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I.

INTRODUCTION

IN three of the four major cast irons, ductile, gray, and compact, the morphology of the graphite is controlled by the solidification process. In all of these irons, the initiation of solidification is by the formation of austenite dendrites, even in many hypereutectic irons. I~JThe graphite then forms in the interdendritic liquid by a eutectic reaction. The eutectic reaction occurs in the divorced mode in ductile iron, with graphite nodules forming first in the interdendritic liquid and later surrounded by an austenite shell. In gray and compact irons, the eutectic reaction occurs in the coupled growth mode, with graphite and austenite growing together into the interdendritic liquid along roughly spherical growth fronts which form eutectic cells that engulf the austenite dendrites. The difference in the mode of eutectic formation has a large effect on the physical distribution of the graphite and, hence, upon physical properties. With the divorced mode of ductile irons, the individual nodules are isolated, whereas in the coupled mode of flake irons, the graphite is interconnected throughout the relatively large volumes of the eutectic cells. In the production of these irons, it is well known that the graphite morphology is controlled mainly by very low levels of impurity elements, particularly, S, O, H, Ca, Mg, and Ce. This is dramatically illustrated by the development of ductile cast iron, which is sometimes considered to be one of the major metallurgical discoveries of this century.t2I The key factor was the discovery that minor additions of Mg or Ce would convert the flake graphite of gray irons to spherical graphite. In studies of this effect, it is sometimes stated

J.S. PARK, formerly Graduate Student, Department of Materials Science and Engineering, Iowa State University, is with the Department of Metallurgical Engineering, Hong-lk University, Chochiwan, Korea 339800. J.D. VERHOEVEN, Professor, is with the Department of Materials Science and Engineering and Ames Laboratory, Iowa State University, Ames, IA 50011. Manuscript submitted July 5, 1995. 2741Y--VOLUME27A, SEPTEMBER 1996

that spherical graphite is the normal or characteristic growth morphology of graphite in cast iron,t3.4.5] because experiments on high-purity Fe-C-Si alloysIS~l often produce spheroidal graphite structures. This idea can be somewhat misleading because it assumes that the eutectic reaction in these high-purity cast irons will normally grow in the divorced mode. There have been numerous experiments on high-purity cast irons in which the eutectic reaction was observed to form by the coupled growth mode, and in these cases, the graphite morphology has a form that is called coral graphite.t9 ~21In addition, as this study will show, the morphology of graphite in irons nearly free from impurities depends on the solidification rate as well as the mode of the eutectic reaction. The flake morphology always occurs at the lowest rates. This work presents a directional solidification study of high-purity cast irons to which S