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 cast iron invented in 1943 by Keith Millis. While most varieties of cast iron are brittle, ductile iron has much more impact and fatigue resistance, due to its nodular graphite inclusions.
In 1943, in an effort to create an iron with better mechanical properties than either gray or malleable iron, Keith Dwight Millis made a ladle addition of magnesium (as a copper-magnesium alloy) to cast iron while working in the International Nickel Company Research Laboratory. The resulting castings contained not flakes, but nearly perfect spheres of graphite. This discovery opened the door to dramatic new possibilities in metal applications. Ductile iron offers versatility and high performance at low costs. Since 1948, ductile iron castings have proven to be an effective, lower cost alternative to malleable iron castings, steel forgings and steel fabrications. High Tensile Strength, yield strength and elongation combine to give ductile iron a superior strength to weight ratio that adds up to more strength for less expense.
When compared to steel and malleable iron castings, ductile iron offers further cost savings. Like most commercial cast metals, steel and malleable iron decrease in volume during solidification, and as a result, require attached reservoirs of liquid metal to offset the shrinkage and prevent the formation of internal or external shrinkage defects. The formation of graphite during solidification causes an internal expansion of ductile iron as it solidifies. This property of ductile iron reduces requirements for feed metal as well as material and energy requirements, resulting in substantial cost savings.
Ductile iron has high plasticity and good toughness, the tensile strength of ductile iron is 60k psi and the yield strength of ductile iron is 40k psi, it can be comparable to carbon steel in strength. So it is often used to produce iron parts which require high strength, toughness and complex shapes.
Its cast performance is better than steel, mainly used to manufacture machine parts with mechanical complexity and a larger load, such as crankshaft, connecting rod, gear, camshaft. Iron foundry can also make recirculating ball type, worm fan steering parts in trucks, buses, construction vehicles. Sometimes it is made into the base and large ductile iron castings.