Metallography of ductile iron

This practical paper provides a valuable insight into the artistry and science used to determine the metallographic features of ductile iron. It first considers the effect of section sensitivity on the mechanical and microstructural properties of pearlitic and ferritic ductile irons. Examples of the various matrix structures commonly encountered, from pearlitic to ferritic, high silicon-molybdenum, phosphides and carbides, are provided. The important role of magnesium and cerium on graphite morphology, as well as the methods used to maximize and achieve high nodule counts are then considered. A number of graphite forms, from the normal to the unusual, are presented and discussed. The author finally details commonly found casting defects in ductile iron, together with methods of prevention. Extensive use of optical and scanning electron microscope photographs and techniques are used to highlight the various aspects of ductile iron metallography. SEM is also shown to be an important defect analysis tool for the metallurgist. All of the metallographic features described by the author have been collected, prepared, photographed and identified in a large ductile iron foundry. It is expected that this will become a useful reference tool for the foundry metallurgists and technicians, and will provide them with the stimulus to search for and recognize similar features in their own ductile iron castings.