TIG Welding Technology in Stainless Steel Flange
TIG welding technology is used to fix stainless steel flat welding flange and stainless steel pipe. Self-fusing welding of flange sealing surface and wire welding of flange non-sealing surface are carried out to realize the function of flange connection.
Tungsten Argon Arc Welding (TIG) is a welding method using tungsten rods as electrodes, inert gases such as argon and helium as shielding gases, and refractory metals as electrodes. During the welding process, the electrodes do not melt. Therefore, the welding process is stable, the welding seam is well formed, and the high quality welding seam can be easily obtained. Suitable for welding aluminium and its alloys, stainless steel, superalloys, titanium alloys and refractory active metals (such as molybdenum, niobium, zirconium, etc.). Because helium is much more expensive than argon, argon is mainly used as protective gas in industry.
Flange connection is to fix two pipeline equipments and fittings on one flange, and between the two flanges, add flange gasket, and fasten them together with bolts to complete the connection. Flange connection is easy to use and can withstand greater pressure. It is an important connection mode in pipeline construction. TIG welding is the preferred way to achieve the fixing of stainless steel flange and stainless steel tube parts.
Selection of Welding Equipment
1.1 Welding Machine: Tungsten Argon Arc Welding Machine, such as WSM Series DC Tungsten Pulse Argon Arc Welding Machine, has the functions of slow current rise and automatic current attenuation.
2. Welding torch: commonly used torches are divided into water-cooled and air-cooled. The former is used for high current (> 150A) welding and the latter is used for low current (< 150A) welding.
Selection of Welding Materials
2.1 Argon: It has excellent protective effect. It neither acts chemically with metals nor dissolves in metals. It makes the metallurgical reaction of molten pool in welding process simple and easy to control. Therefore, it provides good conditions for obtaining high quality welds. Arc combustion is very stable in argon, and it still burns stably under the condition of low current welding (< 10a). It can weld active metals which are easy to oxidize and nitride, ferrous metals with high melting point and dissimilar metals. It is widely used in aviation, atomic energy, petrochemical industry, power plant boiler, machinery and other fields. In order to ensure the best welding quality, the purity of argon is usually required to be over 99.99%. < div= >
2.2 Welding wire: Argon welding wire model is more common 304, 308, 308L, 316L, etc. The selection principle is that the mechanical properties of weld metal should be higher or equal to the limit specified by base metal, and other properties should not be lower than the corresponding requirements of base metal when needed; or mechanical properties and other properties meet the technical requirements specified in the design documents.
2.3 Tungsten Pole: Because pure tungsten is not easy to start arc, thorium tungsten or cerium tungsten poles are generally used. Cerium-tungsten electrode has the advantages of long arc beam, concentrated heat, long life and easy arc initiation. More importantly, the radiation hazard is much smaller than that of thorium-tungsten. Therefore, Cerium-Tungsten electrode is recommended as the electrode for gas shielded tungsten welding. If the surface of tungsten electrode is brown, yellowish-green or blue, it indicates that the oxidation of the end is serious, and it should be re-polished before use.
Welding steps:
3.1 Base Material Requirements: Flat welded flange and pipe fittings are made of 304, 304L, 316 and 316L stainless steel, and the same type stainless steel is preferred for flange and pipe. The inner hole diameter D1 of the flat welded flange is slightly larger than the outer diameter D2 of the pipeline, and the ideal value is d1=D2+0.5mm+0.1mm. Pipeline wall thickness t=0.6-5.0 mm and pipe outer diameter D2=6-500 mm.
Pre-welding cleaning: TIG welding process is very sensitive to pollution, so it is necessary to clean the welding position before welding. Firstly, the burrs of pipes and flange welds are cleaned to prevent oxide scales from occurring during welding. Secondly, the oil contamination of welds is cleaned. Acetone or alcohol is usually used to remove oil and degrease in the range of 20-50 mm on both sides of welds. Then the pipeline passes through the inner hole of the flange from the non-sealing surface of the flange until it is 2-3 mm away from the sealing surface of the flange.
Fig. 1 Schematic diagram of flange and pipeline welding
Welding at position 1 (see figure 1):
Welding position 1 implements self-fusion welding without wire addition.
Adjusting welding current: The welding current of tungsten argon arc welding is usually selected according to the material, thickness and space position of the workpiece. When the welding current increases, the penetration increases, the width and residual height of the weld slightly increase, but the increase is small. Excessive or too small welding current will cause bad weld formation or welding defects. It is better to adjust the welding current to 80-110A at this position. The ideal current value is 90A. It is better to adjust the welding current to 150-180A, and the ideal current value is 160A.
Adjusting argon flow rate: too small argon flow rate, weak protection gas flow, poor protection effect, easy to produce defects such as blowhole and weld oxidation; if the gas flow rate is too large, easy to generate turbulence, poor protection effect, but also affect the stable combustion of the arc. The welding of position 1 does not need to be protected by argon gas. It only needs to adjust the gas flow rate of the torch. The argon gas flow rate can be adjusted to 7-10 l/min.
Determine the welding speed: When the welding speed increases, the penetration and width decrease, the welding speed is too fast, and it is easy to produce non-fusion and non-penetration. If the welding speed is too slow, the welding seam is very wide, and it may also produce defects such as welding leakage and burn-through. In manual TIG welding, the welding speed is usually adjusted at any time according to the size of molten pool, the shape of molten pool and the fusion of both sides.
Adjust the length of tungsten pole protrusion: In order to prevent arc overheating burning nozzle, usually the tungsten pole should extend outside the nozzle. The distance between the tungsten extremity head and the end face of the nozzle is the tungsten pole extension length. The smaller the tungsten pole extension length is, the closer the distance between the nozzle and the workpiece is, the better the protection effect is, but too small will hinder it.
