Study On The Microstructure And Fracture Behavior Of The Welded Joint Of L415/N08825 Bimetallic Composite Pipe

The bimetal clad pipes can fully combine the mechanical properties of the base layer and the corrosion resistance of the clad layer,which can effectively reduce the cost.It is widely used in the fields of petroleum,chemical industry,water conservancy,environmental protection,seawater desalination.During the welding process of the bimetal clad pipes,due to the difference of linear expansion coefficient and thermal conductivity between two metals,which will easily produce stress concentration to promote crack initiation and propagation,thereby increasing the risk of pipeline safe operation.lt is of great significance to understand the fracture behavior of the welded joint and the effective safety assessment of bimetal clad pipes.According to the fracture behavior of welded joint of L415/N08825 bimetal clad pipe,the microstructure,composition distribution and crack propagation resistance of the welded joint were studied.The main conclusions are as follows:(1)The microstructure,composition distribution,inclusion composition and distribution of the welded joint of the bimetal clad pipes were analyzed by OM and SEM(EDS).The results show that the microstructure of the welded joint have a large inhomogeneity.The microstructure of N08825,L415 and the weld seam is a single phase austenite,the complex phase of acicular ferrite,quasi polygonal ferrite,pearlite and bainite and dendritic austenite,respectively.There is unmixed zone in the weld/N08825 interface,the weld/L415 interface and the N08825,and the interface between the base layer L415 and the clad layer of the three phase interface,which are caused by the difference of the melting point of the material.Cr,Ni,Mo and Fe and other major elements in the interface transition still exist mutation and show a larger concentration gradient.The inclusion size is uneven at the weld seam,HAZ of L415 and HAZ of N08825.(2)The universal testing machine,impact testing machine and the microhardness tester were perfomed to study the mechanical properties(strength,stress-strain and micro-hardness)of the weld joint of bimetal clad pipes.The results show that the mechanical properties of welded joints are of great heterogeneity.The average yield strength and tensile strength were 432MPa and 582MPa,respectively.The fracture occurred at the base metal.The welded joint has good impact performance,and the average impact energy of the HAZ and weld seam is 123J and is 123J,respectively.The fracture characteristics are ductile fracture.The average hardness of the weld is higher than that of the base metal and the heat affected zone.The welded joint has good bending resistance.(3)The grain characterize of the three-phase region of the bimetal clad pipes weld joint were studied by EBSD.The welded joints are composed of two crystal structures of BCC and FCC.The unmixed zone between N08825 and L415 consist of two parts which are a large misorientation distribution,and the grain distribution shows obvious directionality.The small angle grain boundaries of BCC and FCC structures are 74.81%and 79.91%,respectively.The CSL grain boundary of BCC and FCC structures are ?3,?11,?17b and ?3,?5,?17b,respectively.The grain size of BCC and FCC structure is about 0.5m,which is the largest proportion.(4)The crack initiation and propagation process in notched specimens made from the weld joint were investigated by using in-situ tensile tests via SEM.The load-displacement curve does not appear obvious yield platform and shows the shape of dome.The crack initiation point is located in the unmixed zone between L415 and N08825.Then,there are many crack initiation points on the N08825 side,The N08825 side preferentially fracture and then growth to the L415 side,eventually fracture.The crack propagation path which is less affected by the interface is approximately linear.The fracture is dominated by ductile void fracture mechanism.(5)According to ASTM E1820-15 Standard Test Method for Measurement of Fracture Toughness,fracture tests and microscopic observations of the weld joint of the bimetal clad pipes were analyzed fracture mechanism of different regions within the weld joints.The HAZ has higher fracture resistance than the weld seam.The crack propagation path of the weld seam and HAZ is approximately linear,and the crack propagation processes do not appear cross region.The fracture mode of the weld seam and HAZ are typical ductile void fracture mechanism and ductile fracture mechanism,respectively.