Study On Microstructure And Properties Of Welded Joints Of Bimetal Composite Pipes

Bimetallic composite tube because of its outstanding performance,now more and more widely used in oil and gas field construction and transportation.But the differences between grassroots and cladding of composite tube in chemical composition and physical properties are very big,the transition layer belongs to the welding of dissimilar metal in the pipeline welding connection,which is more difficult compared with the same metal.Mechanical pipe,in particular,there is a certain gap between grassroots and cladding,which may contain air,water and other inclusions in the welding of the transition layer is easy to cause interlayer porosity,formation of weld flash because of molten pool collapsing,welding opaque and a series of weld defects.Therefore,this article in view of the mechanical bimetallic composite tube welding problem,formulates three different kinds of welding scheme,and respectively adopts V-shape groove,sealing groove and overlaying groove on X65/316L bimetallic composite tube welding,the welding joints were obtained for organization and composition analysis,mechanical property testing and corrosion performance testing,evaluates stand or fall of performances of three kinds of welded joints according to the test results,so as to determine whether the design of three kinds of welding scheme is reasonable.Microstructure observation results show that the weld microstructure of composite tube welded joint under three kinds of welding scheme are ferrite + austenite biphase organization,and cellular crystal,branch crystal,isometric crystal and other grain shapes in different ways to grow appear at high Ni,Cr element content.Transition layer weld phase analysis shows that the phase of transition layer weld of three composite tube welded joints is composed of?+? two-phase,and no ? phase,M23C6 phase and other harmful phases separate out.Fusion zone element line scanning analysis shows that all alloy elements such as Ni,Cr,Mo produce a concentration gradient in this region,but there is no obvious segregation phenomenon is observed,C element in the V-shape groove welding joint and sealing groove welding joint has a slight diffusion phenomenon.Mechanical property test results show that the microhardness maximum of V-shape groove welded joint,sealing groove welded joint and overlaying groove welded joint are 231 HV,213 HV,222 HV respectively,all are lower than the limit hardness of pipeline steel 250 HV under acidic medium,which indicates the hardness of three kinds of welded joint is qualified,the microhardness of weld area is higher than heat affected zone and base area.The average yield strength of three kinds of welded joint are 443.7 MPa,462.3 MPa,481.7 MPa respectively,the V-shape groove welded joint don't reach the yield strength minimum value of 448 MPa of the composite tube grassroots X65 parent metal,while the other two kinds of welded joint meet the requirement.The average impact toughness of three kinds of welded joint all reaches the requirement of X65 carbon steel,and the impact toughness of weld area generally higher than the impact toughness of heat affected zone.The impact fracture of weld seam and heat affected zone of three welded joints present toughening nest morphology with different size and depth,and accompany with ridge torn edges,which belongs to the typical ductile fracture mode.Corrosion performance test results show that electrochemical test results and chemical immersion test result are consistent,which indicates that three kinds of composite tube welded joint have good corrosion resistance,the corrosion resistance of both sealing groove welded joint and overlaying groove welded joint are better than that of 316L stainless steel material,and the corrosion resistance of V-shape groove welded joint is worse than 316L material.In conclusion,the composite tube welding scheme with v-shape groove can't meet the requirement of practical engineering,and the design of welding parameters with sealing groove and overlaying groove are reasonable and meet the requirement of practical production.