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 paper, two different quantitative measurement techniques, including optical metallography and a Scanning Electron Microscopy/Energy Dispersive X-ray (SEM/EDX) with automated feature analysis of graphite distribution were compared. It was shown that the SEM/EDX with automated feature analysis provides more realistic nodule size by separating non-metallic inclusions from the graphite phase. A special conversion algorithm was developed based on the use of an inverse method with an EXCEL 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 distributions: normal with mainly mono-sized nodules and abnormal which has two or three different graphite size nodules. Graphite nucleation and growth conditions both relate to the final three-dimensional nodule size distributions in castings.