The Influence of Boss Welding on Residual Stress of ERW Pipe

During the station construction of gas and oil pipelines, valves, instrument and flanges installation requires tapping in the main pipeline. It is hardly to identify visually the base pipe straight seam location of high frequency electric resistance welding. Some boss mounting positions are very close to electric welding pipe seam. Hence, it is very necessary to study the deep research and analysis for the boss installation influence on the base pipeline. The paper analyses the residual stress distribution on the high frequency straight seam resistance welding pipeline by hole drilling, and original residual stress impact about the different boss welding positions on erw pipe. There is a sharp rise in stress and the longitudinal weld residual stress impact after the boss installation on the erw pipe is much larger than the mother pipeline.

The distribution of residual stress in erw pipe is closely related to its welding and forming process. HFW resistance welding is the strip after rolling through the formation of a cross-section of a circular tube blank, the use of high-frequency current skin effect and proximity effects so that the induced current is highly concentrated in the edge of the opening edge of the tube to so that the temperature rapidly rose to the welding temperature, extrusion roller squeeze completed. Sizing and hydrostatic testing after welding are all important processes leading to axial residual stress values greater than the circumferential residual stress values. After high frequency welding, the on-line heat treatment improves the microstructure and properties of the weld and heat-affected zone, resulting in lower residual stresses in the weld and in the vicinity of the weld. The overall residual stress on the erw pipe is smaller, residual stress is greater than the value of the circumferential residual stress, the maximum axial and circumferential yield strength of about 38% and 29% of the base metal. The residual stress of longitudinal weld in erw pipe and its vicinity is lower than that of the pipe body. The range of variation in axial and circumferential direction is about 24.5% -33.6% and 16.4% -21.5% of the yield strength of base metal, there is stress concentration.