| Big diameter bent pipes (big diameter bent pipes) is the bent pipe that the diameter is more than 508mm, and the wall thickness is more than 8mm. As important component in pipeline system, big diameter bent pipes is widely used in construction machinery, power machinery, and boilers, petrochemical, aerospace, pipeline and other pressure vessel. Pipe bending technology mainly is bend at home and abroad currently, it can be divided into cold-formed and hot bending according to heated or not when it being bent. Bending process is a complex deformation process of elastic and plastic. So there will be cracks appear on external wall, wrinkling on internal wall, distortion on cross-section and so on in the process of bending, resulting in a serious accident. In view of the above, we achieve the forming of big diameter bent pipes by welding technology in this paper, that is the plate is bent based on usage requirements at first, and then the two bending parts are achieved weldinged by the appropriate welding procedure and welding parameters. Further in this paper study the performance of welded joints of big diameter bent pipes, and carry out some associated numerical simulation with ANSYS software.Microstructure test suggest that there are a large number of acicular ferrite, granular bainite, black pearlite exists in base metal, the microstructure of fusion zone near the weld was mainly granular bainite, microstructure of mapping weld is ferrite and a few of pearlite, microstructure of filling weld is ferrite and a few of pearlite; microstructure of root weld is ferrite and pearlite.The forming properties of big diameter bent pipes are studied by the tensile test,fracture surface scan and crack tip opening displacement. Tensile tests show that the fracture location of welded joints mostly in the base metal, which indicate that the quality of the welding is good, and the tensile strength values are up to standard. The scan of fracture surface of tensile specimen display: welding line fracture surface have lots of deeper dimples, and the impact energy is larger during fracture. The fracture at the base metal mainly covered with small dimples, the dimples are shallower and the number is fewer, and the impact energy is lower than welding line during fracture. Testing the fracture toughness of welded joints with crack tip opening displacement and the test results show that: the average CTOD of weld department at minus 20°is 1.39, while the average CTOD of base metal is 0.82. Both are greater than 0.15。It can be concluded that the welded joints of high strength pipeline steel have more excellent low-temperature toughness and good welding process.Two program of ANSYS software were used to simulate temperature field and stress field of big diameter high-strength steel bent pipes. The first case, there is a welding seam welded together. The second case is a six welding seam welded. Using the life and death cell technology, first to "kill" the unit in the weld area, and then gradually make it "resurrection" in the welding simulation process, they can achieve the effect of the simulation of welding process. The results showed that: the temperature field and stress field distribution received by two kinds of programs are same, that is when all the jointing completed, the maximum temperature point appears on the top of the welding materials, at the final part of the welding, the temperature gradually reduced from the middle to the ends. Maximum residual stress point appears in the junction of the base metal and welding materials, and residual stress gradually reduced from the middle to the ends. The results showed that the simulation results of big diameter bent pipes are basically consistent with the test. |
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