Influence of Primary Austenite on the Nucleation of Eutectic Cells

Abstract

The solidification of gray cast iron starts with the precipitation of primary austenite. This phase nucleates either as columnar or equiaxed dendrites depending on whether nucleation occurs on the mould wall or on particles and impurities in the melt. In this work, the nucleation of primary austenite and its influence on the eutectic solidification has been investigated using different amounts of iron powder as inoculants. Besides, the influence of different cooling rates was also examined. Within each austenite grain there is a microstructure, and this microstructure was investigated using a color etching technique to reveal the eutectic cells and the dendritic network. It is shown how the cooling rate affects the dendritic network and the secondary dendrite arm spacing, and how the microstructure can be related to the macrostructure through dendrite arm spacing. The secondary dendrite arm spacing is a quantification of the primary austenite belonging to the primary solidification, and it will be shown how the eutectic cell size is related to the secondary dendrite arm spacing. The total amount of oxygen influences the microstructural dimensions. This effect, on the other hand, is influenced by the cooling rate. The number of eutectic cells versus eutectic cell size show two distinct behaviors depending on whether being inoculated with iron powder or a mixture of iron powder and commercial inoculant. The addition of a commercial inoculant decreases eutectic cell size and increases the number of cells, while iron powder almost only changes cell size.