Ductile Cast Irons (Nodular Cast Irons)

Ductile iron, also known as nodular or spheroidal graphite iron, is cast iron in which the graphite is present as tiny spheres or nodules. Iron with nodular graphite is much stronger and more ductile than gray iron of similar composition. Unalloyed ductile iron typically contains 3.50 to 3.80% total carbon, 0.30 to 1.00% Mn and 2.00 to 2.80% Si.

Hardness and strength of all standard grades of ductile iron are relatively constant up to about 425°C (800°F), with the pearlitic ductile irons surpassing the ferritic grades in terms of elevated-temperature properties.

Use of molybdenum for strength retention at high temperatures is effective in unalloyed or low-alloy ductile irons up to about 650°C (1200°F) and in Si-0.5% Al ductile irons up to 800°C (1500°F). Molybdenum also can be used to boost rupture strength and additions of molybdenum or copper can enhance creep strength.

Oxidation resistance of ductile irons increases with increasing silicon content and the standard grades of ductile iron are more oxidation resistant than cast steel, gray iron, or pearlitic malleable iron. Dimensional growth of ductile iron is much less than that of gray iron at elevated temperatures as well.