The Slag Adhesion Mechanism,and The Microstructure And Properties Of The Deposited Metal Of EniCrFe-9 Nickel-based Covered Electrode For Welding 9Ni Steel

The ENiCrFe-9 electrode has been widely used for the welding of 9 Ni steel due to its excellent properties at cryogenic temperature and good weldability with 9 Ni steel.But so far,there is no mature ENiCrFe-9nickel-based electrode at home to meet the rapid development of the LNG industry.The imported ENiCrFe-9 electrode is very expensive and the welding problems of covered electrode during the welding process can't be resolved efficiently,and the the supply cycle of the covered electrode is restrained by the foreigners.Therefore,it is urgent for our country to develop excellent properties ENiCrFe-9 nickel-based covered electrode and solve the welding problem scientifically.According to the welding metallurgy knowledge and the welding metallurgy characteristics of mineral powders,and the practical production manufacturing experience.The strong basic ENiCrFe-9 nickel based covered electrode with the basicity of 4.25 and alkalescent ENiCrFe-9 nickel based covered electrode with the basicity of 1.59 have been successfully developed by using a three step development approach,i.e.,“initial coating designed by experiences–optimized coating selected by the orthogonal test–final coating determined by the single factor test”,the mechanical properties of the deposited metal meet the requirement of the AWS A5.11 ENiCrFe-9.The welding performance and the mechanical properties of the deposited metal could be comparable to the well-known foreign products.The tensile strength and yield strength of the alkalescent covered electrode deposited metal are 666 MPa and454 MPa respectively,and the elongation is 30%.The tensile strength and yield strength of the strong basic covered electrode deposited metal are 695 MPa and454 MPa respectively,and the elongation is 35%.The tensile strength and yield strength of the Kobelco Ni-C70S deposited metal are 706 MPa and 468 MPa respectively,and the elongation is 34%.The average cryogenic impact value of the alkalescent covered electrode deposited metal is 67 J,the average cryogenic impact value of the strong basic covered electrode deposited metal is 83 J,the average cryogenic impact value of the Kobelco Ni-C70S deposited metal is 58 J.The welding performance of the alkalescent Ti O₂-Si O₂-Sr O type covered electrode was better than that of strong basic CaF₂-Ca O-Si O₂ type covered electrode.The mass transfer coefficient of Mn,Mn,Mo,Cr,Ni and W in the strong basic covered electrode were higher than those in the alkalescent covered electrode,but the mass transfer coefficient of Nb and Si were lower than those in the alkalescent electrode.The content of O,H and S in the deposited metal of alkalescent covered electrode were higher than those in the deposited metal of strong basic covered electrode.Lots of Mn-rich oxides and Al-rich oxides contained in the deposited metal of alkalescent covered electrode,and which lead to the mechanical properties of the deposited metal of alkalescent covered electrode was lower than that of strong basic covered electrode.The slag adhesion mechanism was investigated by changing the ration of Ca CO₃/(CaF₂+Ca CO₃)in strong basic covered electrode and Ca CO₃/(CaF₂+Ca CO₃)in alkalescent covered electrode.The results indicated,when the Ca CO₃/(CaF₂+Ca CO₃)=0.19 in the strong basic covered electrode,the slag covered on the weld metal uniformly with green color,the slag was consisted of blocks with dimension of 100-150?m,and which interconnected with each other.The EDS and XRD analysis results suggested,the Ca₄Si₂O₇F₂was the main composition but less Ca Ti O₃ and CaF₂,the slag detachability rate was 50.7%.The slag detachability rate was increased following the increment of the Ca CO₃/(CaF₂+Ca CO₃).Meanwhile,the color of the slag changed from green to black.When the Ca CO₃/(CaF₂+Ca CO₃)=0.50,the slag detachability rate was97.1%.The slag with black color and consisted of long strip block with the characteristic of radioactive,and the main phases of the slag was CaF₂,and contains small amount of Ca₄Si₂O₇F₂ and Ca Ti O₃.However,when the Ca CO₃/(CaF₂+Ca CO₃)=0.66,the slag appeared with the feature of bamboo like block,the slag were composed of CaF₂ and Ca₄Si₂O₇F₂ but less Ca₂Si O₄.Meanwhile,the Ca-Si-O-F fluorides were distributed on the CaF₂,and the slag detachability rate fall to 92.6%.When the Ca CO₃/(Ti O₂+Ca CO₃)=1 in the alkalescent covered electrode,the slag covered on the weld metal unevenly with black color.The EDS and XRD analysis results suggested,the CaF₂ was the main composition but less Ca Si O₄ and Ca₂Si₂O₇F₂,and the slag detachability rate was 75%.Following the reduction of the Ca CO₃/(Ti O₂+Ca CO₃),the color of the slag changed from black to green.When the Ca CO₃/(Ti O₂+Ca CO₃)=0.36,the slag detachability rate reached up to 98%,the main phase of the slag was Ca Ti O₃.However,when the Ca CO₃/(Ti O₂+Ca CO₃)=0.21,the slag detachability rate fall to 91%,the slag was green and the main phase were Ca Ti O₃,CaF₂ and some unknown phases.The microstructure and the mechanical properties of the deposited metal were investigated by changing the Nb/Cr and Nb/Mo in the deposited metal,respectively.The results indicated,the grain morphology changed from colummar crystal to cellular and equiaxed crystal when the Nb/Cr ration changed from 0.05 to 0.11 and 0.19,and the grain boundary precipitates changed from continuous to semi-continuous and un-continuous.When the Nb/Cr=0.05 and 0.11,the intragranular precipitates were Nb-rich carbides and oxides but Laves and Nb-rich carbide when the Nb/Cr=0.19.The volume fraction and the average size of the precipitates of the precipitates increased from 0.6%and 0.38 mm when Nb/Cr=0.05 to 2.3%and 0.48 mm when Nb/Cr=0.19,which lead to the HV of the deposited metal increased but the average cryogenic impact toughness reduced from 91.3J to 83.7 and 75.0J.Although the Nb/Mo increased from 0.12 to 0.33 and 1.15,the grain morphology was colummar crystal.For Nb/Mo=0.12 and 0.33,the intragranular precipitates were Nb-rich carbides but Laves and Nb-rich carbides when Nb/Mo=1.15,while the grain boundary precipitates changed from continuous to semi-continuous and un-continuous.The volume fraction and the average size of the precipitates increased from 1.29?m and 1.59%when Nb/Mo=0.12 to 1.42?m and 1.9%when Nb/Mo=1.15,which lead to the HV of the deposied metal increased slightly and the reduction of the average cryogenic impact toughness from 91.3J to 80J and 69.0J...