The transition from gray to white cast iron during solidification: Part II. Experimental verification
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. INTRODUCTION
THE structural quality of cast iron produced in the foundry industry is commonly assessed through evaluations of its chill and chilling tendency (CT). Among the chilltesting methods available are the chill-plate, the chill-wedge (A 367-55T ASTM), and the pin-test casting.[1] These methods are employed in testing castings of various sizes in order to simulate thin and thick cross sections. Since the number of potential applications for cast iron is strongly dependent on its inherent CT, considerable efforts have been made in correlating its chill and CT with metallurgical processing factors[2,3,4] or thermal analysis.[5,6] In Part I of this work, an expression was derived which can be used for predictions of the critical casting module below which a chill develops. This condition is given by Mcr
Vc Fc
[1]
where Vc and Fc are the volume and surface area of the casting, respectively. The previous expression can also be described by Mcr pCT
[2]
where p
25/6 aT1/2 s 1/6 pf1/2 c1/3 ef Le
[3]
In the previous expression, a is the ability of the mold to absorb heat, Ts is the equilibrium temperature of the graphite eutectic (Table I in Part I of this series of articles), cef is the effective specific heat of proeutectic austenite (Eq. [A10] in Part I), is the heat coefficient of the metal (Eq. [A17], Part I), and Le is the latent heat for graphite eutectic.
E. FRAS´ , Professor and Director, and M. GÓRNY, Assistant Professor, are with the Foundry Institute, University of Mining and Metallurgy, 30-059 Krakow, Poland. H.F. LÓPEZ, Professor and Chair, is with the Materials Engineering Department, University of Wisconsin, Milwaukee, WI 53201. Contact e-mail: [email protected] Manuscript submitted November 6, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
In addition, the chill width for wedge-shaped castings can be given by wcr
4np CT cos (b/2 )
[4]
where n is the wedge coefficient and is the wedge angle (Figure 4, Part I) Two cases are clearly distinguished, depending on the available information. (1) The nucleation coefficient (b, in °C) (Eq. [13], Part I) and the density of sites (substrates) (Ns, measured in 1/cm3) for graphite nucleation are known. Under these conditions, the CT can be described by CT c
1/6 1 b bd 3 8 exp a Tsc Ns (1 fg)mg Tsc
[5]
(2) The number of graphite eutectic cells per volume, the cell count (Ng), is directly available at T Tc; then, CT c
1/6 1 d Ng (1 fg)m3g Tsc8
[6]
where Tsc is the temperature range under which only graphite eutectic solidifies (Tsc Ts Tc) (Figure 1(a) and Table I, Part I), Tc is the temperature for cementite eutectic formation (Table I, Part I), f is the volume fraction of proeutectic austenite (Table I, Part I), and g is the growth coefficient for graphite eutectic (Table I, Part I). In cast iron, the chill and CT are closely related to the solidification conditions. It is well known that the CT depends, among other factors, on the number of nucleation sites available for the formation of graphite eutectic cells du
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