Phase Composition And Thermodynamic Of Spheroidal Graphite Nucleus In Ductile Iron

Ductile iron is one of the most important materials which are essential to our national industry and even the whole world industry as well as one of the most widespread casting materials. It is becoming more and more widespread because of its high tensile strength、high elongation、good resistance to fatigue and casting properties and lower cost of production, so it gradually replaces cast steel.However, defects such as Spheroidal Graphite (abbreviated SG) decay and graphite nucleation ratio decreas have seriously affect on performance of ductile iron, it not only increase production costs, but also don’t conducive for china to power casting country.Therefore, it is necessary to research chemical composition of SG nuclei and stability of nucleus of SG, which can provide theoretical basis for SG decay.In this paper, residual magnesium and the spheroidal ratio of graphite in the ductile cast iron were studied by casting a series of similar composition ductile iron sticks with different holding time of spheroidizing; in purpose of researching nucleus of SGs, Wedge-shaped castings with similar nodulizer and different inoculant were cast. Thermodynamic of nucleus of SGs were calculated using Thermo-calc software. At the same time disregistry8between graphite and nucleus were also calculated. With experimental results and theoretical calculations, the nucleation mechanisms of nucleus were inferred.The results showed that with increasing holding time of molten iron, residual Mg content decrease linearly, and the relationship between residual Mg content and the holding time was fiffting the quantitative evaluation equation. With reduction of the residual Mg content, the morphology of the SGs was deteriorating. When the residual Mg content was higher than0.02%, the morphology of graphites are spheroidal. It was at the7th minute that the SG distortion and the vermicular graphite generating, at the same time the residual Mg content was0.02%. A large number of distortion graphites start generating as residual Mg content decline. When the magnesium content was0.013%, there was still SG. But SG was seriously distort and did not exist in ductile iron when residual Mg content is0.007%.The residual Mg content has an important influence on spheroidal ratio of graphite. It is clearly that the spheroidal ratio change law is not continuous as increasing the residual Mg content. Low Mg and high Mg content areas are divided by the residual Mg content is0.02%. In the high Mg area, the residual Mg content has unobvious effect on spheroidal ratio which retains high and stable as increasing the residual Mg content and the morphology of graphites are spheroidal or ellipsoidal. In the low Mg area, spheroidal ratio of graphite is improved as increasing the residual Mg content, at the same time there are vermicular garphites and SGs coexisting when the residual Mg content is less than0.013%.Although nucleus of SGs are slightly different, they are mainly reaction compounds of nodulizer and inoculant such as oxides:SiO2, CaO, MgO, FeO, A2O3, CeO2, MoO3, Cr2O3, BaO, CuO, La2O3; sulfide:MgS, CaS, FeS, MnS, CeS2, BaS, La2S3; carbide SiC, TiC, V3C2; nitrides:AlN, Si3N4; phosphide:Mg3(PO4)2; silicide:FeSi, Fe2Si, Fe5Si3; Intermetallic compounds:Ca5Sb3, Ce3Bi4; γ-Fe.Nucleus of SGs is either single sulfides or oxides or other compounds, but there are two or more components in most case. The morphology of nucleus of SGs are flake, cylindrical, tongue-shaped, diamond, conical, polygonal and irregular polygons.The size of nucleus of SGs are in micron level and are about0.9～7.05μm.Nucleation mechanisms of nucleus of SGs are direct nucleation and induced nucleation. The Disregistry8between nucleus of SG and graphite is low, which indicates that nucleus compounds are good nucleating site for SGs and can be nucleation substratios of SG directly.They are mainly MgS, CAS, MgO, CeO2and so on. But when disregistry8between nucleus of SG and graphite is higher than12%, it can not be nucleation substratios of SG in theory. Although disregistry8between SiO2and graphite is15.41%, it can be the nucleus of SG combination with SiC, AlN and Al2O3, which means indirect nucleation.There are mainly face-centered cubic compounds in nucleus of SGs such as MgS, CaS, MgO, CeO2.On one hand face-centered cubic compounds are relatively stable in the molten iron, and components of these compounds are mainly from nodulizer and inoculant, as a result, the content is relatively high; On the other hand, the mismatch between (111) of face-centered cubic and (0001) graphite is good, which indicates that they are good nucleating site for SGs.