Communication: Characterization of Spatial Distribution of Graphite Nodules in Cast Iron

Abstract

Important properties of cast iron, such as fatigue strength, wear resistance, and low-temperature toughness, relate to spatial distribution of graphite nodules. Characterization of spatial distribution can also provide insight into the solidification sequence in casting. An automated SEM/EDX analysis was utilized to distinguish graphite nodules from other structural features (pores and inclusions). The two-dimensional near-neighbor distance (NND) between nodule centers was calculated for three equal sets of nodule diameters (small, medium, and large) in each cast iron. Comparison of measured spatial distributions and ideal random distribution was executed by plotting the mean and variance ratios of NND on a spatial distribution quadrant. This method was used to clarify clustering or ordering tendencies of graphite nodules in studied cast irons. The suggested procedure was used to verify the effects of inoculation and the cooling rate on spatial distribution of graphite nodules. Inoculation of sand casting increased nodule counts, decreased mean NND, and eliminated clustering of small graphite nodules precipitated at the solidification end. Intensive surface cooling of a continuously cast bar significantly increased nodule count near the external surface and decreased NND without changing spatial distribution. The suggested analysis can be used as a tool for cast iron quality control and process development.