and is heated by the tungsten arc and melted together to form the bottom weld.
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