The transition from gray to white cast iron during solidification: Part III. Thermal analysis
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. INTRODUCTION
THE transition from gray to white cast iron during the solidification process is often determined by the resultant structure obtained in standard wedge-shaped castings. Figure 1(a) shows schematically the effect of cooling rates and cooling range on the structure of wedge-shaped castings. Accordingly, two cooling ranges are clearly distinguished. In the first range, gray cast iron is the stable structure, while in the second range, the solidification of white and/or mottled cast iron is found to take place. In particular, notice that as the second range of cooling rates widens, the potential for chilling in cast iron drastically increases.[1] Alternatively, a dual thermal analysis in combination with chilled wedgeshaped castings can be employed for predictions of the chill of cast iron.[2] In this case, simultaneous thermal analyses are carried out on cast iron in tellurium eutectometer cups and in plain (nontellurium) eutectometer cups. In tellurium (the element used to increase the chilling tendency) eutectometer cups, cast iron solidifies as a white structure and the cementite eutectic formation temperature (Tc) can be recorded. In addition, in plain eutectometer cups, cast iron solidifies as a gray structure, enabling the determination of the graphite eutectic formation temperature (Tm) and of the liquidus temperature (Tl). Moreover, through statistical analysis, a relationship can be found between the chill width (w) and the Tl, Tm, and Tc temperatures, as well as the carbon equivalent (CE, equal to pct C 0.25 pct Si 0.5 pct P, where C, Si, and P are the carbon, silicon, and phosphorous contents in cast iron, respectively). From the published literature,[2,3,4] various statistical relationships have been proposed to relate the chill width in ´ , Professor and Director, and M. GÓRNY, Assistant ProfesE. FRAS sor, are with the Foundry Institute, University of Mining and Metallurgy, 30-059 Cracow, Poland. H.F. LÓPEZ, Professor and Chair, is with the Materials Department, University of Wisconsin, Milwaukee, WI 53201. Contact e-mail: [email protected] Manuscript submitted November 6, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
W2-ASTM wedge standards with the cast iron chemistry or characteristic temperatures. Among these correlations are the following: W2 ao a1C a2 Si,
R2 0.85[3]
[1]
W2 ao a1Tl a2 Tc [4]
[2]
W2 ao a1CE a2 Tc [4]
[3]
W2 ao a1 Tc a2 T 2c a3 T 3c , R2 0.98[2] [4] W2 ao a3
a1 Tm , a2 Tc
R2 0.98[2]
[5]
where Tc Tm Tc
[6]
In the previous expressions, W2 is the width of the chill, i (with i 0, 1, 2, and 3) is an empirical constant and R is the regression coefficient. According to the previous expressions, w can been described by various statistical relationships, even though there should be only one chilling mechanism. Hence, the proposed relationships might be statistically correct, but they are only valid for the established set of experimental data. Therefore, as the database changes, it forces, in principle, the dev
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