Cast iron is an alloy of iron and carbon. In addition to these components it also contains Silicon, Sulpher, manganese and Phosphorus. Steel and cast iron are mainly separated by the carbon content. Steel usually contains less than 1% carbon whereas cast iron contains 2 to 4% carbon.
Cast iron are classified on the basis of distribution of carbon content as below:
(1) Grey Cast iron: Grey cast iron specified by FG followed by its tensile strength. I.e., FG200 that means grey cast iron with ultimate tensile strength 200 N/mm2. Cylinder block, brake drum, clutch plate etc. are made from Grey Cast iron.
(2) White cast iron: White cast iron formed when most of carbon content forms iron Carbide. Malleable cast iron is obtained from white cast iron.
(3) Malleable cast iron: As said earlier malleable cast iron is obtained from white cast iron via heat treatment. Malleable cast iron has three basic types:
                    (i) Blackheart- superb machinability and used to manufacture axle, wheel hub etc.
                    (ii) Pearlitic- this material can be hardened by heat treatment and suitable for 
                                          engineering component
                   (iii) Whiteheart- this material is suitable in thin casting.
Malleable cast iron represented by the Letter BM, WM, PM followed by tensile strength.
i.e., BM 300 – Blackheart Malleable cast iron with 300 N/mm2 minimum tensile strength             WM 420 – Whiteheart Malleable cast iron with 420 N/mm2 minimum tensile strength                     PM 550- Pearlitic Malleable cast iron with 550 N/mm2 minimum tensile strength
Apart from above mentioned cast iron, ductile cast iron or nodular cast iron or spheroidal cast iron comes into the cast iron family. When a component of ductile cast iron fails, it shows a bright surface at the fractured surface. It is designated by SG followed by minimum tensile strength and minimum percent of elongation. i.e., SG 850/3 is spheroidal cast iron with a minimum tensile strength of 850 N/mm2 and 3% minimum elongation.