Ductile iron – also known as nodular or ductile iron – is actually a group of irons that exhibit high strength, flexibility, durability, and elasticity due to their unique microstructure. Ductile iron typically contains more than 3% carbon; it can be bent, twisted or deformed without cracking. Its mechanical properties are similar to steel and far exceed those of standard cast iron.

Ductile iron castings are solid metal objects formed by pouring molten ductile iron into the cavity of a mold. The ductile iron then cools and solidifies into the shape of the void it occupies.

What is Ductile Iron?
Invented in 1943, ductile iron is the modern iteration of cast iron. Cast iron and ductile iron have very different physical properties due to differences in their microstructure.
The graphite and carbon contained in cast iron exist in flake form; cast iron has a positive compressive load capacity, but tensile loads above its natural tensile strength cause cracks to form and propagate rapidly from stress points in the flaky graphite microstructure. As a result, cast iron has little to no elongation. It is a brittle material, so its use in tensile and impact load applications is limited.
Graphite in ductile iron is spherical and is therefore sometimes called nodular graphite. Also, the term ductile iron comes from the fact that the carbon contained in ductile iron is in the form of nodules. This unique microstructure allows ductile iron to withstand bending and impact loads better than conventional cast iron.

What is ductile iron made of?
Pig iron is used to produce ductile iron
Although ductile iron can be produced from steel or scrap iron, pig iron is the primary source of feed for most modern ductile iron foundries. Pig iron refers to primary iron produced in a blast furnace with an iron content of more than 90%.
The term "pig iron" originates from the old-fashioned method of casting blast furnace iron into molds arranged in a bed of sand that could be fed from a common runner. Since the die set resembles a litter of suckling pigs, the individual iron pieces are called "pigs" and the runners are called sows. Modern pigs are produced by continuous pig casting machines.
Pig iron is used as the main source of pure iron for ductile iron production. It has many unique advantages: pig iron contains low residual or harmful elements, has consistent chemical properties, promotes optimal slag conditions, and improves process control by providing consistent charge characteristics. Demand for pig iron has increased in recent years as ductile iron foundries take advantage of its advantages over alternative sources of iron such as scrap or melting scrap and adding carbon.

Advantages of Ductile Iron
Ductile iron offers several advantages to designers:
Ductile iron is easy to cast and machine.
It has an excellent strength to weight ratio.
Ductile iron is much cheaper to manufacture than steel.
It has excellent castability and machinability.
Ductile iron offers designers an excellent combination of toughness, low-cost manufacturing, and reliability.

Ductile Iron Application
Ductile iron has higher strength and ductility than gray iron. These properties allow it to be effectively used in a variety of industrial applications including pipes, automotive components, wheels, gearboxes, pump casings, machine frames in the wind power industry, and many more. Because it will not break like gray iron, ductile iron can also be safely used in impact protection applications, such as bollards.