Effect Of La On Microstructures And Mechanical Properties Of 800H Nickel-based Corrosion Resistant Alloy

With the rapid development of scientific technology and industrial production,800 H nickel-based corrosion resistant alloy is widely used in aerospace,petrochemical industry,Nuclear industry,etc.Traditionally,the production process of 800 H alloy is prepared by ingot casting and electroslag remelting,which features lower yield,lower efficiency and higher cost.More recently,the vertical slab continuous casting technique has been adopted in order to overcome these problems above,but the slabs produced by continuous casting has serious quality issues with coarse grains,which could deteriorate the strength and toughness of the alloy.Therefore,this paper takes 800 H nickel-based alloy produced by Baosteel as its subject investigated.In order to solve the problem of the solidification of800 H alloy coarse grains,four steel samples with different La content were melted in medium frequency vacuum induction furnace.The solidification refinement,Characteristics of inclusions,Mechanical properties have been investigated in this paper.It is of great practical significance to provide basicexperimental data for the development of low cost production technology of800 H alloy,to solve the problems existing in the production process,to reduce the surface defects,to reduce the rejection rate and to adjust the structure of China’s steel industry.The methods and contents of this research are as follows:(1)Four steel samples with different La content were melted in medium frequency vacuum induction furnace,chemical composition of alloy samples were analyzed and the mechanism of purifying molten alloy by La addition was analyzed and discussed combined with modern metallurgical theory.(2)The experimental data were characterized by modern detection techniques,such as metallographic microscope,scanning electron microscopy and energy dispersive spectroscopy.,describing the evolution mechanism of La contained submicron inclusions in the refining process of molten steel through thermodynamic calculation and dynamic analysis.(3)To explain the mechanism of microstructure refinement by La contained submicron inclusions,the lattice misfit between different inclusions and 800 H alloy were calculated using two-dimensional lattice misfit formula.(4)The mechanism of La improving mechanical properties of 800 H alloy was analysed and discussed using experimental data collected by mechanical property experimentThis paper draws the following conclusions through experiment and research:(1)Additions of 0-0.047% La to alloys resulted in 0.003-0.001% retainedsulphur while oxygen was reduced from 0.013% to 0.007%,which demonstrate La has strong desulphurization and deoxidation ability.(2)La can significantly refine the solidification structure of 800 H alloy,When the content of [La] in 800 H alloy increased from 0% to 0.047%,the ratio of equiaxed grains increased from 0% to 75%,the size of equiaxed grains decreased from 6.0um to 3.2um,the morphologies of dendrites became thinner and shorter,the primary dendrite decreased from 77.5μm to 43.8μm,the secondary dendrite arm spacing unchangs.(3)La can significantly refine the austenite grains of 800 H alloy,When the content of [La] in 800 H alloy increased from 0% to 0.047%,the austenite grain size is reduced from 121.32μm to 42.57μm.(4)After adding La into alloy,angular Al2O3 particles with large size and elongated MnS particles were modified into small-size and spherical La contained inclusions,Great amount of high-melting point compounds(La2O3and La2O2S)formed in the alloy.Based on the theory of lattice misfit,the lattice misfit between the face(0001)of La2O3 and the face(111)of the matrix of800 H is only 6.7%,the lattice misfit between the face(0001)of La2O2 S and the face(100)of the matrix of 800 H is only 5.42%,these high-melting point compounds all could serve as the core of heterogeneous nucleation during the solidification process,so as to refine the solidification structure.(5)With the increase of the content of La in 800 H alloy,the mechanical properties of 800 H alloy showed significantly increased,when La contentincreases from 0 to 0.047%,the alloy,s yield strength,tensile strength,elongation and impact energy were increased by 46.9%,22.3%,21.1% and 23%,which was mainly attributed to the microstructure refinement and inclusions modification.