Unlike grey cast iron, the graphite in ductile iron occurs as nodules. This difference means that ductile iron is characterised by much better mechanical properties than grey iron, in many instances comparable with steel. The conversion of graphite shape from flakes to nodules is achieved by treating the melt with magnesium. Ductile iron does not damp vibrations as well as grey iron but it is still better than steel in this respect. Furthermore, the machinability of ductile iron while still good is not as outstanding as as grey iron. The major advantage of ductile iron as a constructional material is its low weight with a density 10% less than steel.
GJS-500-7C is standardised in SS-EN 16482. The (discontinued) SS-designation is 0727. The figures 500-7 indicate that the material shall have a tensile strength >500 MPa at the same time as the tensile elongation exceeds 7%. The addendum “C” tells that the ductile iron has been continuously cast to distinguish from form-cast components. The continuous casting process is very flexible in terms of the section which can be manufactured. Round is most common but square, rectangular, half-rounds and other shapes are possible.
Through annealing GJS-500-7C can be converted to GJS-400-15/18C with lower strength but better elongation and impact toughness. The mechanical properties of ductile iron can be enhanced considerably by austempering in a salt bath.
The microstructure and associated mechanical properties of ductile iron are controlled principally by the cooling rate during solidification. In continuous casting, the surface of the bar is subjected to very fast cooling and the microstructure differs considerably from that closer to the bar centre. Hence, the standardised mechanical properties cannot be guaranteed in this surface zone which must be discarded. Relevant machining allowances are stipulated in SS-EN 16482.