Cast iron is a group of iron-carbon alloys with a carbon content greater than 2%. Its usefulness derives from its relatively low melting temperature. The alloy constituents affect its color when fractured: white cast iron has carbide impurities which allow cracks to pass straight through, grey cast iron has graphite flakes which deflect a passing crack and initiate countless new cracks as the material breaks, and ductile cast iron has spherical graphite inclusions which stop the crack from further propagating. Here’s how grey cast iron compares with other types:
Grey Cast Iron
- Composition: Contains 2.5% to 4% carbon and 1% to 3% silicon.
- Microstructure: Characterized by its graphite flakes, which give it a grey appearance on the fracture surface.
- Properties: It has good thermal conductivity, excellent damping properties, and is easily machinable due to the graphite flakes which act as a lubricant. However, it has lower tensile strength and shock resistance compared to other forms of cast iron.
- Applications: Used for engine blocks, cookware, pipe fittings, and where vibration damping is important.
White Cast Iron
- Composition: Lower silicon content than grey cast iron.
- Microstructure: Contains carbides, making it hard and brittle. The carbides prevent the formation of graphite, giving it a white appearance on the fracture surface.
- Properties: High hardness and abrasion resistance but very brittle. It lacks the damping capacity of grey cast iron.
- Applications: Wear-resistant applications, such as rolling mill rolls and crushers.
Ductile (Nodular) Cast Iron
- Composition: Similar carbon content to grey cast iron but with small amounts of magnesium or cerium added to modify the graphite structure.
- Microstructure: The graphite forms into spherical nodules rather than flakes, contributing to higher tensile strength and ductility.
- Properties: Combines the machinability of grey cast iron with much of the strength and ductility of steel.
- Applications: Used in parts requiring high strength and toughness, like automotive components, gears, and axles.
Malleable Cast Iron
- Composition: Initially similar to white cast iron, but it is heat treated in a process called “annealing” to convert the carbides into graphite clusters or nodules.
- Microstructure: The heat treatment process results in a more malleable iron with graphite clusters.
- Properties: Good ductility and toughness, with better shock resistance compared to grey and white cast irons.
- Applications: Suitable for small castings or castings with thin sections that require a good degree of toughness, like brackets, chain links, and hardware.
Each type of cast iron offers a unique combination of properties that make it suitable for specific applications, depending on the requirements for strength, ductility, wear resistance, and machining capabilities.