discussion
The higher nodule count observed in re-solidified material from the with-CHG areas as compared to no-CHG samples is considered to be due to metallurgical differences between the materials. As no significant chemical heterogeneities are ex- pected,8-11
this suggests that elements such as magnesium that
favour spheroidal growth got bound as compounds in the with- CHG area when the block solidified. This could be in line with the observation of Källbom et al.9
who noticed the presence of
numerous inclusions in the heavy section castings they studied, that contain Mg and S but not O in the chunky cells, while in areas with spheroidal graphite and at the with-CHG cell borders particles containing Mg, O and Si were observed. The fact that the solidification behaviour of with-CHG and no-CHG materi- als is identical after the melt has been held for some time at 1200C (2192F) suggests that the precipitates considered above appear below 1200C (2192F) during the slow cooling of heavy section castings. A tentative schematic of metallurgical evolu- tion during DTA experiments could be that remelting the mate- rial led to immediate volatilization of magnesium in the case of no-CHG material, while this step was preceded by the dissolu- tion of the compounds in the case of with-CHG material. This extra time gave rise to a higher level of magnesium available during re-solidification of with-CHG samples than of the no- CHG ones, thus explaining that more spheroidal graphite was observed in the former. The sensitivity of the DTA records to the cooling rate in the case of the with-CHG material should thus be related to time-dependent dissolution/precipitation pro- cesses of compounds in the liquid cast iron.
conclusion
After holding the with-CHG and no-CHG materials in the liquid state long enough for them to be freed of any nodular- izing element, their solidification proceeds similarly by de- position of a coupled eutectic with lamellar or undercooled graphite. The eutectic reaction starts with similar and low undercooling with respect to the stable eutectic temperature, and no significant change in the rate of growth could be de- tected from the DTA curves which could be related to the change from lamellar to undercooled graphite.
When the material is kept for a very short period in the liquid state before cooling, solidification starts again with lamel- lar graphite but then this reaction slows down until a high enough undercooling is reached when spheroidal and ver- micular graphite may grow. Re-solidification of with-CHG material gives rise to a higher nodule count than no-CHG material, and this suggests that nodularizing elements be- come bound in compounds during the lengthy eutectic so- lidification period of heavy-section cast blocks. While this does not give rise to any overall chemical heterogeneity when comparing with-CHG and no-CHG areas, the process does affect the amount of nodularizing elements actually available in the remaining liquid. The exact process and the nature of the compounds involved are still to be investigated.
International Journal of Metalcasting/Winter 2012
acknowledgements
The authors would like to thank the financial support ob- tained from the Industry Department of the Spanish Govern- ment for (ref. PROFIT FIT-030000-2007-94).
rEfErEncEs
1. Elliott, R., Cast Iron Technology, Butterworths (1988). 2. Javaid, A., Loper, C.R., “Production of Heavy-Section Ductile Cast Iron,” AFS Transactions, vol. 103, pp. 135-150 (1995).
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4. Strizik, P., Jeglitsch, F., “Contribution to the Mechanism of Formation of Chunky Graphite,” AFS International Cast Metals Journal, vol. 1, pp. 23-30 (1976).
5. Liu, P.C., Li, C.L., Wu, D.H., Loper, C.R., “SEM Study of Chunky Graphite in Heavy Section Ductile Iron”, AFS Transactions, vol. 91, pp 119-126 (1983).
6. Asenjo, I., Lacaze, J., Larrañaga, P., Méndez, S., Sertucha, J., Suárez, R., “Microstructure Investigation of Small-Section Nodular Iron Castings with Chunky Graphite,” Key Engineering Materials, vol. 457, pp. 52-57 (2011).
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8. Gagné, M., Argo, D., “Heavy Section Ductile Iron Castings – Part I: Structure and Properties,” Proceedings of the International Conference, Advanced Casting Technology, Easwaren J. ed., ASM Int., pp. 231-256 (1987).
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