Microstructure and mechanical properties of welding joints of X100 line pipe by double-wire submerged arc welding

In order to study the effect of La₂O₃ on joint microstructure and mechanical properties,tungsten insert gas (TIG) welding of MGH956 alloy was performed without La₂O₃ and with 2%,4% La₂O₃ as filler materials respectively.The joint microstructure and fracture surfaces were analyzed by testing method such as optical microscope(OM),scanning electron microscope(SEM) and transmission electron microscopy(TEM).Tensile strengths were also tested.The results show that grain refinement occurs in joint when the content of La₂O₃ increases properly.When the joint contains 2% La₂O₃,joint grain is the smallest and uniformly distributed,particle-phase increases and it is uniform distributed in the joint.The strengthening mechanism of joint are fine-grain strengthening and Orowan strengthening.When the content of La₂O₃ is 4%,particles partially agglomerate in joint.The joint is strengthened mainly by piling up of dislocations.When the joint contains 2% La₂O₃,tensile strength is the largest and reaches 628 MPa.The fracture surface of joint exhibites brittle-ductile mixed fracture.