Unidirectional solidification studies of ingot mould cast irons

A furnace has been developed in which liquid iron, supplied from an external source, may be held at elevated temperature whilst solidifying in a cylindrical ceramic mould whose base is supported and closed by a water cooled copper chill. Readings from a series of thermocouples, monitoring temperatures within the iron, have enabled the establishment of a controlled solidification technique, under unidirectional heat flow conditions, with cooling rates varying from 623°C/min.(at the chill/metal interface) to 4.35°C/min.(near the feeder head). Statistical analysis of cooling rate reproducibility between castings has indicated insignificant variability. The solidification technique has been used to study the microstructure of an ingot mould cast iron at three sulphur levels (0.02-0.10 mass %) two nitrogen levels (0.005-0.012 mass %) and the interaction of these 6 variations with increasing titanium additions (0.09-0.49 mass %). Thermodynamic data indicates that the manganese content of the iron (0.8 mass %) should reduce free sulphur to very low values but, even so, the higher sulphur composition (0.1 mass %) showed retardation of graphitic eutectic formation compared with the other two levels. Titanium's greater affinity for nitrogen rather than sulphur, promoted graphitisation at all sulphur levels in the high nitrogen melts at low titanium concentrations, by virtue of the Ti/N interaction whilst, at low nitrogen, titanium graphitised the 0.10, and possibly 0.05, mass % sulphur melts by virtue of titanium sulphide formation. Increasing titanium promoted the carbide eutectic at contents less than previously suggested by other authors. Titanium also induced complex inclusion clusters as identified by S.E.M. examination. The castings exhibited Hong's Type I, II and III mottled structures in regions where graphite morphology has been designated streamer 'grey' and cellular respectively. It is suggested that streamer and 'grey' areas result primarily from direct formation from liquid iron as well as decomposition of eutectic cementite.