Cast Iron is an iron alloy (Fe-alloy). Carbon (C) and silicon (Si) are its main alloying elements. The mass percentage (wt%) of carbon may range from 2.06 to 4.5 wt% and the silicon content may range from 0.5 to 3.5 wt%. Small amounts of manganese (Mn), sulfur (S) and phosphorus (P) are always present in cast iron. Cast iron is produced by mixing and heating pig iron with large amounts of scrap metal, limestone and coke (carbon) until the desired level of every element is reached. Elements like chromium (Cr), copper (Cu), molybdenum (Mo) and nickel (Ni) may be added in small quantities to improve the properties of the material. Cast iron allows the creation of complex shapes in a single production step. Cast iron is heated to liquify and poured into a mould to solidify.
The mechanical properties of cast iron depend on the morphology of the carbon. The morphology is determined by composition and process parameters. In general, the mechanical properties of cast iron are lower than those of cast or wrought steels, especially when loaded in tension or when sudden impacts are applied. Its relatively low melting point and good fluidity beyond that point make it very suitable for casting. The materials high compressive strength, its excellent machinability due to the carbon lubricating the cutting tools, the heigh resistance to deformation and wear and reasonable resistance to corrosion make up for its brittleness in many applications.
Grey cast iron
The silicon in grey iron will force the carbon out of the solution during the melting process and allows the carbon to form plates of pure graphite. Therefore this type of iron is also referred to as lamellar graphite iron. The graphite gives the material a grey color at fracture surfaces. The forming of graphite reduces the shrinkage of castings during cooling down. Sulfur prevents the forming of graphite and increases the hardness of the castings. Adding more manganese to the melting process reduces the effect of sulfur. Grey iron is the most used casting material in the world.
White cast iron
White iron is created by lowering the alloys silicon content and cooling at a much faster rate after casting. This allows the carbon to bond with the iron as cementite (Fe3C) rather than forming graphite. Cementite gives the material a whitish color at fracture surfaces. Cementite formes larger particles than graphite and increase the overall hardness of castings at the expense of toughness. For this white iron is even more brittle than grey iron and less suitable for structural applications. Reducing the cooling rate at a certain point will result in castings with a white iron exterior and grey iron interior, combining the hardness of white iron with the toughness of grey iron. Standard white iron allows only small sized castings. A high chromium (Cr) content in white iron allows very large castings and reduces the need for fast cooling.
Malleable cast iron
Malleable iron is created through an annealing process. Heat is applied to white iron castings. This treatment causes the graphite to form spheroidal particles instead of flakes. These spherical or nodular shaped particles allow castings with relatively thin walls. They improve the materials malleability which allows shaping of castings at low temperatures (cold forming). They also improve the materials ductility which means a better resistance to tensional loads and allows the material to flex more before breaking. Malleable iron is only suitable for small sized castings with thin cross sections. The cooling rate may slow down in larger sections, allowing graphite to form and lower the overall fracture strength of the workpiece.
Ductile cast iron
Ductile iron is also known as nodular iron, spheroidal grafite iron or spherulitic graphite iron (SG iron). This type of iron is much less brittle than other types of iron because of the nodular graphite inclusions. The nodular shaped graphite particles improves the materials impact and fatigue resistance significantly compared to other types of iron. The graphite nodules are created by adding nodulizing elements like magnesium (Mg) or cerium (Ce) before the casting process. Ductile iron allows castings with larger cross sections than malleable iron.
Dr.ir. B.M. Korevaar - Delft University of Technology, The Netherlands
Prof. J. Orkas - Aalto University, Finland
Ir. B. Pennekamp - Delft University of Technology, The Netherlands