Numerical Simulation Of Temperature Field In Single-wire And Multi-pass Welding Of Grade X80 Pipeline Steel

It is significant for welding quality to make accurate calculation of welding temperature field which is premise of metallurgy analysis, welding stress and strain. The change of temperature field through welding thermal process has been explored. In this paper, X80 pipeline steel of West-East Gas Pipeline Project was used to conduct multi-layer welding. Numerical simulation technology was used to explore the change of three-dimensional transient temperature field and welding thermal cycling of weld bead zone and heat affected zone. Microstructure and mechanical property in different zones of welding joint were predicted. The research is important to both science theory and engineering application.Pipeline steel of grade X80 monofilament multi-layer welding procedure test was carried out. Welded joints were processed and analyzed. The microstructures of weld bead zone and heat affected zone are mainly ferrite and granular bainite. Grain structures of each weld layer were affected by heat input and thermal effect of subsequent welding. The grain structure which is affected by small heat input or follow-up welding thermal is relatively small. Conversely, grain structure would be relatively coarse. Hardness of heat affected zone declines than that of base material. The phenomenon of tissue softening has appeared in HAZ. The maximum hardness of HAZ is in coarse grained region. The width of HAZ declines with the increase of welding layer from 3.5 mm to 2.5 mm to1.5 mm.According to the actual welding procedure test, pipeline steel of grade X80 multi-layer welding three-dimensional numerical analysis model was established. Because the arc stiffness of the flux-cored wire semi-automatic arc welding is relatively powerful, double ellipsoid distribution pattern was chose as heat source. Solid70 and solid90 were chose as welding heat transfer element. According to the need, gridding lengths of weld bead zone, heat affected zone and base material zone were divided into 1 mm, 2 mm and 4 mm with the computational efficiency and accuracy considered. ―Element birth and death technology‖ was used to simulate gradual formation of the multilayer weld bead process. Three-dimensional moving coordinate system was used to realize the movement of the welding heat source load.3-D transient welding temperature field of starting stage, stable stage and cooling stage were analyzed. Welding molten pool was generated at 1.5 s. Length, width and depth of welding molten pool increase over time. Quasi-steady state was achieved at 3.5 s. Temperature field of different layers and thermal cycling of different points was analyzed. Thermal cycling and CCT diagram of pipeline steel of grade X80 were used to predict microscopic structure and hardness of weld bead zone and heat affect zone. Microstructures are ferrite and bainite. Average hardness is 218~229 HV10.