A model for the graphite formation in ductile
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I.
INTRODUCTION
D U C T I L E cast irons are prime examples of materials where the properties achieved depend upon the characteristics of the microstructure, u,zm This microstructure is determined in part during solidification and in part during cooling in the austenite (y) and austenite + ferrite (y + o~) regimes. In Part I of this investigation, L41the inoculation mechanisms in ductile cast iron and the conditions for graphite formation during solidification have been examined. The purpose of the present study is to develop a verified quantitative understanding of the solid state transformation reactions in ductile cast iron. In particular, attempts have been made to rationalize spatial variations in the graphite nodule density with models based on well-established concepts from heat treatment and welding of iron and steel. II. TRANSFORMATIONS IN D U C T I L E CAST I R O N In this section, the solid state transformation behavior of ductile cast iron will be briefly reviewed.
A. Solidification and Cooling For a given cast iron, the cooling rate encountered during solidification and subsequent cooling is a significant factor in the microstructural development. The effects of cooling rate are quite complex, since this parameter affects both the graphite morphology and the segregation pattern of alloying elements within the austenite, as well as altering the kinetics of the subsequent austenite to ferrite transformation. However, increased solidification cooling rates will generally increase the graphite nodule count and the graphite nodularity of ductile iron due to the associated increase in the nucleation rate. It is a general observation that the ferrite content of T. SKALAND, formerly Ph.D. Graduate Student, Division of Metallurgy, The Norwegian Institute of Technology, is Senior Research Metallurgist, Elkem a/s-Research, N-4602 Kristiansand, Norway. O. GRONG, Professor, and T. GRONG, Professor and Head, SINTEF-Metallurgy, are with the Division of Metallurgy, The Norwegian Institute of Technology, 7034 Trondheim, Norway. Manuscript submitted June 19, 1992. METALLURGICAL TRANSACTIONS A
the ductile iron matrix depends on the chemical composition of the iron, the cooling rate through the eutectoid transformation range, and the volume fraction and number of graphite spheroids. Slower cooling of the iron through the eutectoid transformation temperature range results in a higher fraction of fen'ite in the microstructure. Alloying elements, such as Cu, Sn, Mn, Cr, Mo, and V, are considered to be pearlite promoters since they increase pearlite hardenability, while silicon is a ferrite promoter, lSj Small additions of elements, such as Sn and Sb, can completely suppress the formation of ferrite shells regardless of the cooling rate. Brown and Hawkes 161 found that the growth of the ferrite shell proceeds by diffusion of carbon from the a / y interface through the ferrite to the graphite spheroids, which act as carbon sinks in the structure. Tin and Sb segregate to the graphite/metal interface, where the impurity-rich
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