The influence of boss welding on residual stress of electric welding longitudinal pipes

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 high-frequency straight seam resistance welding pipe. There is a sharp rise in stress and the longitudinal weld residual stress impact after the boss installation on the electric resistance weld pipes is much larger than the mother pipeline.

The distribution of residual stress in HFW welded pipes is closely related to the welding and forming process. HFW resistance welding is to form a circular tube blank with a broken cross section after rolling forming. The skin effect and the proximity effect of the high-frequency current are used to concentrate the induced current on the edge of the opening corner of the tube blank. The temperature is quickly raised to the soldering temperature and is completed by extrusion of the squeeze roll. Both the sizing and the hydrostatic test after welding are important processes that cause the axial residual stress value to be greater than the hoop residual stress value. After high-frequency welding, the microstructure and properties of the weld and heat-affected zone are improved by in-line heat treatment, resulting in residual stress values in the weld and its vicinity being lower than the pipe body area. The overall residual stress value on the HFW electric resistance welded pipe is small, and the axial direction is small. The residual stress value is greater than the hoop residual stress value, and the axial and hoop maximum values are approximately 38% and 29% of the base material yield strength. The residual stress value of the longitudinal weld of the HFW welded pipe and its vicinity is lower than that of the pipe body. The axial and circumferential variation range is about 24.5%~33.6% and 16.4%~21.5% of the yield strength of the base metal. The weld area is not There is a stress concentration phenomenon.

The horizontal line indicates the average residual stress of the welded pipe weld zone, and the curve is the axial and circumferential residual stress of the welded pipe. It can be seen that after the Φ 26 mm and Φ 50 mm hole welding bosses are opened by hot cutting at the center of the weld, the influence law and degree of the original residual stress amplitude on the HFW longitudinal weld are basically the same. And as the size of the opening increases, the stress concentration area increases. The residual stress value in the vicinity of the boss on the longitudinal weld of the HFW welded pipe increases, and the increase of the axial residual stress is much larger than the axial residual stress. As the distance from the boss weld increases, the residual stress increases gradually. In both cases, the axial residual stress amplitudes are 444 MPa and 427 MPa, which is about 3.86~3.71 times higher than the original state of the HFW resistance welded pipe, which is about 84% of the base material yield strength. The magnitude of the circumferential residual stress is 201 MPa and 260 MPa, which is about 3.10~2.33 times higher than the original state of the HFW electric resistance welded pipe, which is about 49% of the yield strength of the base metal. Relative to the original distribution of the residual stress of the HFW resistance welded pipe, according to the change of the axial residual stress, the stress concentration on the longitudinal weld in both cases
The range of the sound is: 92 mm and 167 mm.

The boss welding changes the original distribution state of the residual stress of the electric resistance welded pipe, and the regularity is basically the same as that on the longitudinal welded joint, that is, the residual stress at the edge of the boss rises rapidly, and the stress concentration region exists locally. However, unlike the residual stress distribution on the HFW longitudinal weld, the increase in circumferential residual stress on the HFW resistance welded pipe body is much larger than the axial residual stress. It can be seen that the magnitude of the hoop residual stress in both cases is 433 MPa and 273 MPa, respectively, which is about 2.4 and 1.5 times higher than the maximum value of the HFW electric resistance welded pipe body, which is about 82% of the yield strength of the base metal. And 52%. After the boss is welded, the axial residual stress of the pipe body changes little. According to the variation of the circumferential residual stress, the stress concentration in the two cases is in the order of 51 mm and 113 mm.