Cast irons are popularly classified on the basis of three factors: their carbon and silicon content, alloying with other elements, and what ‘melting, casting, and heat treatment processes’ were used to make them.
In the past, cast iron was initially categorized into two types based on its fracture: white cast iron and gray cast iron. However, as knowledge about cast iron grew and metallurgy advanced, other classifications based on microstructural features emerged. Later alloy content-based further classification was added.
Based on the above factors, cast irons are popularly classified as Gray Cast Iron, White Cast Iron, Ductile Cast Iron, Malleable Cast Iron, and Compacted graphite iron.
In this article, we will discuss the metallurgical classification of cast iron types and will evaluate them to find the best one!
Table of Contents
Types of Cast Iron
The types of cast iron depend primarily on carbon content and silicon. For instance, increasing the carbon content makes cast iron harder and more brittle. Lower carbon levels result in greater ductility.
Varying carbon content can influence the iron’s hardness and wear resistance. Silicon promotes graphite formation, which enhances cast iron’s ability to dampen vibrations and reduce shrinkage during cooling.
Alloying elements like nickel, chromium, molybdenum, and copper can be added to cast iron to modify its properties. For instance, nickel and chromium enhance corrosion resistance and high-temperature strength, while molybdenum improves wear resistance. Copper is added for improved machinability.
The way iron is melted and cast also affects its microstructure and mechanical properties. Heat treatment processes, such as annealing, quenching, and tempering, can further change the cast iron’s hardness, strength, and toughness.
Overall there are 5 major types of cast iron categories
Gray Cast Iron
Gray cast iron is a type of cast iron known for its dark grey fracture color, which is due to its unique graphitic microstructure.
This iron contains graphite flakes because of the presence of silicon, which helps stabilize carbon in the form of graphite rather than iron carbide.
Gray cast iron is commonly used in various applications where its properties like stiffness, machinability, vibration dampening, heat capacity, and thermal conductivity are advantageous.
Key Features of Gray Cast Iron
- Gray cast iron typically contains 2.5% to 4.0% carbon and 1.0% to 3.0% silicon.
- It has a gray fracture surface with ferrite and dispersed graphite flakes.
- Known for high compressive strength, fatigue resistance, wear resistance, and good machinability.
- Commonly classified according to its tensile strength.
Major Applications of Gray Cast Iron
- internal combustion engine cylinder blocks
- gearbox cases
- disk brake rotors
- It is also used in vibration damping and moderate thermal shock applications.
White Cast Iron
White cast iron is another type of cast iron with a white fracture color, primarily due to the presence of iron carbide, or cementite (Fe3C).
This differs from gray cast iron due to its lower silicon content, which prevents the formation of graphite during solidification, resulting in the presence of massive carbides.
White cast iron is known for its extreme hardness but is also brittle.
Key Features of White Cast Iron
- White cast iron typically contains 1.8% to 3.6% carbon, 0.5% to 1.9% silicon, and 1.0% to 2.0% manganese.
- It has a white, crystalline fracture surface with iron carbide plates.
- High compressive strength and excellent resistance to wear and abrasion.
Major Applications of White Cast Iron
White cast iron is used in parts that require extreme wear resistance, such as shell liners, slurry pumps, ball mills, and abrasion-resistant components like crushers and pump impellers.
Ductile Cast Iron or Nodular Iron
Ductile cast iron, also known as nodular cast iron or spheroidal graphite cast iron, is characterized by the presence of spherical nodules of graphite.
Unlike white cast iron, ductile cast iron is not formed through heat treatment but has a specific chemical composition that encourages the formation of spherical graphite inclusions.
Key Features of Ductile Cast Iron or Nodular Iron
- Ductile cast iron typically contains 3.2% to 3.6% carbon, 2.2% to 2.8% silicon, and 0.1% to 0.2% manganese.
- Contains spheroid nodular graphite particles in a ferrite or pearlite matrix.
- Combines high strength with ductility, good load strength, wear resistance, shock resistance, and a high modulus of elasticity.
Major Applications of Ductile Cast Iron or Nodular Iron
Ductile cast iron is used in a wide range of applications, from automotive parts to infrastructure components due to its unique combination of strength and ductility.
Malleable Cast Iron
Malleable cast iron is formed through a slow annealing heat treatment of white cast iron. This process converts the iron carbide (Fe3C) in white iron to graphite, resulting in a matrix of ferrite or pearlite with spherical or nodular graphite shapes.
Malleable cast iron offers good malleability and ductility, and it has lower silicon content compared to other cast irons.
Key Features of Malleable Cast Iron
- Malleable cast iron is similar to white cast iron but with graphite in spherical or nodular shapes.
- It offers good malleability, ductility, and fracture toughness, especially at low temperatures.
- Microstructure consists of ferrite and free graphite particles.
Major Applications Malleable Cast Iron
Malleable cast iron is used in applications such as
- electrical fittings
- hand tools
- pipe fittings
- farm equipment
- mining hardware.
Compacted Graphite Iron
Compacted graphite cast iron, also known as vermicular iron, has a unique graphite structure with interconnected eutectic cells. The graphite in this iron is coarser and more rounded compared to gray cast iron, and its microstructure is intermediate between gray cast iron and ductile cast iron.
Key Features of Compacted Graphite Iron
- Graphite in compacted graphite cast iron is coarser and more rounded, forming interconnected eutectic cells.
- Offers a unique combination of properties, making it superior to both gray and ductile cast irons.
- Produced by controlling the amount of magnesium added during casting.
- Properties include high strength, ductility, wear resistance, shock resistance, and good tensile strength.
Major Applications of Compacted Graphite Iron
Compacted graphite cast iron is commonly used in critical components like disc brake rotors and diesel engine heads, where its unique properties are highly advantageous.
Conclusion: Types Of Cast Iron
Cast iron, a group of iron-carbon alloys, comes in various types, each with distinct characteristics and uses. The best type of cast iron will depend upon the particular application and the desired properties. It’s imperative to consider factors such as tensile strength, hardness, ductility, and resistance to wear and heat.
For instance, while gray cast iron would be suitable for applications requiring high damping capacity, ductile or nodular cast iron would be preferred for parts that need to endure heavy loads. Thus, understanding the specific requirements and desired properties is crucial in making the right choice.
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FAQ | Types Of Cast Iron
What is the most popular type of cast iron?
Gray iron and Ductile iron are the most popular cast iron types.
What is cast iron and its classification?
Cast iron is an iron-carbon alloy with over 2% carbon content, classified into five main types: gray, white, ductile, malleable, and compacted graphite iron types.
What is the color of cast iron?
Cast iron typically appears as a dark gray or black color due to its graphite content and type
What are the four types of cast iron?
The most recognized four types of cast iron are gray iron, white iron, ductile iron, and malleable iron.