Research On Fatigue Crack Growth Of Welded Joints Of L360MS Pipeline Steel

Bimetal composite pipe a typical application of dissimilar metal connection for oil-gas transmission pipelines,which has been developing rapidly recently because of its excellent properties and the competitive cost advantage.However,due to the great difference in physical and chemical properties between base metal and weld metal,stress concentration inevitably occurs at dissimilar metal welded joint,which promotes the initiation and propagation of cracks.Therefore,it is very important to understand the change of microstructure and fracture behavior of dissimilar metal welded joints for the safety assessment and safe operation of oil and gas pipelines.In this paper,the microstructures,mechanical properties and fatigue growth behavior of L360MS pipeline steel welded joint filled with Inconel 625 wire were investigated.The main conclusions are as follows:(1)The microstructure and composition distribution of the welded joints were investigated by optical microscopy(OM)and scanning electron microscopy(SEM).The results show that the microstructures of L360MS pipeline steel welded joint have a large inhomogeneity.The base metal is composed of banded pearlite and ferrite and the welds include columnar austenite and equiaxed austenite.Besides,Type I boundary and Type II boundary are also observed near the fusion line.The heat affected zone(HAZ)consists of coarse grained zone consisting of coarse ferrite,pearlite and lath martensite,and fine grained zone consisting of uniform fine ferrite and pearlite.There is an element concentration gradient at the interface between base metal and weld,in addition,there is another significant element concentration change at Type II boundary:the content of Fe and Cr decreases slightly,while the content of Mo,Nb and S increases sharply,which shows the low corrosion resistance of Type II boundary.(2)The mechanical properties of the welded joints were tested by microhardness tester and universal testing machine.The results show that the hardness distribution of welded joints is not uniform.The hardness of base metal is about 170-185HV,and that of weld metal is about 220-250HV.More importantly,there are the jump zone of hardness between the weld and base metal.The highest hardness of welded joint occurs in the carbon-rich zone near the fusion line,and the lowest hardness occurs in the high temperature tempering zone of HAZ.The average yield strength and tensile strength of L360MS pipeline steel welded joint are 463 MPa and 557 MPa,respectively.The fracture positions of the tensile specimens are all located at the base metal,which indicates the welded joint is in a high strength matching state.(3)The fatigue crack propagation tests were carried out with CT specimens by fatigue test system for each area of the welded joint,including base metal,HAZ and weld,respectively.The a-N curves of specimens in each region and the rate equations of fatigue crack growth in each region are fitted.The material constants C and n of each region are calculated.Moreover,according to the fatigue crack growth rate equation obtained from each group of specimens,the residual fatigue life of each zone of welded joint is roughly estimated,which is 15.4 years for base metal,10 years for heat affected zone and 214.8 years for weld metal.To sum up,HAZ is the weakest welding joint of L360MS pipeline steel.(4)The fatigue crack propagation paths of base metal,weld and HAZ were observed by OM and SEM.The results show that the fatigue cracks in the base metal mainly propagates in the way of intergranular or transgranular.The fatigue crack propagation paths in welds are relatively tortuous due to the effect of equiaxed austenite and multi-pass welding.For HAZ specimens whose crack growth rate is slightly lower than that of base metal,the observation results show that the fatigue crack propagates in the fine grain region of HAZ.For HAZ specimens with crack propagating across welds and fusion line,Electron Back-Scattered Diffraction(EBSD)was used to scan and analyze the crystallographic information such as phase distribution,grain orientation,local orientation and grain size around the crack.The results show that the fatigue crack zigzags into HAZ along the high angle Type ? boundary and Type I boundary in the weld.(5)The fatigue fractures of the welded joint in different regions after test were observed and analyzed by SEM.The results show that the fracture morphology of the base metal is basically the same as that of the HAZ specimens,which contains a large number of fatigue steps and secondary cracks,and there is no fatigue striations.For the weld specimens,the initial crack fracture has the quasi-cleavage characteristics of river and tear ridge.There are a large number of fatigue striations in the stable propagation period.When the crack turns into the rapid propagation,it follows the slip mechanism and dimple formation mechanism,and finally shows a single dimple-type ductile fracture feature in the instability propagation stage.