Study On Friction Stir Welding Of X65 Pipeline Steel

Pipeline steel as the main body of pipeline transportation engineering,is the representative of high strength low alloy steel,has excellent strength,toughness,corrosion resistance and welding performance,and is widely used in long distance transportation of petroleum,natural gas,oil and other energy sources,which has unique advantages of economy,safety and environmental protection.In recent years,with economic growth and increasing demand for energy,construction of global oil and gas pipeline continues to accelerate.X65 pipeline steel occupies a large amount of pipeline engineering in our country,especially the submarine pipeline construction of marine development,therefore,the pipeline welding method,environmental protection and other performance requirements continue to improve.According to the research focus on foreign and domestic advanced solid material almost blank of friction stir welding(FSW)technology,and the requirements of the State Key Laboratory of scientific instruments and equipment related work,based on the pipe friction stir welding machine developed by the research group,a series of FSW test of X65 pipeline steel(specification:?273mm×7mm)were conducted successfully.The microstructure and mechanical properties of the joint especially the impact toughness were analyzed,and the welding process and properties were optimized.This research mainly includes the following five parts:(1)The pipe friction stir welding machine consists of mechanical system and control system.The mechanical system consists of support system,power system,mixing stool system and hydraulic system.The control system includes hardware configuration and software system.The welding machine is suitable for all positions of the ring seam FSW,welding diameter:168?273mm,and can avoid the keyhole defect of conventional FSW.(2)The FSW test of X65 pipeline steel was carried out,and the weld with good surface forming,dense fish shape and no internal defects was obtained.The microstructure in heat affected zone to fine equiaxed ferrite,there are fine pieces scattered pearlite and a small amount of acicular ferrite(AF)and polygonal ferrite,no coarse grain zone;the microstructure of the nugget zone is more fine and compact,dominated by AF.Comparison of the nugget zone,the content of AF in the retreating side is the most,as the transition region,the content of AF in the middle part is the second,while the forward side is mainly the equiaxed fine blocky ferrite,and the AF content is the least.(3)The rotation speed increases from 400,500 to 600rpm,the micro structure first thinning again increasing relatively,namely too small or large rotation speed is not conducive to the transformation of organization recrystallization and dynamic recrystallization process;the hardness and strength of joints increases continuously with decreasingly increase degree,the impact power of welding seam increases then decreases,and impact energy accounted for more than 66%of parent material,which has excellent joint toughness.The welding speed decreased from 150,100 to 50mm/min,the grain size increases,too large welding speed results in coarse microstructure affecting the performance;hardness and strength of the joints decreases and the impact energy of the weld seam decreases,and the impact energy accounted for more than 68%of parent material,which has excellent joint toughness.(4)The appropriate welding process is lined with screened pipe;the appropriate FSW tool is to ensure sufficient mixing length size,such as No.B stirring stool;the appropriate welding parameters:the rotation speed 400?600rpm and the welding speed 50?150mm/min,and the optimal welding parameters were:rotational speed 500rpm and welding speed 100mm/min.(5)Compared with traditional welding,there is no cold crack and inclusion in the pipeline steel FSW,and the deformation is small,the residual stress after welding is low,the degree of microstructure refinement is very high,the mechanical properties are generally good and stable,especially the impact toughness is extremely excellent.