Graphite shape characterisation in cast iron—from visual estimation to fractal dimension

Abstract

Thin wall iron castings are characterised by a much finer structure than that obtained in regular commercial castings. Existing microstructure standards do not cover this fine range of microstructures. Although computer aided image analysis may provide some answers, limitations exist. Graphite shape in spheroidal and compacted graphite iron is particularly sensitive to the measurement method. This paper offers a discussion on the various shape factors commonly used for evaluation of graphite shape. Selected shape factors are compared in terms of their ability to fully describe graphite shape across the whole spectrum of graphite morphologies in thin wall iron castings. While several shape factors can be used, it appears that sphericity is a better descriptor of spheroidal graphite shape than compactness. Compactness cannot distinguish well between exploded and spheroidal graphite, or poorly shaped and well-shaped spheroidal graphite. Our experimental data demonstrate that the shape factor method, otherwise intuitive, accurate and fast, cannot be used indiscriminately, as it is pixel size dependent. The posibility of using the fractal dimension method, which is pixel size independent, to quantify the morphological complexities of graphite shape was also investigated. Unfortunately, it was found that there is partial overlapping of the fractal dimension values for different graphite shapes (flake versus spheroidal graphite or compacted versus exploded graphite). Thus, it is recommended to use shape factors for graphite shape measurement in thin wall iron castings. The optimum pixel size for graphite shape evaluation in thin wall spheroidal and compacted graphite iron castings is 0.5 to 0.6 μm.