Cast Iron

Cast Irons are alloys of Iron containing, at the time they are cast, carbon in the range from about 2% to 4.5% and usually with about 1-3% silicon. This contrasts with Steel which is also an iron-carbon alloy but which contains considerably less carbon. It is common for the term `cast' to be omitted in descriptions such as `grey (cast) iron', `malleable (cast) iron' or even `(cast) irons'.

Iron containing about 3% carbon solidifies over a temperature range from about 1350C to 1130C, i.e. significantly lower than the 1535 C or so melting point of pure iron. This wide freezing range coupled with a high fluidity ensures that complex shapes are readily cast. Other favourable characteristics of the simple irons include good corrosion resistance (relative to carbon steel) and erosion resistance, a high Damping capacity and good compressive strength. Cast irons can develop, under appropriate cooling conditions and by subsequent heat treatment, the structures described in the entry on Steel. However, the very high carbon content has a significant effect and in the as-cast condition, the structure will contain either soft, weak, free carbon, i.e. carbon not combined with the iron, or large quantities of hard, brittle cementite (iron carbide, Fe3C). Factors such as precise composition and cooling rate during casting influence the structures that initially develop. In many irons the free carbon will exist as graphite flakes. This usually forms as an Eutectic with the austenite at 1130 C but in adverse circumstances a coarse form, termed Kish graphite, can be precipitated at an earlier stage of solidification. However, even normal graphite flakes have a pronounced weakening effect although some improvement can be obtained by Inoculation which involves adding a finely particulate refractory powder, such as calcium silicide, to the molten iron. The particles provide a large number of nucleation sites for precipitation thereby refining the graphite. When irons containing graphite flakes are broken the fracture follows the weak flakes producing a characteristic grey fracture surface.

Historically, irons have been categorized on the appearance of their fracture surface and hence such irons are termed Grey irons. Apart from the graphite the microstructure will comprise a matrix of other phases such as Pearlite or Ferrite hence terms such as Pearlitic grey iron and Ferritic grey iron. Where the composition and cooling rate are such that no graphite is formed and the structure comprises cementite and pearlite the material will be very hard and brittle with a bright fracture surface, hence the term White iron.

The mechanical properties, particularly Ductility and Malleability, of white irons can be improved by various heat treatments to form Malleable irons. One process involves heating a white iron of fairly low carbon content, about 2.2-2.9%, at about 850 to 950 C in a non-reactive, i.e. non-decarburizing, environment for a long period (up to a week). This causes the cementite to decompose, producing a microstructure of ferrite with Temper carbon, i.e. graphite in a nodular or rosette form which is not so damaging as flake. These carbon nodules are more prolific towards the center of the section so the appearance of the fracture surface leads to the term Blackheart (malleable) iron. A variation on this process requires a second heating to 900 C allowing some of the carbon to redissolve in the austenite so that pearlite is produced on cooling, hence the term Pearlitic malleable iron. Such irons can be heat treated as described for steel. An alternative malleabilizing treatment involves heating white iron, of relatively high carbon content, about 3-4%, at about 950 C in an oxidizing environment for up to 100 hours. This allows most of the carbon to diffuse to the surface where it reacts with oxygen, forming carbon dioxide which is released into the atmosphere. The resultant structure is largely ferritic or pearlitic, with a little temper carbon at the center (more in large castings), hence the name Whiteheart (malleable) iron. This non-homogeneous structure usually results in inferior mechanical properties compared with the Blackheart grade. An alternative approach to producing ductile irons is to modify the form of the graphite in the as-cast state, usually by small additions of magnesium or cerium to the molten iron. These promote the formation of graphite in the nodular or spheroidal form which, compared with the flake form, has only a slightly adverse effect on the mechanical properties. These materials are referred to by various terms such as Ductile irons, Nodular irons, Spheroidal graphite irons (SG irons). The descriptions may also be extended by referring to other microstructural features as in terms such as Pearlitic SG iron and so on.

For more metal terminologies, please refer to our Metal Dictionary - Tech Terms.