Abstract
Ductile iron casting production is strongly affected by austenite and graphite distribution obtained after the solidification process. At the same time it is accepted that solidification behavior can be considered as hypo-, hyper- or eutectic depending on the chemical composition; there is still some misconception about the growth evolution of graphite nodules and about solid fraction progression. Quenching experiments were performed on two different carbon equivalent compositions using inoculated and non-inoculated thermal analysis standard samples with the aim of freezing the existing phases at different solid fractions for each alloy. As a result of these experiments, it was possible to study the structural features found at different locations of each sample and at different stages of solidification. Additionally nodule evolution during the liquid-solid transformation was also analyzed and discussed regarding the chemical and processing characteristics of the prepared alloys.
Highlights
Ductile iron solidification is a complex process in which the mechanisms that occur during the mushy zone development have not yet been fully mastered [1]
It can be observed that solidification starts at higher temperatures for the A cast irons than for the B ones, which can be recognized by higher liquidus temperature values (Tliq)
While A alloy shows a clear hypo-eutectic behavior, B alloys, considered as hyper-eutectic by chemical composition, show a slightly hypo-eutectic behavior according to their solidification model
Summary
Ductile iron solidification is a complex process in which the mechanisms that occur during the mushy zone development have not yet been fully mastered [1]. A proper understanding of solid fraction evolution, regarding graphite and austenite nucleation and growth, together with the corresponding contraction-expansion phenomena is of special interest in order to establish the most appropriate industrial procedure to manufacture sound castings and to avoid the formation of shrinkage defects [2] This task is not easy as there are a wide variety of solidification possibilities for a given chemical composition. The quenching experiments performed at that time were adapted and repeatedly used with the aim to freeze the structural changes that occur under specific conditions and to determine their evolution during solidification In addition to these aspects quenching methods have been used to validate the mathematical and thermodynamic bases applied to develop the obtained solidification models. More advanced quenching procedures have been reported in the literature such as the one where portions of melt are extracted using a quartz tube in order to obtain different solid fractions in the same sample [8]
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