In the former article, we have talked much on the main production process of big straight welded pipe, and now I’d like to give a detailed information on the analytic description of the dry welding technology of large diameter thick wall welding.
Dry welding technology of large diameter thick wall welding
Automatic welding of large diameter and thick wall (more than 21 mm) pipeline often use u-shaped groove or complex groove, because the u-shaped groove and composite groove machining effects processing time and efficiency. V groove processing is simple, time saving and energy saving, but when having V groove automatic welding to process large diameter and thick wall pipe, if the welding technology is not proper, the welding defects will happens.
Welding methods adopt STT root welding + CRC-P260 automatic welding machine including hot welding, filling, covering. Welding equipment: Lincoln STT welding machine, DC – 400, the CRC – Lincoln P260 automatic welding machine. Gas protection: STT root welding, automatic welding protective gas for Ar + 80% and 20% CO2.
Automatic welding commonly adopts complex groove or u-shaped groove, and V groove also can be used in small wall thickness pipe. Their common characteristic is with smaller groove flowing. The second line of west-east gas pipeline wall thickness of 21.0 mm, the top width of the V groove is about 22 mm, the width is close to the CRC – P260 welding torch swing limit. The groove type of automatic welding is a huge challenge. Based on past experience in automatic welding test welding process parameters were determined.
Using the above parameters to have automatic welding test, test found not fusion between layers, sidewall incomplete fusion, dense hole and overhead welding parts, etc. are more likely to happen in welding automatic welding.
In order to guarantee good cover surface forming effect, the cover surface of weld in choosing the smaller welding speed at the same time try to reduce the welding torch swing frequency, make the cover of weld thin and wide, so as to reduce the presence of molten pool time, achieved the purpose of reducing overhead welding position. According to the welding test result, the STT root welding + CRC automatic welding filler are analyzed and finally determined to be used in the second line of west-east gas link.
Weld mechanical properties nad been tested by the China institute of petroleum and natural gas pipeline welding technology testing center, and the indicators in accordance with the second line of west-east gas tie line to meet the construction requirements. The successful application of STT root welding + CRC – P260 automatic welding in large diameter and thick wall (V groove) pipe fully embodies the automatic welding technology, high quality, high efficiency and low labor intensity.
The technical requirements and quality inspection of straight seam welded pipe:
According to GB3092 standard for low pressure fluid conveying in welded steel pipe, welded pipe nominal diameter for 6 ~ 150 mm, nominal wall thickness of 2.0 ~ 6.0 mm, the length of the pipe is usually 4 ~ 10 meters, can produced according to the length which is required. Steel tube surface quality shall be smooth, not allowed to have folded, crack, delamination and lap welding defects. Steel tube surface is allowed no more than negative deviation of wall thickness, scratches, minor defects such as weld misalignment, burns and scarring. Allow thickening of seam thickness and inside seam welding steel. Welded steel pipe should be tested for mechanical properties test and flattening test, flaring test, and to achieve the requirements of the standard.
Steel pipe should be able to withstand the internal pressure, the pressure test 2.5 Mpa when necessary, and no leakage in a minute. Allows for eddy current flaw detection method instead of water pressure test. Eddy current flaw detection by GB7735 standard for steel tube eddy current flaw detection method. Eddy current flaw detection method is to probe fixed on the rack, inspection and weld 3 ~ 5 mm distance, depending on the rapid movement by the steel tube to have a full scan of the pipe, and to achieve the purpose of inspection by the automatic processing and automatic sorting of the eddy flaw detector. After inspection of welding, using the flying saw to cut off with the regular length. And using the hexagonal strapping for package.