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Fig. 5. Stress–strain curves of four ductile iron test bars removed from various locations in a large cylindri- cal ductile iron casting are shown.


nodule clustering. With ductile iron fractures, cracking begins with void formation around the graphite nodules. Voids form more easily in castings with coarse graphite nodules.


T e stress intensity around the nodules increases with increased nodule size, and the fracture strength of the pearlite liga- ments between the nodules is reached at a lower bulk stress, resulting in lower tensile


strength and tensile elongation. T e ten- sile properties in 1-in. (2.5 cm) Y-blocks and 3-in. (7.6 cm) Y-blocks of a recent study illustrate the reduction in strength and elongation with section size. As nodule count is increased and nodule size is reduced, tensile strength and elongation both increase. Heat treatment can be used to enhance the strength and elongation in ductile iron with coarse graphite nod- ules. T rough refi nement of the grain structure the mechanical properties can be improved. A recent study on ductile iron samples from 3-inch (76-mm) Y-blocks showed that normalizing just below the upper critical temperature can produce a fi ne-grained ferritic–pearlitic microstructure. T e refi nement of the microstructure resulted in a signifi cant increase in strength and elongation. Another form of degradation in


properties is regularly displayed in ductile iron castings. In pearlitic–ferritic irons,


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