Study of microstructure development in hypoeutectic grey cast iron

Licentiate thesis, 2011

The avoidance of shrinkage porosity and metal expansion penetration defects as well as the control of the size and number of eutectic colonies and the tensile properties of the casting are issues of major importance in the production of lamellar cast iron. These defects, characteristics and properties are related to the microstructure that develops during solidification. An improved control of this features derive from our understanding of how the microstructure changes until its final state and our capacity to accurately measure such changes. The aim of this work is to enhance such understanding in hypoeutectic grey cast iron and provide experimental and analytical techniques to facilitate a deeper investigation of the solidification phenomena. Interrupted solidification, natural solidification, direct austempering after solidification treatment, cooling curve analysis, Fourier thermal analysis, different etching reagents, non-standard image processing techniques and stereological relations are combined for the first time to study the development of the microstructure from the beginning of the solidification to the as-cast state. Stereological relations are successfully combined with bi-dimensional observations to approximate tri-dimensional parameters of the microstructure in single or multiple sections analysis. The fundamental phenomenon of the coarsening of the primary phase (reduction of the primary austenite surface) and the increase of the secondary dendrite arm spacing are observed to occur prior, during and after the eutectic solidification in different cooling conditions. The validity of the secondary dendrite arm spacing as a measure of coarsening is assessed. The ease of flow for the interdendritic melt is characterized in terms of the hydraulic diameter of the interdendritic phase, which captures the improved flow conditions during the eutectic phase. The change of and interdependence between the inverse surface area per unit volume, secondary dendrite arm spacing, the cube root of time, solidification time, eutectic colony diameter and cooling rate as well as the change of fraction of primary austenite is presented. A tri-dimensional solid reconstruction of the microstructure of grey cast iron is attempted for the first time.