Butt welding tees are elbows formed by hot pressing or forging of steel. The connection form of butt welded tees is directly to steel pipes. The main manufacturing standards of butt welded tees are generally GB/T 12459, GB/T 13401, ASME B16.9, SH3408, SH3409, HG/T 21635, HG/T 21631, SY/T 0510. Butt welding tees generally have equal diameter tees, different diameter tees and so on. The materials are carbon steel, alloy steel and stainless steel.
The weld of welded tee joint is on the intersection line between main pipe and branch pipe. The shape and position of weld joint are more complex than those of weld joint. The quality of weld joint is not easy to control during welding. According to statistics, the leakage rate of this kind of welding seam accounts for 80% of all process seam leakage rate. The main defects are impermeability, porosity, slag inclusion and so on.
At present, the weld groove on the intersection line is mainly made by manual gas cutting or plasma cutting. The processing accuracy is not high, the groove angle is small, and the welding process parameters are not selected properly, resulting in the lack of penetration. Another main reason for the quality defect of welded tee is the wrong cutting process, the outside groove of branch pipe is processed into the inside groove, and the branch pipe is directly overlapped on the main pipe for welding. Unpenetrated welds reduce the strength of welds. For pipelines transporting corrosive media, with the increase of production cycle, the heat affected zone of welds will be thinned, leading to weld leakage. The causes of blowhole and slag inclusion are that oil, rust, moisture and other impurities in the groove and its vicinity are not cleaned up before welding, and the skin is not cleaned during welding. In addition, when welding operation, the welding speed is too fast and the welding current is too small, which accelerates the cooling speed of the weld.
