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...

Ductile Cast Iron

Ductile Cast Iron  Ductile cast iron also known as nodular cast iron, spheroidal graphite iron or SG iron, and spherulitic cast iron. The ductile iron process was developed by The International Nickel Company in 1948. As the name ductile iron suggests this grade of cast iron has a degree of ductility. The main characteristic of this material is the structure of the graphite. Ductile iron is a family of cast graphitic irons which possess high strength, ductility and resistance to shock. Annealed cast ductile iron can be bent, twisted or deformed without fracturing. Its strength, toughness and ductility duplicate many grades of steel and far exceed those of standard gray irons. Yet it possesses the advantages of design flexibility and low cost casting procedures similar to gray iron. The difference between ductile iron and gray iron is in the graphite formation. Ordinary gray iron is characterized by a random flake graphite pattern in the metal. In ductile iron the addition of a few hundredths of 1 % of magnesium or cerium causes the graphite to form in small spheroids rather than flakes. These create fewer discontinuities in the structure of the metal and produce a stronger, more ductile iron. This nodular graphite structure inhibits the creation of linear cracks hence the ability to withstand distortion. Fig 1 shows typical micro structure of ductile iron. Fig 1 Typical micro structure of ductile iron  With ductile iron, the safety and reliability of process equipment is improved. The improved mechanical properties increase its resistance to breakage from physical load, or mechanical and thermal shock far above that of gray iron. The corrosion resistance of ductile iron is equal or superior to gray cast iron and to cast steel in many corrosives. Its wear resistance is comparable to...