Chilling Tendency and Chill of Cast Iron

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182 The chilling tendency CT can be calculated from Eq. (7) as a function of the chemical composition of the cast iron, Ns and b, listed in Table 3, the mean temperature of the metal just after pouring, Ti≈1260℃, and any relevant thermophysical data (shown in Table 1). Table 4 shows results for plate-shape castings reported in the literature [4] with various chemical compositions and wall thicknesses, as well as the number of nodules and the cementite fraction. In these cases, the nucleation coefficients b and Ns are not known, so CT is calulated using Eq. (8). The results in Table 4 show that in melts I and II, chill starts to occur in walls with thicknesses between 3 and 6 mm, while in melt III chill starts to occur for wall thicknesses between 1.5 and 2 mm. scr must be calculated using Eq. (1) to compare these results with the theoretical predictions. These calculations assumed that a=0.11 J/(cm 2 ·s 1/2 · ℃ ) and Ti=1250℃ with other information taken from Table 1. The results in Table 4 show that in melt I the Tsinghua Science and Technology, April 2008, 13(2): 177-183 change of the wall thickness, s, from 6 mm to 3 mm is closely linked to the change in the number of nodules from 588 to 1039 mm −2 . As a result, an average nodule count of NF,cr=813 mm −2 was used in this work. Similar determinations were made for melts II and III with NF,cr values of 945 and 1959 mm −2 , respectively. scr for melts I, II, and III were then 3.4 mm, 3.1 mm, and 1.8 mm as shown in Table 4. In addition, comparison of s and scr in Table 4 indicates that the predictions from the theoretical analysis are in good agreement with the experimental determinations of the wall thickness at which chill occurs. Table 4 shows the CT for the three melts calculated using Eq. (8) and data in Table 1. The results in Table 4 show that as the chilling tendency increases from 0.34 to 0.68 s 1/2 ·℃ –1/3 , the critical wall thickness, scr, increases from 1.8 to 3.4 mm. Table 4 Chemical composition, wall thicknesses, nodule count, cementite fraction, and chilling tendency for three ductile cast iron melts Melt No. C/% Si/% P/% I 3.40 2.70 0.046 II 3.45 2.91 0.044 III 3.31 4.42 0.051 4 Conclusions Wall thickness (mm) Experimental s Calculated scr (1) A simple theoretical analysis is presented which predicts the chilling tendency and chill in flake graphite and ductile cast iron based on The chemical composition of the casting which for flake graphite cast iron is characterized by ∆Tsc, fγ, T, µ, Ncr or Ns, and b, and for ductile cast iron is characterized by ∆Tsc, D, Tl, Ncr or Ns and b; The inoculation and spheroidization method, the superheating temperature, and bath holding Nodule count (NF/mm −2 ) Fraction of cementite (%) CT/( s 1/2 ·℃ –1/3 ) 6.0 588 0.0 0.68 3.4 3.0 1039 9.0 — 2.0 1380 24.0 — 1.5 1311 34.0 — 6.0 854 0.0 0.60 3.1 3.0 1037 7.3 — 2.0 1100 24.0 — 6.0 1127 0.0 — 3.0 1726 0.0 — 2.0 1890 0.0 0.34 1.8 1.5 2027 9.3 — times as characterized by Ncr or Ns and b; Selected thermophysical data of the metal and mold material as well as the metal temperature just after pouring into the mold. (2) Theoretical calculations of the width of the total chill in wedges for flake graphite and of the critical wall thickness for nodular cast iron compare well with experimental data. (3) The absolute values of the chilling tendency of cast iron are calculated to range from 0.3 to 1.1 s 1/2 ·℃ –1/3 .
E. Fraś et al:Chilling Tendency and Chill of Cast Iron 183 References [1] Javaid A, Thompson J, Davis K G. Critical conditions for obtaining carbide-free microstructures in thin-wall ductile irons. In: Transactions of the American Foundry Society and the One Hundred Sixth Annual Casting Congress. Kansas City, USA, 2002: 889-898. [2] Javaid A, Thompson J, Sahoo M, Davis K G. Factors af- fecting the formation of carbides in thin-wall DI castings. In: One Hundred Third Annual Meeting of the American Foundrymen’s Society. Rosemont, USA, 1998: 441-456. [3] Ruxanda R E, Stefanescu D M, Piwonka T S. Microstruc- ture characterization of ductile thin wall iron castings. In: Transactions of the American Foundry Society and the One Hundred Sixth Annual Casting Congress. Kansas City, USA, 2002: 1131-1147. [4] Giacopini A, Boeri R E, Sikora J A. Carbide dissolution in thin wall ductile iron. Mats. Sci. Tech., 2003, 19(12): 1755-1760. [5] Labrecque C, Gagne M, Javaid A, et al. Production and properties of thin-wall ductile iron castings. Int. J. Cast Metals Research, 2003, 16(1-3): 313-317. [6] Choi J H, Oh J K, Choi C O, et al. Effect of Bi on formation of microstructure and mechanical properties of ductile iron castings with thin-wall section. AFS Transactions, 2004, 112: 831-840. [7] Mampaey F, Xu Z A. Mold filling and solidification of thin-wall ductile iron casting. AFS Transactions, 1997, 105: 95-103. [8] Showman R E, Aufderheide R C. Process for thin-wall sand castings. AFS Transactions, 2003, 111: 567-578. [9] Fraś E, Górny M, López H F. The transition from gray to white cast iron during solidification: Part I. Theoretical background. Metallurgical and Materilas Transactions A, 2005, 36: 3075-3082. [10] Fraś E, Górny M, López H F. Eutectic transformation in ductile cast iron. Part I. Theoretical background. Metal- lurgy and Foundry Engineering, 2005, 31: 113-136. [11] Merchant H D. Recent Research on Cast Iron, Gordon and Breach. New York: Science Publishers, 1968. [12] Rys J. Stereology of Materials. Cracov: Fotobit, 1995. [13] Osher J, Lorz U. Quantitative Gefuengenanalysie. DVG: Leipzig-Stuttgard, 1994. [14] Wojnar L. Effect of graphite size and distribution on fracture and fractography of ferritic nodular cast iron. Acta Stereologica, 1986, 5(2): 319-324. [15] Wiencek K, Rys J. The estimation of Fe3C particle density in steel by simple counting measurements made in plane sections. Materialas Engineering, 1998, (3): 396-399. [16] Fraś E, Górny M, López H F. Nodule count in ductile iron: Theoretical model based on Weibull statistics. Interna- tional Journal of Cast Metals Researches, 2005, 18(3): 156-162. [17] Fraś E, Górny M, Tartera J. Nucleation and grains density in grey cast iron—A theoretical model and experimental verification. International Journal of Cast Research, 2003, 16: 99-104.
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