Solidification of cast iron - A study on the effect of microalloy elements on cast iron

The present thesis deals with the heat transfer and solidification of ductile and microalloyed grey cast iron. Heterogeneous nucleation of nodular graphite at inclusions in ductile iron during eutectic solidification has been investigated. A series of ductile iron samples with two different inoculants in four different thicknesses has been produced and studied; chemical analysis, metallographic investigation and thermal analysis of the specimens have been carried out.

A numerical model for solidification of ductile iron has been implemented and the results (i.e. cooling curve, cooling rate, nodule count and fraction of solid phases) have shown a good agreement with experimental studies; following this, inoculation parameters in the model have been studied and discussed.

The effect of Ti and S on the microstructure of grey iron is studied. Optical and electron microscopy are used to examine the unetched, colour-etched and deep-etched samples. It was confirmed that in irons with high sulphur content (0.12 wt%) nucleation of type-A and type-D graphite occurs on Mn sulphides that have a core of complex Al, Ca, Mg oxide.

An increased titanium level of 0.35% produced superfine interdendritic graphite (~10µm) at low (0.012 wt%) as well as at high S contents. Ti also caused increased segregation in the microstructure of the analysed irons and larger eutectic grains (cells). The inclusions have been identified in an effort to explain the nucleation of the phases of interest.

The reasons for increase in the fraction of primary austenite and formation of superfine interdendritic graphite have been investigated using Thermocalc simulations and metallographic studies. TiC did not appear to be a nucleation site for the primary austenite as it was found mostly at the periphery of the secondary arms of the austenite, in the last region to solidify.

The superfine graphite which forms in this type of irons is short (10-20µm) and stubby. The microstructure of this kind of graphite flakes in titanium alloyed cast iron is studied using electron microscopy techniques. The methods to prepare samples of cast iron for comprehensive transmission electron microscopy of graphite and the surrounding iron matrix have been developed and explained.

Dual beam microscopes are used for sample preparation. A TEM study has been carried out on graphite flakes in grey cast iron using selected area electron diffraction (SAED). Based on the SAED pattern analysis, crystallographic orientations are identified and compared. Subsequently, the orientation relationship between iron and graphite crystals at the interface is studied and discussed.

Based on this information, growth models for the platelets in the fine graphite flakes in cast iron are suggested and discussed.