Speed of Recalescence as a Measure of Graphite Nucleation in Spheroidal Graphite Cast Iron Castings

The solidification of spheroidal graphite cast iron (SGI) plays an important role in production of sound quality castings. However, owing to the complex solidification behaviour of SGI, prepouring melt quality evaluation technique such as thermal analysis is used to predict quality of liquid metal by analysing the cooling curve pattern. In this study, an attempt has been made to investigate the role of speed of recalescence (SOR) in nucleation of graphite during solidification of SGI. Cooling curve analysis of liquid samples was carried out at different inoculation degrees. Recalescence and SOR data were obtained from thermal analysis, whereas nodule counts were obtained from microstructural examination of respective sample. It was observed that recalescence and nodule counts (nucleation sites) have a statistically significant inverse linear relationship; with the increase in recalescence, the nodule count was found to get reduced. However, the multiple values of nodule count at same recalescence degree were to be understood, which was clearly justified by SOR studies. After analysing the SOR vs nodule count relation, almost each SOR value corresponds to different nodule counts and the relation was statistically more significant. This infers that for an in-depth nodule count estimation, SOR analysis will be more helpful as compared to increment Tr; hence, it is envisaged that speed of recalescence is true indicator of graphite nucleation as compared to recalescence in SGI.