Chemical composition and structural identification of eutectic carbide in 1 pct Mn ductile iron
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INTRODUCTION
IN the production of austempered ductile iron (ADI), Mn may be added to the iron to enhance hardenability, because it is a potent and inexpensive hardenability promoter. Although higher Mn is desirable for castings of heavier sections, the amount of addition is limited to a quantity low enough to avoid the excessive interdendritic segregation of Mn I~-4j that leads to carbide formation, t51 This carbide precipitates in the intercellular/ interdendritic region and forms a brittle network which considerably reduces the ductility of ADI. t6"71 Exact knowledge of the nature of carbide is needed for the purposes of understanding and controlling the phenomena. In particular, it will be helpful in evaluating the results of various practices, if we can distinguish the structure and composition of the eutectic carbide from those of carbides precipitated during heat treatment. Crystal structures of complex carbides of Fe and Mn in steels have been well documented, tS'9,1~ Westgren and Phragmen r9J have reported that Mn atoms may replace about one-quarter of the Fe atoms in cementite without appreciably changing the size of the unit cell of cementite. In steels very high in Mn, the presence of MnaC (or Mn23C6) and M n 7 C 3 has been reported, where some Mn may be replaced by Fe, giving (Mn, Fe)4C or (Mn, Fe)TC3 .[8,9[ Kuo and Parsson lZ~ studied ternary system Fe-Mn-C at isothermal sections at 1050 ~ 910 ~ and 690 ~ They found that no other carbide than cementite M. NILI AHMADABADI, formerly Graduate Student, Tohoku University, is Research Associate, Institute of Metallurgy, Tehran University, Tehran, Iran. E. NIYAMA, Professor, and T. OHIDE, Research Associate, are with the Department of Materials Processing, Tohoku University. M. TANINO, Professor, is with the Institute of Materials Research, Tohuko University, Sendai 980, Japan. T. ABE, Section Head, is with the Government Industrial Research Institute of Tohoku, Sendai 983, Japan. Manuscript submitted January 5, 1993. METALLURGICALAND MATERIALS TRANSACTIONS A
is to be expected in steel with Mn less than 20 pct. However, when the Mn content increases beyond this value, (Mn, Feh3C6 first appears together with (Mn, Fe)3C and later replaces the latter completely. Cast iron is characterized by its high Si content, which tends to make cementite less stable. [lu Behavior of carbides in high-Si steel may be suggestive in this regard. Because of the very small solubility of Si in cementite, precipitation of cementite during tempering of martensite in high-Si steel is retarded, and also, its growth is controlled by the diffusion of Si away from the cementite/ ferrite interface, t~2-171 Carbides of types other than cementite have been known to precipitate during tempering of martensite of high-Si steel. Konoval e t al. t~81 have reported that in a steel with 0.062 pct C and 3.5 pct Si, after brine quenching from 1395 ~ and aging at 150 ~ to .450 ~ a carbide that was neither cementite nor e appeared. They proposed that the crystal structure of this needle carbide w
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