Experimental And Numerical Investigation On Twin-wire And Multi-pass Girth Welding Of Grade X80 Pipeline Steel

Tandem Gas Metal Arc Welding(T-GMAW)is an efficient welding technique,which is widely used for the welding of High-Strength Low-Alloy steels,stainless steels and alloyed aluminums.API X80 pipeline steel has many advantages such as high strength,good performance,less consumption and low cost and has been greatly used in pipeline construction.The study is focused on improving welding efficiency and quality.T-GMAW was chosen to weld 18.4-mm-thickness and 1219-mm-diameter X80 pipeline steels and appropriate weld joints have been formed.Macro-morphology,microstructure and micro-hardness were observed and measured as well as temperature profiles and thermal cycles were numerically analyzed.T-GMAW was used to get well forming welded joint which has only 5 welding layers.To compare with combined welding process of 7 welding layers that was applied in the second west to east gas pipeline project,a 30% increase in welding efficiency.Optical microscope and scanning electron microscope were used to study the microstructure of weld metal(WM),heat affected zone(HAZ)and base metal(BM).The microstructure of WM in root pass was composed of 80% acicular ferrite(AF)and a little polygonal ferrite(PF)whose edge length is less than 10 ?m.The microstructure in hot pass had feathery ferrite(FF)that length-width ratio is greater than 20.More than 50% PF existed in fill pass and gap pass,the grain size of PF in fill pass was much smaller.The microstructure of HAZ in root and hot pass was composed of granular bainite(GB)and lath-shaped bainite(LB),LB was less in hot pass.The microstructure in fill pass and gap pass was composed of GB.Grain size of gas pass had doubled than that of fill pass because of heat treatment after welding.There will be softening behavior of varying degrees in the welded joint,especially in the HAZ.The hardness distribution presents like “W”.Temperature field in quasi-steady state from root pass,hot pass,fill pass to gap pass indicate that the highest temperature increase with welding heat input improving.Take the second fill pass to study the double molten baths stage,the combining molten pool and the cooling stage of tandem welding temperature field.The quasi-steady stage developed in 10.0 s.The highest temperature was in the front of the rear molten pool and the maximum heat flux present in the front of welding pool under the first wire,and the maximum temperature gradient under the second heat resource.The welding pool got longer and thinner with welding speed increasing.When the distance between welding wires got larger,there would be forming two welding pool independently.The thermal cycle of 5 layers was researched through picking a series of points in HAZ.It would take the most heat effect if the selected points were near the welding layer.The thermal cycle curves of twin-wire welding and single wire welding were compared.The twin-wire welding process was through two peak temperature and got slower cooling speed.Make prediction of microstructure and hardness of the HAZ through cooling time t8/5 and the continuous cooling transformation curve,and the result was compared with the one from the experiment.