Chemically this material is the same as grey iron and is iron-carbon-silicon alloy. It is one of the more recent developments in cast iron technology and has been around since 1948. As the name suggests, it was developed to overcome the brittle nature of grey and white irons. It is also quite ductile in the as-cast form and negates the need for long heat treatments such as those required to produce malleable iron.
Like grey iron, ductile iron is an iron-carbon-silicon alloy. Typical carbon content is in the range 3 to 4%, while silicon content is slightly higher than grey iron at 2 to 3%. It is also common for ductile irons to contain significant additions of nickel.
Ductile iron is often made from scrap, pig iron and recycled ductile iron. Formation of flake graphite (and a grey iron structure) is avoided through the addition of small amount of magnesium to the melt. The interaction is extremely vigorous as the melt temperature of iron is higher then the boiling temperature for magnesium.
Ferrosilicon and other additives may also be used to promote graphitisation, control nodule size and promote formation of spherical graphite particles. Such additions may be added to the ladle rather than the furnace, a process called ladle inoculation.
The main difference between ductile iron and grey iron is the morphology of the graphite particles which take on a nodular or almost spherical form after suitable treatments are made to the melt. They resemble those found in malleable iron, but are more spherical.
Similar to grey iron the matrix may be ferritic, pearlitic or martensitic depending factors such as chemistry and other process variables.
The most commonly used system for these materials is outlined in ASTM A536. Designations consist of three numbers and cover 5 grades. The fist number relates to the minimum tensile strength (in kips per square inch), the second to the minimum yield strength (in kips per square inch), while the last refer to elongation during the tensile test (in per cent). The grades of ductile iron are as follows:
- Grade 60-40-18
- Grade 65-45-12
- Grade 80-55-06
- Grade 100-70-03
- Grade 120-90-02
Separate ASTM standards pertain to austenitic ductile irons (ASTM A439 and A571) and special purpose ductile irons (ASTM A476, A716, A395 and A667).