Ductile iron, also known as ductile cast iron, nodular cast iron, spheroidal graphite iron, spheroidal graphite cast iron and SG iron, is a type of graphite-rich cast iron discovered in 1943 by Keith Millis. While most varieties of cast iron are weak in tension and brittle, ductile iron has much more impact and fatigue resistance, due to its nodular graphite inclusions.
Non-malleable cast iron is not deformable under compressive stress, while spheroidal cast iron is deformable under tensile stress and also under compressive stress to a certain degree. Therefore spheroidal graphite cast iron as such cannot be considered non- malleable iron. Spheroidal graphite cast iron and malleable cast iron differ in terms of their composition and method of their production. Even if spheroidal graphite cast iron has characteristics similar to those of malleable cast iron (EN-GJM), none the less it constitutes a separate category (EN-GJS).