Microstructure And Mechanical Properties Of Welded Joint Of Low-carbon Bainitic Steel

Transporting oil and gas over long distances under harsh conditions requires excellent mechanical strength and toughness steels.With the development of low-carbon bainitic steel and the demand of high performance pipeline,it is essential to develop welding materials for high efficiency submerged arc welding to produce weld metal,which can greatly match the properties of the base metal.It is also vcry necessary to study strengthening and toughening mechanisms of welded joint.In this paper,the low carbon bainitic steel(carbon content is 0.044%)was the base mental,the sintered flux CaF2-MgO-Al2O3-CaO-ZrO2 matching Mn-Ni-Mo-Ti-B alloys welding wire was developed,and the two surface welding two-sided forming was adopted to weld the base mental by using KangWang SAW welding machine.Microstructure and mechanical properties of welding joints were analyzed and the mechanism of strengthening and toughening of the weld seam were studied.The results shows that the microstructure of weld zone is acicular ferrite(AF),granular bainite(GB)and a small amount of lath bainite(LB).The microstructure of heat affected zone is mainly poly ferrite(PF),granular bainite and bainite ferrite(BF).AF is the toughening mircostructure of the weld.GB and LB is Strengthening microstrue of weld seam.The content of element in weld seam has a great fluctuation,such as Mn、Si、Ti、Mo and Ni.On the one hand,it is because of the oxidation loss during the welding process.On the other hand,the concentration of solid-liquid interface is different,which result in the difference of diffusion velocity during solidification.There is a certain relationship between toughness and the content of acicular ferrite in weld seam.The effect of a small amount of AF is not obvious to separate and refine the grain.When the amount of AF reaches a certain value,the AF has greater effect to prevent the crack.EDS and EBSD results shows that several ferrite laths or plates are nucleated on Al2O3 inclusions.The crystallographic orientation of acicular ferrite is not Acicular ferrite is separated by high angle boundaries.Some acicular ferrite laths or plates which are formed on one inclusion and grow in opposite directions have the same crystallographic orientation,presumably because it has some crystallographic orientation relations with the prior austenite.There is a certain relationship between the content of Al and AF in weld seam.The tensile strength changes with the content of GB by a functional relation.It can be found that GB has an effect on grain refinement strengthening and strengthening of second phase M-A island,and LB affect grain boundary strengthening and organization strengthening by analyzing the pictures of SEM and TEM in welding region.There is a certain functional relation between the content of B and Mn equivalent and the strength of welding joint.