Development Of Self Protective Flux Cored Wire For X100 Pipeline Steel And Research On Strength And Toughness Of Joint

X100 pipeline steel is used widely in the construction of oil and gas pipelines because of its high strength,toughness and good weldability.China has had the ability to produce X100 pipeline steel,but the research of self-protection flux cored wire which can match with it is still at an early stage.Therefore,to develop a high quality self-shielded flux cored wire for X100 pipeline steel is imminent and has great application prospects.The self-shielded flux cored wire for X100 pipeline steel with low hydrogen and low alkaline slag system centered on BaF2 and Mn-Ni-Mo-Ti alloy system was designed in this study,which is based on the theory of self protective flux.cored wire slag system and the toughness mechanism of microalloy.The formulation of slag system was designed by orthogonal experiment,the welding process performance of welding wire were studied,then the mathematical models of slag system components and the welding process performance of welding wire were established by DPS.The welding process performance of welding wire is better when the mass amount of BaF2,Al-Mg,TiO2,ZrO2 is 30wt.%,8wt.%,10wt.%,8wt.%respectively;with the increase of TiO2 and BaF2,the proportion of BaTiO3 in slag increases,the slag detachability of weld becomes better;with the increase of Al-Mg,the kinds of spinel compounds in the slag increase and the binding force inside the slag decreases,causing slag adhesion and furtherly leading to the slag detachability became worse.Based on the slag system,the formulation of the wire alloy system was designed,the microstructure and mechanical properties of welded joint were analyzed,the best alloy formulation which is made up with 16wt.%Mn,6wt.%Ni,7.5wt.%Al-Mg and 2wt.%Mo is determined from the analysis results;the micro structure of the weld is Acicular Ferrite(AF),Granular Bainite(GB)and a little amount of Pro-eutectoid Ferrite(PF);the tensile strength of the welded joint can reaches 803MPa and the impact energy can reaches 156J,which meets the requirements of the petroleum pipeline construction project standard.Whereafter,the corrosion resistance of welded joints in different acidic environment was studied,it is found that the corrosion rate of the weld zone is lower than the corrosion rate of the parent material,besides,there occurs serious pitting in the weld zone in 5%HCl and serious uniform corrosion in 5%H2SO4,the results provide data basis for the subsequent research on corrosion protection of welded joints.The relationship between microstructure and strength and toughness of welded joints was analyzed,it's found that the content of acicular ferrite and granular bainite has great influence on the strength and toughness of welded joints,the disorderly distribution of AF and GB in the joint segmentation of weld structure which enhances the strength and toughness of the joint by fine-grain strengthening,the M-A island on GB distributes in the weld and plays the role of second-phase strengthening.The nucleation mechanism of acicular ferrite was further studied,the AF in the weld zone take Al2O3 inclusion as the core and grow in multidimensional nucleation,and they are not completely random distribution and have certain preferential orientation.The research results provides a theoretical basis for improving the mechanical properties of welded joint by controlling weld organization methods.