Microstructural Evolution And Mechanical Properties Of Friction Stir Welding Oil-gas Pipeline Steels Joints

With the rapid development of China's economy,the demand for energy is increasing,the exploitation and transportation of oil and natural gas resources have become the core concern.The demand of the oiled tubing for the exploiting oil and gas,the land and marine oil and gas pipelines used for transportation are very large,and the welding problem in builuding those pipeline is particularly important.At present,the fusion welding is widely used to join oil and gas pipelines.It is very difficult to avoid solidification defects such as cracks and blowholes,the grain coarsening is also very serious,and the mechanical properties and corrosion resistance of joints are seriously reduced.The pipelines in service often fail from the weld joints.In addition,the complex welding process in the construction of oil and gas pipelines also greatly extended the period of time and increased the cost.Therefore,an efficient and stable welding technology is urgently needed for joining oil and gas pipelines.Friction stir welding(FSW)is a new type of solid-state bonding technology.The joint quality is excellent and it is an ideal technology for welding oil and gas pipeline.Therefore,girth weld FSW machine is developed,the onshore,offshore oil and gas pipeline steel and coiled tubing steel were friction stir welded,the microstructural evolution of the joint and mechanical properties were studied systematically.The main work and results of this dissertation are as follows:(1)In order to apply FSW to actual welding engineering of oil and gas pipeline,the girth weld FSW machine,which was specially used for welding pipeline,was successfully developed by our research group.This machine can be used to weld girth butt joint of oil and gas pipeline in an efficient and high quality way.No preheat or slow cooling processing was taken during FSW,therefore the welding efficiency was greatly improved.No defect was produced at suitable range of welding parameters,and the quality of welded joint is stable.(2)The onshore pipeline steel-X100 pipeline steel was friction stir welded at high,medium and low heat input parameters.The grain size,bainite transformation,texture components and impact toughness on the advancing side and retreating side of the stir zone were studied.The experimental result suggested that the strong plastic deformation in friction stir welding has an important effect on the phase transition of the joint.The stir zone at low heat input parameters mainly contained lath-bainite and impact toughness of the stir zone was excellent and almost equal to that of the base metal.At high and medium heat input conditions,massive granular bainite formed,showing poor toughness in those zones.(3)The joint of FSW contained fiber microstructure with clear direction,and the direction of the fiber microstructure and microstructure at different locations of the stir zone were quite different.The fiber structure direction on the advancing side,center and the retreating side of the joint in FSW offshore oil and gas pipeline steel-duplex stainless steel was 0°,90° and 45° to the welding direction,respectively.Dynamic recrystallization occurred in most materials on the advancing side and the grain refinement was also obvious.There was no obvious deformation occurring on the advancing side during tensile test,and most of plastic deformation and train induced martensitic transformation taken place on the center and retreating side,crack easily formed at the boundaries between ferrite and transformed martensite on the retreating side,and the tensile strength of the welded joint was similar to that of the base metal.(4)Base on the low heat input characteristic of FSW,the coiled tubing steel-austenitic stainless steel with Mn was friction stir welded at wide range of welding parameters.The experimental results noted that the heterogeneous distribution of the temperature and strain rate had great influence on the microstructural evolution of the stir zone.Both discontinuous and continuous dynamic recrystallization occurred on the advancing side and the retreating side was mainly continuous dynamic recrystallization.At lower rotation rate,no precipitated phase and ?-ferrite formed in the stir zone.The ?-ferrite formed in the stir zone with the increase of rotation rate,and the ?-ferrite can promote dynamic recrystallization and prevent the grain growth.As the content of the Mo element was very low in austenitic stainless pipeline steel,no harmful ? phase formed in the stir zone.The tensile strength of the welded joint was almost similar to that of the base metal at optimum parameter.(5)In order to alleviate the wear of the stir tools during FSW steels,this paper used gas metal arc welding(GMAW)to preheat the steels,the heat assisted FSW process was simplified,and the X100 pipeline steel was friction stir welded.In this case,the welded joint was substantially preheated by the GMAW before FSW and then the steel was fully softened,therefore the friction force of the stir tool was greatly decreased and the wear of the stir tool was greatly alleviated.With the increase of the distance between the stir tool and the welding torch,microstructure on the advancing side was shifted from tiny granular bainite to coarsen lath-bainite.