Effects of impurities on graphite shape during solidification of spheroidal graphite cast ions
Résumé
Since the discovery that magnesium and cerium (and more generally rare earths) added at low level to cast iron melts lead to spherodized graphite, it is known that some other elements are detrimental even when present as traces. In all practicality, it has soon been recognized that adding rare earths to the melt helps counteracting the effect of these detrimental elements. Accordingly, only few works have been devoted to studying the effect of trace elements in melts without any rare earths. This is the first aim of the present work to review those studies as they contain the material to understand the mechanism for spheroidal graphite degeneracy. From this review, three types of degeneracy have been defined which show up when the critical level of any particular element is exceeded. These results are then discussed to show that all degeneracies certainly proceed in the same way. To substantiate this discussion, the growth of compacted graphite as obtained by low level treatment of cast iron melt with magnesium is also presented. Finally, a mechanism is suggested for describing the action of trace elements on spheroidal graphite degeneracy. This mechanism is partly substantiated by first-principles calculations which showed that all elements can strongly adsorb on the prismatic planes which are the planes on which carbon atoms add on during graphite growth.
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