Cast irons and their Classification...

Cast irons and their Classification  The term ‘cast iron’ represents a large family of ferrous alloys. Cast irons are multi-component ferrous alloys, which solidify with a eutectic. The major elements of cast irons are iron, carbon (2 % or more), silicon (1 % to 3 %), minor elements (less than 0.1 %), and often alloying elements (less than 0.1%). Cast iron has higher carbon and silicon contents than steel. The structure of cast iron displays a richer carbon phase than that of steel because of its higher carbon content. Cast iron can solidify according to the thermodynamically metastable Fe-Fe3C (iron carbide) system or the stable iron-graphite system depending principally on composition, cooling rate, and melt 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 does 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 which is pure carbon 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 (Fe3C, cementite) which is hard and brittle. During the solidification process, when the metastable route is followed, the rich carbon phase in the eutectic is the iron carbide and when the stable solidification route is followed, the rich carbon phase is graphite. Referring only to the binary Fe-Fe3C or...