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Examination of the Eutectic Crystallization of Cast Iron by Thermal Analysis

Thermal analysis of cast iron is traditionally used for quick determination of the C%, Si% and CEL% values on the basis of measured values of the liquidus temperature (TL) and solidus temperature (TS) and the estimated phosphorus content (P%). The metallurgical quality of cast iron is determined by the factors influencing the crystallization of the eutectic. At a given chemical composition and cooling condition, the metallurgical quality depends on the nucleation status. There is no generally accepted plant-scale process for the representation of the nucleation status. For a type of cast iron the less the supercooling prior to the eutectic crystallization, the better is the metallurgical quality. For estimating the metallurgical quality we have introduced a new method based on the determination of the nucleation factor, which is the relationship of the difference of the measured stable and metastable eutectic temperatures to the difference of the equilibrium stable and metastable eutectic temperatures, expressed in percent. The nucleation factor can be determined by thermal analysis of the carbidic and graphitic cast iron. Tests have shown that adding an equal quantity of modifier, the value of the nucleation factor will be higher in case of a higher Si content or a bigger graphitization tendency. The nucleation factor is appropriate for determining the effects of the modification and for detecting the different effects of the different modifiers. The data collection system and the developed software, enable the user to determine the value of the nucleation factor parallel to the chemical composition of the cast iron in real time. In case of graphitic crystallization, the supercooling which is connected to the eutectic solidification, is related to the nucleus state of the cast iron. If the nucleus state is better, the crystallization starts with a lower supercooling [1]. In figure 1. we can see the cooling curves of a hypoeutectic gray cast iron, recorded by thermal analysis. ‘Basic’ indicates before modification, ‘superseed’ 0,3% indicates after modification.