Cast iron is an alloy of carbon and iron where the graphite is solidified out of the matrix. In gray cast iron the graphite is in the form of flakes. In 1948 it was discovered that the flake graphite structure could be modified where the graphite was in the form of spheroid or nodules by the addition of magnesium to the molten metal. This iron is now typically known as Ductile Iron or S.G. Iron. The mechanical properties of ductile iron as compared to gray cast iron show superior tensile strength, yield strength, ductility and impact resistance. The use of ductile iron has grown at a rapid rate over the past fifty years.

The much improved properties were recognized to be of particular benefit in the centrifugal pipe industry and as such most of the pipe production is now in ductile iron.