Graphite Nodule Size Distribution in Ductile Iron

Important microstructural features including alloying element segregation during eutectic solidification and ferrite/pearlite formation during the eutectoid reaction as well as mechanical properties (strength, ductility and low temperature toughness) are linked to the real three-dimensional size and spatial distributions of the graphite phase in ductile iron. In this article, two different quantitative measurement techniques of graphite distribution using metallography and automated SEM/EDX ASPEX analysis were compared. It was shown that the ASPEX analysis provides realistic nodule size by separating non-metallic inclusions from the graphite phase. A special conversion algorithm was developed based on use of an inverse method with the EXSEL solver to correlate two-dimensional nodule graphite size distribution to the real three-dimensional nodule diameter.  The effects of carbon equivalent, cooling rate and inoculation on three-dimensional nodule size distribution were investigated in no-bake mold and continuously cast ductile irons. The analysis revealed two different types of nodule size distribution: normal with mainly mono-sized and abnormal having two or three different graphite nodule size modes.  Graphite nucleation and growth conditions both relate to the final three-dimensional nodule size distributions in castings.