Cast Iron
Cast iron is an important alloy of Fe and C are largely used in industry for its convenience to casting in intricate and good mechanical properties. Steels could also be cast but process is often costlier. Figure clearly shows the region of cast iron in equilibrium diagram. The diagram shows that there is a eutectoid at 4.3% (point C) which has solidification temperature of 1135oC. Alloys within carbon range of 2.3% to 4.2% are sometimes referred to as hypo-eutectic irons, since their carbon content is below the eutectic composition. Although, the melting points of cast irons are much higher than several non-ferrous alloys, yet they are within the reach of simple melting furnaces, several of which are commercially available. This is one reason as to why cast irons are as popular as structural material.
To understand various phases that are present in a cast iron one may consider cooling of a typical Fe – C mixture from melt, say one having y % C .The first material to solidify is austenite which is a solution of carbon in fcc iron. The line PG will give the amount of C in solution in austenite during solidification. This amount is always less than the average percentage in the melt, which means carbon is rejected out of austenite while the liquid phase is enriched in carbon. The last liquid to solidify has the eutectic composition, i.e. 4.3%. The eutectic contains the austenite and carbon.
If the liquid is cooled slowly maintaining near equilibrium conditions, the carbon solidified as flakes of graphite in a matrix of soft ferrite, in pearlite or in a mixture of ferrite and pearlite. If liquid is cooled rapidly then ferrite is suppressed and pearlite and cementite precipitate.
Casting is a process in which molten is poured in a mould and on solidifying the casting of the shape of mould is obtained. Cast iron, as already stated, in a good material for casting. General properties of cast iron are:
a. Cheap material.
b. Lower melting point (1200oC) as compared to steel (1380-1500oC).
c. Good casting properties, e.g. low shrinkage, high fluidity sound casting, ease of production in large number.
d. Good in compression but CI with ductility are also available.
e. CI is machinable is most cases.
f. Abrasion resistance is remarkably high.
g. Very important property of CI is its damping characteristics which isolates vibration and makes it good material for foundation and housing.
h. Alloy CI may be good against corrosion.