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.

Study of the solidification structure of compacted graphite cast iron

“…Recent investigations about the solidification micro and macrostructures of spheroidal graphite cast iron (SGI) and lamellar graphite cast iron (LGI) [1][2][3][4][5][6][7][8][9][10] provided relevant information. Some key results were obtained after the application of specific colour etching techniques to examine the macro and microstructure of LGI and SGI.…”

“…The results described above gave experimental support to an explanation of the solidification of these types of free graphite cast irons. 1,5,7 The proposal states that for both SGI and LGI, the solidification starts with the independent nucleation and growth of austenite dendrites and graphite from the melt. As dendrites grow, they interact with graphite particles, conforming solidification units that can be described as grains of austenite with a dendritic substructure that contain a very large number of graphite particles inside.…”

EBSD Analysis of the Primary Austenite Grains in Lamellar Graphite Cast Iron

“…As dendrites grow, they interact with graphite particles, conforming solidification units that can be described as grains of austenite with a dendritic substructure that contain a very large number of graphite particles inside. This mechanism has also been proposed to be valid for hypo and hypereutectic gray irons [6,13].…”

Primary Structure and Graphite Nodules in Thin-Walled High-Nickel Ductile Iron Castings

“…The published literature provides limited data on the formation of the primary structure in SGI [28][29][30][31][32][33][34][35]. This is key to understanding the microstructural evolution and, ultimately, the optimization of mechanical and other properties.…”