In materials engineering, cast irons are a class of ferrous alloys with carbon contents above 2.14 wt%. Typically, cast irons contain from 2.14 wt% to 4.0 wt% carbon and anywhere from 0.5 wt% to 3 wt% of silicon. Iron alloys with lower carbon content are known as steel. The difference is that cast irons can take advantage of eutectic solidification in the binary iron-carbon system. Eutectic is Greek for “easy or well melting,” The eutectic point represents the composition on the phase diagram where the lowest melting temperature is achieved. For the iron-carbon system, the eutectic point occurs at a composition of 4.26 wt% C and a temperature of 1148°C.
Types of Cast Irons
Cast irons also comprise a large family of different types of iron, depending on how the carbon-rich phase forms during solidification. The microstructure of cast irons can be controlled to provide products with excellent ductility, good machinability, excellent vibration damping, superb wear resistance, and good thermal conductivity. With proper alloying, the corrosion resistance of cast irons can equal that of stainless steels and nickel-base alloys in many services. For most cast irons, the carbon exists as graphite, and both microstructure and mechanical behavior depend on composition and heat treatment. The most common cast iron types are:
Gray cast iron. Gray cast iron is the oldest and most common type of cast iron. Gray cast iron is characterized by its graphitic microstructure, which causes fractures of the material to have a gray appearance. This is due to the presence of graphite in its composition. The graphite forms as flakes in gray cast iron, taking on three-dimensional geometry.
- White cast iron. White cast irons are hard, brittle, and unmachinable, while gray irons with softer graphite are reasonably strong and machinable. A fracture surface of this alloy has a white appearance, and thus it is termed white cast iron.
- Malleable cast iron. Malleable cast iron is white cast iron that has been annealed. An annealing heat treatment transforms the brittle structure as the first cast into the malleable form. Therefore, its composition is similar to white cast iron, with slightly higher amounts of carbon and silicon.
- Ductile cast iron. Ductile iron, also known as nodular iron, is similar to gray iron in composition. However, the graphite nucleates as spherical particles (nodules) in ductile iron rather than as flakes during solidification. Ductile iron is stronger and more shock resistant than gray iron. Ductile iron has mechanical characteristics approaching steel, while it retains high fluidity when molten and lowers its melting point.