Evolution of Dendritic Austenite in Parallel With Eutectic in Compacted Graphite Iron Under Three Cooling Conditions

Abstract

Shrinkage defects are common problems in industrially produced metal cast components. Local density changes occur during freezing, which demand material transport between parts of the casting, often involving flow of liquid through partially solid regions. Cast alloys typically freeze with a dendritic morphology, which large interface against the liquid restricts liquid flow. Recent research also indicates that this dendritic structure has an impact on the mechanical properties of the final material. For these reasons it is important to understand and predict the evolution of this structure through the solidification of cast alloys. In this work, the evolution of the dendritic austenite structure is investigated in a near-eutectic compacted graphite iron solidified under three different cooling conditions. The solidification was interrupted by water quenching, enabling characterization of the dendritic austenite structure at different stages of solidification. Higher cooling rate was found to promote a more coherent dendritic austenite structure which constituted a larger volume fraction. In parallel with growth of the eutectic, the amount of dendritic austenite in extra-eutectic regions continued to rise. This rise was associated with both tip growth of new dendrites and with growth by thickening of existing dendrites.