Study of microsegregation buildup during solidification of spheroidal graphite cast iron

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I. INTRODUCTION

CHEMICAL heterogeneities, which build up during solidification of cast irons at the scale of the solidification microstructure (microsegregations), have been investigated for a long time because of their influence on the properties of cast products.[1–11] Microsegregations affect the microstructure resulting from the solidification step, namely, the gray (stable) to white (metastable) eutectic transition, but also the solid-state transformations during cooling or heat treatments. Charbonnier and Margerie[1,2] have shown that elements that favor the stable system such as silicon, nickel, or copper segregate negatively during stable solidification. The reverse is true for elements that favor the metastable system such as chromium and manganese, which segregate positively during stable solidification. Other authors[3–10] agree on these general features. It is worth emphasizing that Charbonnier and Margerie noted similar results for both lamellar and spheroidal graphite cast irons, and it is accepted that the segregation behavior of alloying elements is the same in gray and nodular cast irons. In irons that solidified in the metastable system, it has been found that segregation of chromium and silicon is inverted with respect to the case of stable solidification, while manganese segregates positively for both gray and white solidification.[1] While the sign of the solidification segregation of the main alloying elements of cast irons is well established, there are a number of controversial features that need further investigation. Concerning stable solidification, studies on the role of the cooling rate and of cell or nodule count on the amplitude of microsegregation led to contradictory results. Concerning white solidification, positive segregation of silicon during the proeutectic deposit of austenite has CHRISTOPHE SELIG, formerly Graduate Student, School of Mines, Nancy, France, 54042, is with the Branche Alliages Le´gers, VALFON, 01602 Tre´vous Cedex, France. JACQUES LACAZE, Senior Scientist, is with the Laboratoire Interfaces et Mate´riaux, Centre Inter-Universitaire de Recherche et Inge´niere des Mate´riaux (CIRIMAT), 31077 Toulouse Cedex 4, France. Manuscript submitted September 2, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS B

been reported,[2,11] which could not be clearly understood. Also, Charbonnier and Margerie[2] observed that segregation is slightly less marked in as-cast malleable irons, but Feest et al.[12] found high positive segregation of silicon in white cast iron. In this line of studies, this work presents an attempt to describe the complex relationship between the development of the solidification microstructures and buildup of microsegregation in spheroidal graphite (SG) cast irons by coupling an experimental investigation and a modeling approach. In most of the works devoted to the characterization of microsegregations, solute distribution was studied by means of microprobe measurements made either in selected areas or along line scans. Using the possibilities of automatic

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