Study On The Effects Of Different Welding Methods On The Yield Ratio Of Welded Joints Of Low Alloy Steel

The yield ratio is an ability to characterize material resistance from yield to plastic instability.The lower the yield ratio of the welded joint,the greater the plastic reserve.From a safety point of view,low yield ratio is an important feature to characterize structural seismic performance and resistance to external impact.Different welding methods,welding heat input and welding metallurgy process are different and welding efficiency is also different.Therefore,there is also a difference in the yield ratio of the weld metal.In this paper,in view of Q345 E low alloy high strength steel,four common welding methods,solid wire gas metal arc welding,flux cored wire gas metal arc welding,electrode arc welding and submerged arc welding,are used to butt plate welding.In order to obtain the mechanical properties of the deposited metal in the weld center,the welding test is carried out using two assembly gaps of 2 mm and 6 mm.Tensile test samples were cut from the cover layer of 2mm assembly gap weld and the central welding layer of 6mm assembly gap weld and then conducted tensile test.When the assembly gap is 2mm and the welding current of the cover layer is 320 A,the yield ratio of the cover layer of gas metal arc welding(GMAW)weld is 0.824,with the tensile strength 608 MPa.And the yield ratio of the cover layer of GMAW weld is 0.813,with the tensile strength 728 MPa.The microstructure of the cover weld is mainly acicular ferrite with relatively high strength,and the pre-existing ferrite and side slab ferrite with poor plastic toughness.The Fluxed-cored arc welding(FCAW)weld has a higher strength and a lower yield ratio because the alloying element content is higher,the ferrite and side slab ferrite are less,and the columnar crystal is narrower.In the development of the welding process,it is considered to control the proportion of the cover layer structure in the weld to control the yield ratio of the weld.When the assembly gap is 6mm,the mechanical properties of the deposited metal at the center of the weld between the welding methods are quite different.Among them,the welding method with the lowest weld yield ratio is GMAW.When the filling and cap welding current are 300 A and 320 A,the weld metal yield ratio is only 0.718,and the tensile strength is 547 MPa.When the filling and cap welding current of FCAW are 300 A and 320 A,the weld metal yield ratio is only 0.718,and the tensile strength is 547 MPa.When the filling and cap welding current of shielded metal arc welding(SMAW)are 200 A and 220 A,the yield ratio of the deposited metal in the center of E5015 weld is 0.750,and the tensile strength is 504 MPa.The welding method with the highest weld yield ratio is submerged arc welding(SAW).When the filling and capping currents are 450 A and 600 A,the yield ratio reaches 0.822 and the tensile strength reaches 629 Mpa.The GMAW weld has a lower yield ratio and lower strength because the content of Mn,Cr and other strengthening elements in the weld is less,and there are almost no elements such as Ti and Nb that can refine grains.At larger heat input,the microstructure of the center of the weld are bulk ferrite and pearlite in the filling layer and acicular ferrite in the cover layer.A combination of more soft phase and a certain amount of hard phase results in a lower yield ratio.The yield strength of the E5015 SMAW weld is higher than that of the GMAW because the weld also contains less strengthening elements such as Mn and Ti,and due to its lower heat input,the cover layer has a shallower depth and Less acicular ferrite.The center of the weld is mainly a low-strength large-sized ferrite and pearlite,which makes the yield strength similar to that of solid wire gas shielded welding,but the tensile strength is lower,thus resulting in a higher yield ratio.The SAW weld has a high yield ratio and strength because the weld contains more strengthening elements and refined grain elements such as Mn,Cr,Ti,Nb,and V.And because of its high heat input,the cover layer has a deep penetration,so the microstructure of the center of the weld is a large amount of high-strength acicular ferrite and a certain amount of needle-shaped sorbite,But there are less blocky ferrite with better plasticity.As a result,the tensile strength of the weld metal is high and the yield strength is also high,so the yield ratio is the highest.The FCAW weld also has more elements such as Mn,Cr,Ti,Nb,V,etc.,but its yield ratio is lower than that of SAW.Because the heat input is smaller than the SAW,resulting in a shallower depth of cover layer.The microstructure of the weld center is mainly composed of fine granular pearlite dispersed in the ferrite matrix in the filling layer and fine ferrite in the lower layer,so the tensile strength is close to the SAW but the yield strength is lower.Therefore,the yield ratio is lower.