The Development Of Self-shielded Flux Cored Wire For Low Carbon Bainite Steel And Study On Toughness Mechanism Of Welded Joint

Due to the harsh transporting conditions of oil and gas,the pipelines have the properties of large diameter,resistance to high pressure and distance transporting.The low carbon bainitic steel is widely used in pipeline for its high strength and high toughness.at present,the research of self-shielded flux cored wire of low carbon bainitic steel is still in the primary stage.Therefore it is necessary to develop the self-shielded flux cored wire of low carbon bainitic steel.In this paper,the base metal is low carbon bainitic steel.The Mn-B-Ni-Mo-Ti alloy system with BaF2-TiO2-ZrO2-(Al-Mg)slag system is determ ined as self-shielded flux cored wire of low carbon bainitic steel according to matching principle between weld zone and base zone and slag system theory of self-shielded flux cored wire.The self-shielded flux cored wire of low carbon bainitic steel was developed when the amount of packaged powder reaches 18%.The mathematical model between factors and welding performance was established by orthogonal design and DPS software,and the results show that the welding performance is the best when BaF2 is 25%,TiO2 is 10%，ZrO2 is 9%and Al-Mg is 8%.The micro structure was characterized by optical microscope(OM),scanning electron microscope(SEl\M),X-ray diffractometer(XRD)and transmission electron microscope(TEM),and the hardness,strength and toughness of welded joint were measured.The results show that the microstructure of weld zone includes acicular ferrite(AF),granular bainite(GB)and a small nuJmber of M-A islands;The microstructure of heat affected zone is GB,AF and some coarse polygonal ferrite(PF);The streng th of welded joint is more than 550 MPa a:nd the impact energy is more than 100J,this welded joint is consistant with the principle of the pipeline steel;The ductile brittle transition temperature of weld zone is between-20℃--40℃ the ductile brittle transition temperature of heat affected zone is betxween-40℃--60℃;The toughness mechanism of welded joint was studied as well.The results indicate that the ratio of AF phase and GB phase has larger influence on the toughness and strength,because AF and GB split the original austenite grains effectively and improve the strength and toughness of weld zone by refined crystalline strengthening mechanism;The large angle grain boundary between AF and GB can effectively prevent the expansion of cracks;AF phase nucleates and grows centered on Al2O3.which changes with the content of Al by a function relation y=7.23+31.53e-2(x-0.08/0.08);the relationship of Mneq and tensile strength can be described as y=753.96-690240ex/0.35;Smaller welding heat input can make microstructure of welded joint more uniform and improve the performance of weld zone and heat affected zone.