| The structure and the solidification cures of the cast iron alloy and thehigh-purity Fe-C-Si alloy with0.5%Ce were determined by the thermal analysistest during the process of solidification in different melting and solidificationcondition. The influence of the undercooling, the recalescence temperaturedifference of primary graphite nucleation, and the liquid-solid interface on thegrowth morphology of graphite in the process of solidification of Fe-C-Si alloyswere investigated.It has been presented that either ductile iron or gray cast iron alloy, if thesolidification undercooling is very small, their growth morphology of graphite areflake, and the solidification characteristics of the graphite nodulizing by adding Ceembodied at the significant reduction in the nucleation temperature. However,giving a small undercooling to the alloy even with the addition of Ce, the graphitemorphology changed from sphericity to flake. This transformation is totallydependent on the degree of undercooling in the crystallization. So the nodulizer isnot sufficient condition of graphite spheroidization，but undercooling is a primaryfactor that promotes graphite to turn into sphericity during the solidification. Thenthe experiment result also shows that the recalescence temperature difference offlaky graphite is higher than that of spherical graphite. It reflects the growth speedof graphite. The speed of flaky graphite is faster than that of spherical graphite. Theliquid-solid interface energy on the (0001) surface is higher than that of (1010)surface after interfacial reaction; then the former growth morphology of graphite isspherical, the latter is flaky.Second，this paper also used the DSC experiment to analyse the crystallizinglatent heat differences between the unit mass of spherical graphite and flakygraphite in an infinite slowly melting and solidification condition, namely withalmost zero undercooling, when spherical graphite and flaky graphite melted andcrystallized near the melting point close to the theoretical melting point.The results show that: The crystallization latent heat ofFe.3.5%C-3%Si-0.15%Ce spherical eutectic alloy is107.4J/g, that ofFe.3.5%C-3%Si eutectic alloy is-115.5J/g，so the crystallizing latent heat in the unit mass eutectic flaky graphite is higher than that in the unit mass eutecticspherical graphite; In the hypereutectic nodular iron, the heat release in the unitmass primary flaky graphite during crystallizing is four times than the heatabsorbing in the primary spherical graphite during melting.It indicates the flakygraphite grows faster than the spherical graphite, which releases the latent heatintensively in short term. Then the heat release in the unit mass primary flakygraphite during crystallizing is higher than the heat absorbing in the primaryspherical graphite during melting.Sessile drop remelting method was use to study the effects of0.15%purecerium,0.5%pure magnesium,2%rare earth magnesium and sulfur in the alloys onthe interface energy and graphite morphology, comparing liquid-solid interfacialenergy of the same sample melting on different graphite substrates.The results show that the spherical grows along the polar of base surface, andflaky graphite grow along along the polar of edge surface. Graphite always growsalong the polar of the crystal face which has a smaller liquid-solid interfacialenergy, and the adsorption situation of different elements on the graphite’s crystalsurfaces will change the interfacial energy of two crystal faces, thereby change thedirection of growth of graphite and then form different growth patterns.As a result, the paper indicates that undercooling is the main factor that causesspherical graphite growth in the crystallization process. When the solidificationtemperature is close to the theoretical melting point, graphite always grow to beflaky, with the solidification undercooling increasing, the graphite morphology turnto be spherical. And the flaky morphology is the normal form of graphite growth. |
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