Modeling of ferrite growth in nodular cast iron
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I.
INTRODUCTION
IN nodular cast iron, the structure after completed solidification consists of graphite nodules dispersed in an austenitic matrix which is nonhomogeneous with respect to alloying elements due to microsegregation during solidification. It is of major importance to know how different alloying elements are distributed in the matrix since they have a very large influence on the solid-state transformations. Elements such as Si, Cu, and Ni are concentrated in the vicinity of the graphite (negative segregation), while the highest levels of Mn, Cr, and Mo are found in the last solidified areas (positive segregation), i.e., halfway between two nodules. EI,2~ Two of the most common alloying elements in nodular cast irons are Si and Mn. Silicon is known to drastically increase both the stable and the metastable eutectoid temperatures, while manganese has the opposite effect.t3] Bearing in mind the segregation pattern of Si and Mn, it is found that there will be a gradient in eutectoid temperatures between the nodules with the highest values at the graphite/austenite interface. From this consideration, it is natural to conclude that ferrite nucleates at the graphite/austenite interface and that growth proceeds by diffusion of carbon to the nodules which then act as carbon sinks. When the metastable eutectoid temperature is reached, there is also a possibility for pearlite to nucleate and grow in competition with the ferrite. The growth of pearlite depends only on short-range diffusion of carbon corresponding to the dimension of the lamellar spacing, and therefore, pearlite growth is considerably faster than that of ferrite. Since pearlite is a metastable structure, it is possible that the pearlite formed might decompose into the stable phases ferrite and graphite during subsequent cooling. However, it has been shown that for the cooling rates found in most castings, this mechanism is of minor importance;C4J consequently, there will only be a relatively small increase in ferrite content after the moment when pearlite has nucleated. In order to predict the ferrite/pearlite ratio in a casting, MAGNUS WESSI~N, Postgraduate Student, and INGVAR L. SVENSSON, Associate Professor, are with the Division of Component Technology, School of Engineering, J6nk6ping University, S-55111 J6nkrping, Sweden. Manuscript submitted December 29, 1994. METALLURGICALAND MATERIALSTRANSACTIONSA
it is therefore necessary to have a good understanding of the mechanisms which govem ferrite growth and to consider models which take all the relevant mechanisms into account. A limited number of articles in the literature have treated this topic, t5 ~21Most recent models for ferrite growth in nodular cast irons are based on the assumption that the growth rate is controlled by the rate of carbon diffusion through the ferrite shell. In one of the most recent publications,tgJ it is suggested that another mechanism operates simultaneously with the one mentioned previously. Assuming that local equilibrium prevails at the ferrite/austenite interface,
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