Use of Thermal Analysis for Porosity Prediction in Spheroidal Graphite Iron

The paper examines the effect of zirconium- and cerium-based inoculants on the porosity and microstructure of thermal analysis cups and an industrial casting in the context of riserless castings. Thermal analysis cups and industrial castings were used to establish correlations between spheroidal graphite count, porosity and elements of cooling curve and its time derivative.

The Ce-inoculated sample exhibited the highest nodule count (512 mm-2), while lower numbers were found for the Zr-inoculated (265 mm-2) and non-inoculated (216 mm-2) thermal analysis (TA) cups. Cooling curve analysis revealed that the Ce-inoculant produces the highest eutectic temperature, followed by the Zr- inoculant, and then by the not-inoculated sample.

Newtonian analysis was conducted to evaluate the fraction solid evolution during solidification and the effect of inoculants on the latent heat. It was found that the chemical composition of the inoculant affects the latent heat as the Ce-inoculant produced significantly higher latent heat than the Zr-inoculant.

Two criteria for evaluating inoculant efficiency in preventing porosity have been documented. The first one is the lower value of the cooling rate at the end of solidification, . The Ce-inoculated sample exhibited the lowest minimum of the first derivative, confirming the correlation high nodule count → low 1st temperature derivative. The second criterion, developed through Newtonian analysis of experimental data, is the time of occurrence of the maximum of the fraction solid growth rate, . It was found that for the Ce inoculant (zero porosity) the highest occurred at the end of solidification, while for the Zr inoculant (37.8% porosity) the highest rate was at the beginning of solidification. The highest at the end of solidification means that the graphite volume grows faster at the end and its expansion prevents porosity formation. This criterion could be used in the selection of inoculants that are effective in decreasing or eliminating porosity, and important finding in the quest of producing riserless castings, a technology of high importance in particular for spheroidal graphite iron.