The aim of this master’s thesis is to describe the mechanism of graphite particle nucleation in spheroidal graphite cast irons in the presence of different types of inoculants. The main focus was placed on the chemical composition of the nuclei responsible for graphite particle nucleation and on the chemical composition of the inoculant, with the aim of identifying a correlation between them. All currently known theories describing the mechanism of graphite particle nucleation were thoroughly examined, and parallels with the obtained experimental results were sought.
This study involved the use of two different types of inoculants containing two key elements, namely zirconium and aluminium. The spheroidal graphite cast iron EN-GJS-400-15 was used for the experiments. A special cell was developed for the experimental work, into which an inoculant grain was mechanically inserted. Molten metal was then poured through the inoculant grain into the casting cavity of the measurement cell. After completion of the experiment, the inoculant grain was initially examined using an optical light microscope and subsequently using a scanning electron microscope (SEM). For the SEM analyses, the samples were additionally sectioned using an ion beam. This enabled the chemical composition of all microstructural constituents in the sample to be determined. For this purpose, EDS area mapping was used, providing information on the distribution of individual chemical elements across the observed sample surface.
In the samples, the distance from the inoculant grain to the area affected by the inoculant was observed, and the microstructural constituents were analysed across the entire cross-section. The entire region affected by the inoculant was analysed. Based on the obtained results, the dissolution of the inoculant in the melt and the strength of its inoculating effect were investigated. Using EDS area mapping, the chemical composition of the microstructural constituents was examined, with particular focus on graphite nodules in order to identify the nuclei responsible for graphite formation in spheroidal graphite cast irons.
It was found that in regions very close to the inoculant, where the concentrations of the chemical elements were very high, highly interesting graphite morphologies occurred. It was observed that the size of the graphite nodules increased rapidly with increasing distance from the inoculant, indicating that the presence of the inoculant has a strong influence on graphite nucleation. In some cases, EDS analysis revealed nuclei within the graphite that could be directly correlated with theories describing the mechanism of graphite nucleation in cast irons. In other cases, however, the nucleation mechanism could not be clearly determined, as no nuclei could be detected within the graphite nodules. The most frequently observed features were graphite nodules without identifiable nuclei at their centres and graphite nodules associated with the contact surfaces of silicon crystals.
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