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Interplay between temperature gradients field and C → E transformation in solidifying rolls

Wołczyński W., Guzik E., Kania B., Wajda W.

Archives of Foundry Engineering

2009

Vol. 9, iss. 4

254-260

At first step of simulation a temperature field for solidifying cast steel and cast iron roll has been performed. The calculation does not take into account the convection in the liquid since convection has no influence on the proposed model for the localization of the C → E (columnar to equiaxed grains) transformation. However, it allows to study the dynamics of temperature field temporal behavior in the middle of a mould. It is postulated that for the C → E transition a full accumulation of the heat in the mould has been observed (plateau at the T(t) curve). The temporal range of plateau existence corresponds to the incubation time for the full equiaxed grains formation. At the second step of simulation temporal behavior of the temperature gradient field has been studied. Three ranges within temperature gradients field have been distinguished for the operating point situated at the middle of mould: a/ for the formation of columnar grains zone, ([...] and high temperature gradient [...]), b/ for incubation of the liquid to the C → E transformation, ([...] no temperature gradient changes [...]), c/ for equiaxed grains growth, ([...] and moderate temperature gradient [...]). T - temperature, r - roll radius. It is evident that the heat transfer across the mould decides on the temporal appearance of incubation during which the solidification is significantly arrested and competition between columnar and equiaxed growth occurs. Moreover solidification with positive temperature gradient transforms into solidification with negative temperature gradient (locally) after the incubation. A simulation has been performed for the cast steel and cast iron rolls solidifying as in industry condition. Since the incubation divides the roll into to parts (first with columnar structure, second with equiaxed structure) some experiments dealing with solidification have been made in laboratory scale. Finally, observations of the macrosegregation or microsegregation and phase or structure appearance in the cast iron ingot / roll (made in laboratory) has also been done in order to confront them with theoretical predictions. An equation for macrosegregation identification is suggested. Additionally, a new equation for redistribution studied across a given grain and its surrounding (precipitates) is also delivered. The role of the back-diffusion parameter is emphasized as a factor responsible for homogenization of the massive roll ingots.