Among the micro-structural defects of ductile iron, the formation of chunky graphite is the most dangerous because it leads to the collapse of the mechanical properties in the areas where it is present. This paper proposes a new approach to explain the formation mechanism of this shape, through the primary and the secondary nucleation and the growth of graphite nodules during the eutectic transformation and the growth thereof at the solid state. The increase of the volume of graphite on a hand, with the contraction of austenite on the other hand, create tensions in the shell of newly formed austenite. The latter has microscopic imperfections derived from non-metallic inclusions, incorporated during the development of the solid phase as well as defects in crystalline structure, such as punctual, linear, plane or spatial defects (Koch-Cohen cluster of defects). The expansion of graphite simultaneously with the contraction of austenite create radial cracks in the shell of austenite where the propagation of these occurs on planes with defects. When the eutectic grains enter into collision, in the remaining liquid, rich in Carbon and non metallic inclusions (oxides and sulphides) "killer" nodules are originated with the second germination generation. Their development is identical to the growth of primary nodules, but the number in the volume unit is much greater in respect to the number of nodules created in the first germination. In the same time they have very small dimensions and therefore a good mechanical strength. They are also surrounded by austenite which breaks following its contraction in connection with the expansion of graphite. The fragments of austenite, especially the sharp-edged ones, are pushed by the fragments of austenite which surrounds the "killer" nodules by means of the graphite expansion (volumetric) in radial direction towards the primary nodules, damaging them ("wedge" effect). The broken primary nodules are the "Chunky Graphite". At the moment of the end of the solidification (TSol), the process of formation of chunky graphite is not completed yet. The nodules diameter increases continuously. The volume increase results in a burst of pressure which continues to damage, even in the solid state, the larger nodules, therefore the metal matrix is dynamic. The mechanical movement induced by the graphitic growth in the solid state fragments even more the existing graphite formations which are already damaged in the mushy zone. In the solid state the graphite growth is always accompanied to the contraction of austenite that increases the tensions during its transformation. Thus the formation of chunky graphite has to start in the mushy zone during the eutectic transformation and continues until the eutectoid transformation in the solid state. The formation of chunky graphite is due to a mechanical movement of the solid metal matrix and consists in a damage of the large fragile nodules. It is well-known that cerium promotes the formation of chunky graphite. The present work also links the excessive graphite expansion with the presence of Ce in the central areas rich in segregated C in heavy section castings.
展开▼
