Statistical Analysis of the Influence of Some Trace Elements on Chunky Graphite Formation in Heavy Section Nodular Iron
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gh the influence of graphite degeneration on mechanical properties of ductile irons has been the subject of several technical works in recent decades, the present knowledge about the mechanisms that cause it is still limited. One of the most important examples among these degenerations is chunky graphite (CHG), which was documented to have a detrimental effect on mechanical properties of nodular irons such as those used in the windmills industry.[1–3] A noteworthy number of studies have focused on the influence of processing variables and of manufacturing conditions on CHG formation,[2,4,5] while other works were aimed at determining the microscopic causes for the appearance of this defect.[4, 6–9] Regarding the processing variables, it was found that both inoculation[1,5,10] and low cooling rates[4,11,12] promote CHG formation in nodular irons. The effect of inoculation, however, is controversial, as previously discussed.[5] Furthermore, it has long been demonstrated that the chemical composition of melts has an important influence on this graphite degeneracy, which depends both on the main alloying elements such as C and Si through carbon equivalent[9,13] and on trace elements such as As, Bi, Pb,
JON SERTUCHA and PEIO LARRAN˜AGA, Research Engineers, are with the A´rea de Ingenierı´ a y Procesos de Fundicio´n, AZTERLAN, 48200 Durango (Bizkaia), Spain. JACQUES LACAZE, Senior Scientist, is with CIRIMAT, Universite´ de Toulouse, ENSIACET, BP 44362, 31030 Toulouse cedex 4, France. Contact e-mail: [email protected] SUSANA ARMENDARIZ, Engineer, is with TS Fundiciones, S.A. Pol. Sansinenea Erreka, 20749 Arroa-Zestoa (Gipuzkoa), Spain. Manuscript submitted October 5, 2012. Article published online January 12, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
Sb, Te, or Ti.[4,14] The interplay between these metallic trace elements and rare earth additions through spheroidization and inoculation treatments was demonstrated.[4,15] The effect of Ce,[16] Sb,[16,17] Bi, and Sb[18] was more particularly studied, and these elements were found to significantly affect this defect. While it is recognized that very low levels of some elements may affect graphite degeneration in heavy-section nodular iron castings, very little quantitative information has been made available so far. The present work was designed to answer the need of quantitative prediction on the risk of CHG appearance in heavy-section castings. Chemical and metallographic data used in the present work were obtained from 68 cubic blocks 30 cm in side cast according to the experimental methodology previously described.[5] The addition of specific alloying elements was performed in the ladle used to achieve the nodularizing treatment so as to have either pearlitic or ferritic grades. The chemical composition of the FeSiMg master alloy used for nodularizing was (wt pct) 42.2 to 45.0Si, 8.5 to 9.1Mg, 2.5 to 2.8Ca, 0.9 to 1.1Al, and 0.8 to 1.1 rare earths (RE). Postinoculation was carried out using a commercial inoculant (70 to 75Si, 3.2 to 4.5Al, 0.3 to 1.5Ca, and 0.4 to 0.
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