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Silicon is essential to the alloy. Without it the product would be a very high Carbon brittle steel. Silicon causes the Carbon to come out of solution as Graphite flakes in a matrix of soft iron. This structure has a several useful properties.
The melting temperature of 1420 to 1470 K, is about 300K lower that the melting point of pure iron. The metal expands slightly on solidifying as the graphite precipitates resulting in sharp castings. The graphite flakes make for wear resistance and corrosion resistance. It is machinable. Cast iron tends to “damp” vibrations in machinery.
Cast iron has less tensile strength and shock resistance than steel and is difficult to weld.
The above describes the usual “grey cast iron”. With a lower Silicon content and faster cooling “white cast iron” results with the Carbon held as Iron carbide. It is too brittle for most uses but is hard and abrasion resistant. Rapid cooling can be used to put a hard white surface on a body of grey cast iron, called a “chilled casting”.
“ Malleable iron” starts as a white iron casting then is held at about 900C for some time. The Carbon separates out but into spheroidal particles rather than flakes, giving it properties more like mild steel. There is a limit to how large a part can be cast in malleable iron since the cooling rate must be fast enough to solidify as while cast iron all the way through.
A more recent development is nodular or ductile cast iron. By adding Magnesium and carefully controlling other elements and timing the carbon can be made to separate as spheroidal particles as cast. The properties are similar to malleable iron but parts can be cast with larger sections.
Cast iron is often used in the making of cookware. Cast-iron pots and pans are sturdy and conduct heat well, but require seasoning -- impregnation of the cooking surface with oil -- to prevent rusting and as a non-stick surface. Modern cast-iron pans are available pre-seasoned, but when cleaning one must take care not to scrub off this protective layer.
See also: Sand casting
Alloys Metalworking