The common welding methods of carbon steel elbow in production are: fully automatic welding large caliber, thick wall (greater than 21mm) pipeline, often using U groove or compound groove. Because U groove and compound groove processing consume time and energy, it restricts pipeline welding efficiency. The processing of V groove is simple, saving time and labor, but if the V groove automatic welding of large diameter and thick wall pipeline is automatic, improper welding process parameters will lead to welding defects.
The welding method adopts STT root welding +CRC-P260 automatic welding machine to heat welding, filling and cover surface. Welding equipment: Lincoln STT welding machine, Lincoln DC-400, CRC-P260 automatic welding machine. Protection gas: STT root welding protection gas 100%CO2, automatic welding protection gas is 80%Ar+20%CO2. Carbon steel elbow automatic welding is usually made of compound grooves or U grooves, and V grooves can also be used in small thick pipelines. Their common characteristics are smaller gap on the groove. The wall thickness of the second line of the west to east gas transmission pipeline is 21.0mm, and the width of the V groove is about 22mm, which is close to the limit of the CRC-P260 welding torch. Such a type of groove is a great challenge to automatic welding. According to the previous experience, the parameters of the automatic welding test and welding are determined. The above parameters are used to carry out automatic welding test. It is found that automatic welding seam is prone to defects, such as no fusion between layers, non fusion of side walls, dense pores and over high welding height.
In order to ensure the good molding effect of the cover surface, the welding speed of the cover welding is minimized while minimizing the welding speed. The welding frequency of the welding torch is minimized and the welding seam of the cover surface is thin and wide, which reduces the time of the weld pool and reduces the remaining height of the welding position. According to the results and analysis of the test welding, the STT root welding of the second line of the west to east gas transmission line is welded by +CRC fully automatic welding and covering process parameters. According to the welding parameters of Table 3, the weld of carbon steel elbow can be detected without defects such as holes, cracks and non fusion, and the forming of the weld surface. The mechanical properties of the welded joints are tested by the welding technology center of CNPC, and all indicators meet the construction requirements of the second west east gas transmission line. The successful application of +CRC-P260 automatic welding for STT root welding on large diameter and thick wall (V groove) pipeline fully embodies the characteristics of automatic welding technology with high quality, high efficiency and low labor intensity.