2009 Volume 81 Issue 10 Pages 482-488
A color-etching method has been proposed to reveal the austenite microstructure that precipitates during eutectic solidification of spheroidal graphite cast iron. In fact, the color-etched microstructure corresponds to a phase with a dendritic, or a layer, shape encompassing the spheroidal graphite. To clarify the identity of this color-etched microstructure, we compared the microstructure ?which had been interrupted-quenched during eutectic solidification with a normally solidified microstructure obtained at the same cooling rate.
The results show that the color-etched microstructure corresponds to the austenite phase at the middle stage of eutectic solidification. The occurrence of color-etched microstructure was also observed in some commercial castings. Austenite dendrites with various morphologies were observed in every spheroidal graphite cast iron sample. With castings of the same shape, the dendrite structure was found to vary ; Some castings had small dispersed austenite dendrite structures, while others are seen to increase to bulky sizes, suggesting that this hidden austenite dendrite structure affects fluidity during the eutectic solidification process and manifests itself in castability as shrinkage behavior.
