Study On Microstructure And Mechanical Properties Of Ce-alloyed Weld Metals Joined By Fulx Aided Backing Submerged Arc Welding

Flux aided backing submerged arc welding(FAB-SAW)is a kind of single side welding method which uses ceramic backing to obtain the double side forming.This welding method has many advantages,such as high production efficiency,simple assembly,good welding quality,etc.It is widely used in the welding of steel structures,such as shipbuilding,offshore platforms and large storage tanks.During FAB-SAW welding process,heat input is usually above 50kJ/cm,and the residence time of high temperature is very long,which generates a large amount of grain boundary ferrite and side plate ferrite in weld metal,thereby reducing the strength and toughness of weld metal,resulting in the performance of the weld metal can not meet the use requirements.Therefore,how to reduce the amount of grain boundary ferrite and side plate ferrite produced by FAB-SAW method under high heat input and improve the strength and toughness of weld metal becomes an urgent problem to be solved.In order to solve this problem,the microstructure and mechanical properties of FAB weld metal under Ce alloying were systematically studied in this paper.The effects of FAB welding process parameters such as welding current,welding voltage,welding speed and wire extensions on the welding quality were analyzed by Taguchi method.The mathematical model on the relationship between FAB welding process parameters and welding quality was established.Finally,the optimal process parameters combination was obtained.It can be concluded by the signal-to-noise ratio(S/N)analysis and analysis of variance(ANOVA)results that the importance order of FAB welding parameters for welding quality of weld quality was: welding current > wire extensions >welding speed > welding welding voltage.The optimum process parameters were obtained with current of 1010 A,wire extension of 35 mm,welding speed of 23cm/min,welding voltage of 32 V.The effect of different heat input on microstructure and mechanical properties of joints was analyzed by FAB welding process experiment of DH36 steel plate in different thickness with Fe powder addition.FAB weld metals are mainly composed of grain boundary ferrite(GBF),side plate ferrite(FSP)and acicular ferrite(AF)composition for steel plate with thickness of 8mm,12 mm,15mm and 20 mm.In FAB weld metals,with the increase of the welding heat input,the grain size becomes larger,the AF proportion decreases while the content of GBF and FSP increases.The microstructure of CGHAZ is coarse austenite grain with a large number of fine lath bainite ferrite in grains,and the microstructure of FGHAZ is ferrite,pearlite and a small amount of bainite ferrite.When DH36 steel with the thickness of 20 mm was joined by FAB-SAW,the tensile property,impact energy at-20 °C and bending performance are not meet the requirements of use because of the maximum heat input.The thermal cycling curves in different regions of the weld heat affected zone under different heat input have been measured.The results show that with the increase of heat input,the residence time and cooling rate are increased significantly.The effect of rare earth Ce element on the microstructure and properties of FAB-SAW weld metal was investigated.By adding different content of CeO2 powder in FAB-SAW welding flux,rare earth Ce can transit to weld metal and modify inclusions,and finally convert the complex inclusion of Al2O3,SiO2,Ti2O3 and MnS for Al2O3,SiO2 to compound of Ti2O3,MnS,Ce2O2 S and CeS.With the addition of Ce,the proportion of inclusions with diameter less than 1?m in weld metal was increased obviously,and the small inclusions increased the nucleation of AF.The low mismatch between Ce2O2 S,CeS and ?-Fe is the main mechanism to induce AF nucleation.Rare earth Ce can improve the AF proportion of weld weld,reduce the proportion of GBF and FSP,and significantly improve the tensile strength,elongation and low temperature impact energy of FAB weld metals.When the content of CeO2 in welding flux is 3.0 wt.%,the density of inclusions is maximum,the proportion of acicular ferrite is 76%,the tensile strength of FAB weld metal is 582 MPa,the elongation is 24%,and the impact energy of Charpy V-notch at-20 °C is as high as 142 J.The precipitation conditions and existing forms of Ce inclusions in FAB weld pool were studied by thermodynamics calculation.In the melton pool,if O and S elements exist at the same time,Ce always participate deoxidation reaction firstly and Ce deoxidation product is only Ce2O3.The product of desulfurization reaction between Ce and S elements depends on the activity of S element.When a[O]<0.13,a[S]/a[O]<12.59,Ce2O3 is generated,while a[O]<0.13,12.59<a[S]/a[O]<263,or a[S]<0.51,97.72<a[S]/a2[O]<134896,the product will be Ce2O2 S.When a[S]>0.51,a[S]/a[O]>263,Ce2S3 will be generated.When a[S]<0.51,a[S]/a2[O]>134896,CeS is generated.According to the results of thermodynamic calculation,together with the chemical composition of weld metals,the type of Ce inclusions is judged in theory and the calculated results is consistent with the results that of EDS test.