Difference between Ductile Iron and Cast Iron

Ductile iron and cast iron are two types of iron alloys that are widely used in a variety of industrial applications. While they are both made of iron, they differ in their microstructures and mechanical qualities, making them suited for different purposes.

Read this article to find out more about Ductile Iron and Cast Iron and how they are different from each other.

What is Ductile Iron?

Ductile iron is a type of cast iron alloy with improved mechanical qualities over ordinary cast iron. It is also known as nodular cast iron or spheroidal graphite iron. Because of its exceptional mix of strength, toughness, and ductility, it is frequently used in a variety of industrial applications.

Let's take a look at the specifics of ductile iron −

• Microstructure − The name ductile iron comes from the peculiar microstructure of graphite nodules or spheroids buried in a matrix of ferrite and pearlite. During the solidification process, graphite nodules are created by adding small amounts of magnesium or other nodulizing chemicals to the molten iron. These nodules operate as "crack arresters," inhibiting crack growth and increasing the ductility of the material.

• Nodularization − To encourage the formation of graphite nodules, a small amount of magnesium (typically in the form of magnesium-containing alloys) or another nodulizing agent is added to the molten metal.

• Casting − To make the desired shapes, molten ductile iron is poured into molds. The cooling rate during solidification helps in the formation of nodules, giving ductile iron its distinctive microstructure.

• Ductility − Ductile iron is more ductile than standard cast iron, which means it can withstand significant deformation before failing. This attribute qualifies it for applications requiring flexibility.

• Grades of Ductile Iron − There are various grades of ductile iron, each designed to meet specific needs. Ferritic-pearlitic, ferritic, pearlitic, and martensitic ductile iron are some of the most prevalent grades. The grade selected is determined by the mechanical qualities required and the intended use.

What is Cast Iron?

Cast iron is a kind of iron-carbon alloy recognized for its outstanding castability, high wear resistance, and machinability. It has been used for millennia and has played an important role in the creation of modern industrial society. Cast iron is frequently used in a variety of applications due to its unique combination of characteristics.

Let's look at the specifics of cast iron −

• Composition − Cast iron is mainly composed of iron and carbon, with the carbon percentage ranging between 2% and 4.5%. Cast iron has trace amounts of other elements such as silicon, manganese, sulphur, and phosphorus, which can affect its characteristics.

• Gray Cast Iron − The most common type is gray cast iron, which is noted for its outstanding machinability, wear resistance, and damping qualities. It has a flakes-like graphite microstructure that gives it its distinctive grey appearance.

• White Cast Iron − White cast iron contains little to no free graphite, resulting in a white, crystalline microstructure. It is extremely hard and brittle, making it ideal for applications requiring great wear resistance, such as grinding balls and mill liners.

• Manufacturing Process − Cast iron is produced by melting iron with a high carbon concentration and other alloying components. After that, the molten metal is poured into molds to make various shapes and forms. The microstructure of the cast iron is determined by the rate of cooling during solidification.

Difference between Ductile Iron and Cast Iron

The following table highlights the major differences between Ductile Iron and Cast Iron −

Characteristics	Ductile Iron	Cast Iron
Versatility	More versatile, suitable for a wide range of applications	Limited applications due to brittleness
Microstructure	Graphite in spherical nodules	Graphite in flake-like or lamellar structure
Machinability	Slightly challenging due to its tough microstructure	Relatively easy to machine
Manufacturing Process	Additives promote nodular graphite formation	Standard casting process with no additives
Mechanical Properties	Higher tensile strength, toughness, and ductility	Brittle with lower ductility and impact resistance
Applications	Pipes, fittings, automotive parts, gears	Engine blocks, manhole covers, heavy machinery

Conclusion

In conclusion, the primary difference between ductile iron and cast iron is found in their microstructures and mechanical properties. When opposed to cast iron, which is stronger in compression but more brittle, ductile iron has higher toughness and ductility, making it ideal for a wider range of applications.

The decision between the two depends on the application's specific needs and the desired mechanical qualities.