The Influence Of Heat Input On Microstructure And Properties Of Low-carbon Banite Weld Metal

With the develop of science and technology,engineering structure with large-scale and light weight have been widely developed.High strength steel with the excellent comprehensive mechanical properties greatly reduces the weight of structure,economic costs and carbon emissions in the process of production and transportation,and further decrease the consumption of energy,which make high strength steel used in the engineering structure.But the toughness of weld metal is lower than that of the base metal at low temperature,which limits the application of high strength steel further.And mechanical properties of welded joint is determined by the microstructure of the joint.When the welding material and base metal are determined,the microstructure and properties of joint are determined by welding procedure.The heat input is one of the most important factors in the welding procedure.It is necessary to study the effects of heat input on the microstructure and properties of weld metal.In this paper,the effects of heat input on the microstructure and properties of weld metal of low carbon bainite were investigated.The weld metal was obtained by arc welding method with metal powder core wire.The influence of Ni element on the microstructure and properties of weld metal was studied for the weld metal obtained with optimized welding variables.Finally,the reason of formation of hot crack and solution of weld metal of the Ni6 were also analyzedThe results show that:(1)The microstructure of original weld metal and reheated weld metal of Ni0,Ni2 were composed of granular bainite and quasi-polygonal ferrite.And the microstructure of original weld metal and reheated weld metal of Ni4 were composed of lath bainite(LB)and quasi-polygonal ferrite.With the increase of heat input,the content of granular bainite or lath bainite of original weld metal and reheated weld metal with various Ni contents increased firstly and then decreased,and when the heat input was 17.9 kJ/cm,the content of granular bainite or lath bainite reached the maximum.With the increase of heat input,the yield strength and tensile strength of weld metal increased firstly and then decreased.When the heat input approached 17.9 kJ/cm,the yield strength and tensile strength of weld metal reached the largest.Impact toughness of weld metal reduced gradually along with the increase of heat input.(2)When Ni content of the weld metal increased from 0 to 4%,the microstructure of original weld metal and reheated weld metal were transformed from granular bainite(GB)+ quasi polygonal ferrite to the lath bainite(LB)+ quasi polygonal ferrite under 15.8kJ/cm.Moreover,with the increase of the Ni content,the content of bainite in the weld metal also gradually increased.With the increase of Ni content,the tensile strength,yield strength and impact toughness,Vickers hardness of weld metal all showed a trend of increasing at-50℃ or room temperature.(3)The elements,such as Fe,Ni,Si,Mn,S were found in the first layer of the weld metal of Ni6.The low melting eutectic can be easily formed due to these alloy elements,especially the segregation of Ni element.These areas would fracture firstly due to tensile stress produced in the process of weld metal solidification and cooling.Ultimately,hot crack appeared in these areas,and gradually extended to other layers.Hot crack was always produced in the weld metal whether the increase or decrease of the heat input.It was inevitable to produce the hot crack at the present welding procedure and welding filled materials.