Study On The Solidification Process And Graphitization Mechanism In Low-carbon Ductile Iron

Low-carbon ductile iron is a kind of new engineering material, the thin and whole graphites are separated out directly without heat treatment, consequently it exploite and make use of the F-C alloy whose carbon content is from 1.2% to 2.0%. In recent years,studies about the application of low-carbon ductile iron gained a rather large evolution; but the fundamental researches especially the studies based on the electronic structure layer are lesser,the undercooling liquid structure in low-carbon ductile iron and its affection to solidification process, solidification process of low-carbon ductile iron and graphite nucleation mechanism.On the basic of EET and its application for the liquid metal reseach, according to the reseach results of the inherit relations between liquid metal fused mass structures and solid structures, the thermodynamic and kinetic conditions of the liquid metal nucleation and growth. In this article, the liquid metal fused mass structure, solidification process and graphitization mechanism are studied.Main conclusions are as follows:1.The electronic structures of graphite, austenite and cementite are calculated and analyzed, according to the heredities between the liquid metal fused mass structures and solid structures,the electronic structures of the segregation atom mass of fused mass in low-carbon ductile iron are analyzed, the solidification process and graphitization micro-mechanism in low-carbon ductile iron are studied by using the structure formation factor S.2.Low-carbon ductile iron fused mass are micro-inhomogeneous nearby liquid line,mainly form the similar face-centred structures such as Fe-C and Fe-C-M, plane layer structure such as C-C shortrange order atom mass, type, number and dimension are related to the temperature.3.Combining with the nucleation theory of the liquid metal,the thermodynamic condition of heterogeneous graphite nucleation in low-carbon ductile iron is put forward:G_L-F_G>G_L-F_H, E_G⁰⁰⁰¹> E_H¹¹¹, that is, the bond-forming energy of unit of structure of nucleate heterogeneous particle F_H is lesser and far from the graphite's F_G, the bond-forming energy of crystal plane E_H¹¹¹ is lesser and far from the graphite's E_G⁰⁰⁰¹. The thermodynamic condition decides that CaS and MnS crystal billet should appear first in low-carbon ductile iron undercooling liquid.4.Combining with the model of coincidence lattice of lange-angle grain boundary that is common used in modern times, the structural conditions of the heterogeneous graphite nucleation in low-carbon ductile iron are put forward: The structural conditions are that there is a certain lattice contact ratio between the crystal structure of graphite and the crystal structure of heterogeneous particle attached by graphite.5.There are conditions that construct coincidence lattice among the graphite, CaS and MnS. The surface(111) of CaS and MnS provides the structural condition for crystal structure attached by graphite, that is, the coincidence lattice exists on the interface of (0001)_G//(111)_CaS and (0001)_G//(111)_MnS, misfit degree of the coincidence lattice is smaller.