Classification of Cast Irons

Classification of Cast Irons

• Cast irons are classified primarily based on their carbon content and the presence of alloying elements.
• The carbon content is a significant factor because it influences the microstructure and thus the properties of the cast iron.
• Different classifications exist depending on the application or desired properties.

Types of Cast Irons

• Gray Iron: Characterized by flake graphite, resulting in a relatively low strength but good machinability.
  • It's commonly used in applications requiring good casting properties and machinability such as engine blocks, cylinder liners, and machine components.
  • Gray iron is known for its dampening effect, making it suitable for parts subjected to vibrations.
• White Iron: Contains high carbon levels in the form of cementite (Fe3C).
  • This type is exceptionally hard and brittle, making it unsuitable for many direct applications.
  • Heat treatment or chemical processes might be employed to improve certain properties or make the material suitable for some instances.
  • White Cast Iron is extremely hard and brittle, but often used as an intermediate material that can further be processed.
• Malleable Iron: A modified version of white iron, undergoing a heat treatment process that transforms cementite into graphite in a more controlled manner.
  • This process significantly improves its ductility and toughness.
  • Malleable iron finds application in parts requiring both strength and ductility, such as gears, engine parts, and structural components.
• Nodular Iron (Ductile Iron): Similar to gray iron in some respects, but its graphite structure is spheroidal rather than flaky.
  • This unique spheroidal graphite structure results in a high strength-to-weight ratio, improved ductility, and toughness.
  • Nodular iron's superior properties compared to gray iron make it suitable for more intricate and demanding applications in the automotive, construction, and machinery industries. It can be used for gears, crankshafts, and pressure vessels.
• Alloy Cast Irons: These contain various alloying elements added to tailor specific properties. Examples include:
  • High-strength low-alloy (HSLA) cast irons: Enhanced strength properties.
  • Heat-resistant cast irons: Withstand higher operating temperatures.
  • Corrosion-resistant cast irons: Resist chemical attack and corrosion.
• Compacted Graphite Iron (CGI): A more recent development in cast iron, characterized by a different type of graphite microstructure.
  • The compacted graphite formation gives similar properties to those of a nodular iron, though CGI tends to be more abrasion resistant, and slightly less expensive than nodular iron.
  • It often finds application in heavy-duty machinery and abrasion or erosion resistant components in engines.

Designation Systems

• Various standards and conventions exist for designating the specific composition within each type and to identify the microstructure and resulting properties.
• Different organizations might have their specific standards for designating and classifying cast irons.
• The standards are based on composition and other properties such as graphite formation.