Difference Between Ductile Iron and Cast Iron

Key Difference – Ductile Iron vs Cast Iron

As the name articulates, both ductile iron and cast iron contains iron as a common element; however, there is a difference between them based on their compositions. The differences in composition lead to various other variances in their properties; so that these two materials are used in different applications. Both of these materials are equally important; but, it is said that ductile iron has more advanced properties compared to cast iron. The key difference between ductile iron and cast iron is, ductile iron is durable, flexible and stronger than cast iron. Cast iron has a great history because it was invented in 4th century BC whereas ductile iron was discovered in 1943.

What is Ductile Iron?

Ductile iron contains many elements such as iron (Fe), carbon (C), Silicon (Si), Manganese (Mn), Magnesium (Mg), Phosphorus (P) and Sulphur (S). Sometimes tin (Sn) and Copper (Cu) are added to get desired properties. In addition, this contains nodular graphite which gives flexibility to the material. Ductile iron materials are strong and durable. Therefore, it is used in sewer and water lines.

Key Difference - Ductile Iron vs Cast Iron

Microstructure of ductile iron

What is Cast Iron?

Cast iron is an alloy which contains Carbon (C), iron (Fe) and Silicon (Si) as the main elements. It is in the iron-carbon alloy group and contains more than 2.1% of Carbon. White cast iron and grey cast iron are two examples in this category, but with different compositions. Cast iron is mainly found in engineering and constructing materials such as pipes, machines and automotive industry parts. In general, cast iron is a brittle material with a relatively low melting point; and also it possesses some other excellent properties such as machinability, deformation resistance and wear resistance.  Difference Between Ductile Iron and Cast Iron

What is the difference between Ductile Iron and Cast Iron?

History of Ductile Iron and Cast Iron:

Ductile Iron: Ductile iron was discovered in 1943 by Keith Millis.

Cast Iron: Cast iron has been used for many years and it has a great history. Chinese people invented this material in the 4th century BC. In the beginning, it was used to make weapons, pots, ploughshares and pagodas. However, westerners discovered cast iron in the late 14th century.

Composition of Ductile Iron and Cast Iron:

Ductile Iron: The composition of a typical ductile iron is as follows.

Element Content
Carbon 3.2 – 3.6%
Silicon 2.2 – 2.8%
Manganese 0.1 – 0.5%
Magnesium 0.03 – 0.05%
Phosphorus 0.005 – 0.04%
Sulfur 0.005 – 0.02%
 Copper <0.40%
Iron (Balance) 15%-30%

In addition, some other elements are added in smaller quantities to improve material properties; Copper or tin is added to increase tensile and yield strength and to reduce ductility. Nickel, Chromium or Copper is added to achieve corrosion resistant properties.

Cast Iron: Cast iron does not contain many elements as in ductile iron. It mainly contains only three elements; iron, Carbon and Silicon. The Carbon content in cast iron is over 2.1%.

Properties of Ductile Iron and Cast Iron:

Ductile Iron: Ductile iron possesses a high strength of ductility and resistant to shock. Annealed cast iron is able to bend, twist or deform without fracturing. It is also strong and durable than cast iron and has corrosion resistant properties.

Cast Iron: Cast iron is a brittle material with a low production cost and breaks when it is bent. Cast iron corrodes faster than ductile iron.

Uses of Ductile Iron and Cast Iron:

Ductile Iron: The main usage of ductile iron is  for water and sewer lines; it is an alternative for polymeric materials such as PVC, HDPE, LDPE and polypropylene. It is also used in automobile industry such as trucks, tractors and oil pumps.

Cast Iron: Cast iron is an engineering and construction material. It is used in the construction of buildings and bridges and to produce some machine parts.

Image Courtesy:
“Cast-Iron-Pan” by Evan-Amos – Own work (Public Domain) via Commons
“Ductile Iron” by Michelshock – McGill University. (Public Domain) via Commons