A comparative study of graphite growth in cast iron

Fe-C-Si alloys show good properties such as castability, appropriate mechanical properties, thermal stability, good heat conductivity and damping capacity, that make these alloys suitable for engineering purposes. Most of these properties are related to graphite morphology and consequently, to the processes of nucleation and growth. The nucleation of graphite is directly related to the number of substrates capable to create stable nuclei, where the carbon atoms can be attached building up the graphite. The objective of this paper is to provide a better understanding the mechanisms that govern the nucleation and growth of graphite.

 
Variables such as melt composition, cooling rate during solidification that all affect local segregation, determine the morphology and nature of the non-metallic inclusions that can serve as nuclei. Silicates, oxides, sulphides and nitrides were found as the main nuclei for graphite formation. In Mg-free iron melts the oxygen level is relatively high (>0.75 ppm at 1420°C) and graphite growth preferentially in the a-direction producing graphite platelets (lamellae).

When the melt is treated with Mg, the free oxygen and sulphur are reduced, encouraging the growth of the graphite along the c-direction and consequently, the formation of spheroidal graphite.