The term “Cast iron” identifies a large family of ferrous alloys. These are primarily iron alloys which contain 2% or more carbon and from 1% to 3% silicon. The properties of cast iron can be varied widely by varying the percentages of carbon and silicon, by alloying with various metallic elements, and by varying the practices of melting, casting, and heat treatment.
Cast iron in its basic form is a brittle material which has a very little impact strength. It has a little or practically no toughness when compared to low carbon steels. It has a fraction of the tensile strength of low carbon steels. When a cast iron piece fails it will not deform in a noticeable way and appears to snap apart or break in a manner consistent with a snap. There is no early warning of a failure.
The graphite phase is pure carbon and acts as a natural defect in the material. The iron is so saturated with carbon that graphite forms (free carbon) and causes the cast iron to be weaker. Much smaller amounts of carbon is combined with iron (Fe) in the form of iron carbide (Cementite) which is hard and brittle.
Cast irons can be classified as either unalloyed cast irons or alloy cast irons. Unalloyed cast irons are essentially iron-carbon-silicon alloys containing small amounts of manganese, phosphorus, and sulfur. There are other specialty cast irons like austenite gray cast iron and inoculated cast irons. Cast iron can be alloyed as in carbon steels with elements like Chromium (Cr) and Nickel (Ni) etc.
There are basically five types of cast irons. These are gray, ductile, malleable, compacted graphite, and white iron. Except in the case of white cast iron, all other cast irons have a microstructure that consists of graphite phase in a matrix which can be ferritic, pearlitic, bainitic, tempered martensitic or combinations of these. The four types of graphitic cast irons are identified according to the morphology of their graphite phase.
Gray Cast Iron – When the composition of the iron and the cooling rate at solidification are suitable, a substantial portion of the carbon content separates out of the liquid to form flakes of graphite. The fracture path of such iron follows the graphite flakes. The fracture surface of this iron appears gray because of the predominance of exposed graphite.
Gray cast iron has several unique properties that are derived from the existence of flake graphite in the microstructure. Gray iron can be machined easily. It has hardness conducive to good wear resistance. It resists galling under boundary-lubrication conditions. It has very good properties for use in vibration damping or moderate thermal shock applications.
Ductile Cast Iron– Ductile iron is also known as nodular iron or spheroidal graphite cast iron. It is similar to gray iron in composition but during casting of ductile iron the graphite is caused to nucleate as spherical particles instead of flakes. This is achieved through the addition of a very small but definite amount of magnesium and/or cerium to the molten iron through a process step called nodulizing. Ductile iron is produced from the same raw materials as gray iron, but these materials are to be purer especially with respect to sulfur. Casting properties of ductile iron such as fluidity are similar to those of gray iron. Main advantage of ductile iron over gray iron is its combination of high strength with ductility. Martensitic ductile irons and austempered ductile irons exhibit even better properties.
Malleable Cast Iron– This cast iron encompasses a form of graphite called temper carbon. This form of graphite is produced by the heat treatment of white cast iron. When a white cast iron is heated for an extended period of time (about 60 hrs) at a temperature of 960 °C, the cementite decomposes into austenite and graphite. By slow cooling from 960 °C, the austenite transforms into ferrite or pearlite depending on the cooling rate and the diffusion rate of carbon. The ductility and toughness of malleable iron falls between that of ductile cast iron and gray cast iron. Now a days malleable irons have been replaced by the more economically produced ductile irons for many uses.
Compacted Graphite Cast Iron– It is also known as vermicular graphite cast iron. It is characterized by graphite that is interconnected within eutectic cells as is the flake graphite in gray iron. The graphite in compacted graphite iron is coarser and more rounded than the graphite in gray iron. The structure can be considered intermediate between those of gray iron and ductile iron. The unique combinations of properties obtainable in compacted gray irons make them superior to either gray or ductile iron. The major applications are disc-brake rotors and diesel-engine heads. Compacted graphite cast iron can be produced by carefully controlling the amount of magnesium added as an inoculant in a process very similar to the process used to make ductile iron.
White Cast Iron– White cast iron is named such since this iron is having characteristic white fracture surface. White iron is formed when the carbon in solution in the molten iron does not form graphite on solidification but remains combined with the iron, often in the form of massive carbides. Instead, the carbon is present in the form of carbides mainly Fe3C and Cr7C3. Sometimes in white cast irons there are present complex carbides such as (Fe,Cr)3C from additions of 3 to 5% Ni and 1.5 to 2.5% Cr, (Cr,Fe)7C3 from additions of 11 to 35% Cr or those containing other carbide-forming elements.
White cast iron is hard and brittle and produces white, crystalline fracture surface. White cast iron has high compressive strength and good retention of strength and hardness at elevated temperature. But it is very often used for its excellent resistance to wear and abrasion. The massive carbides in the microstructure are chiefly responsible for these properties.