Study On The Influence Of Alloying Elements On The Deposited Metal Properties Of Nickel-based Electrode

Liquefied natural gas(LNG)carrier is an important part of liquefied natural gas industry chain.9%Ni steel(9Ni steel)is one of the most important materials for the manufacturing of large LNG carriers.Although the development of China's domestic9Ni steel has been relatively mature,its nickel-based welding materials still rely heavily on imports.One of the issues is that the strength and plasticity of deposited metal of the nickel-based electrodes for 9Ni steel cannot meet the requirements at the same time.Aiming at this problem,this thesis firstly uses the machine learning method to analyze the influence degree of some alloying elements on the mechanical properties of nickel-based electrode deposited metal.Then,based on the results of machine learning,the influence mechanism of Nb and Cr on the mechanical properties of deposited metal are mainly studied.The results of this thesis provide a theoretical basis and experimental support for the development and engineering application of nickel-based welding electrodes for LNG carriers.First,this thesis studies the correlation coefficients between alloying elements such as Nb,Cr,Fe,Mn,Mo and Ti and the plasticity and strength of the deposited metal of nickel-based electrodes.A decision tree model is also used to study the effect of the concentration of alloying elements on the plasticity and strength of the deposited metal of nickel-based electrodes.It is found that the Nb element can increase the strength and plasticity of the deposited metal in a certain content;the Cr element can significantly improve the plasticity of the deposited metal.Therefore,this thesis further studies the effects of Nb and Cr on the mechanical properties of deposited metal.Three kinds of electrode with different content of Nb,noted as A1(1.02%Nb),A2(1.14%Nb)and A3(1.60%Nb),were selected for welding experiments,and the deposited metals were analyzed.It is found that the microstructure of deposited metal was dendritic?-phase,and there were a lot of precipitates between dendrites,including carbide and eutectic structures such as Nb C,Mo₆C and Cr₂₃C₆.With the increasing of Nb content,the Nb C precipitated between dendrites increased,the dendrite spacing and dendrite width decreased,the tensile strength,hardness and plasticity of deposited metal also increased.Compared with A1,the dendrite width of A3 decreased by 1/3,the tensile strength increased by 17%,the hardness increased by 5%,the plasticity increased by 2 times,and the impact energy at low temperature increased by 10%.Both A1,A2 and A3 have good impact toughness at low temperature,and the impact fracture is of dimple shape.By analyzing four kinds of deposited metals with different Cr contents of electrode,A4(11.84%),A5(12.05%),B1(27.28%)and B2(27.61%),it is found that Cr element could facilitate the deformation-induced phase transformation of deposited metal and transform the FCC phase to BCC structure.Compared with A1 and A2,when the Cr content increased by 2 times,the BCC content produced by deformation-induced transformation increased by about 1.8 times,and the plasticity of B1 and B2 increased by about 70%.The low-temperature impact toughness of B1 and B2 decreased by about30%,mainly due to the decrease of Ni content.The low-temperature impact fracture has a quasi-cleavage morphology.