Cast iron and its types
Cast Iron
Cast iron is produced by remelting pig iron with coke and lime stone in a furnace (known as cupola). It is primarily an alloy of iron and carbon. The carbon includes in the cast iron is up 1.7 to 4.5 %. It may be either present as a free shape(as graphite) or as combined form (Cementite Cast iron is a brittle material so that it cannot be used in those parts which are subjected to shocks. Cast iron is low cost in manufacturing, good casting characteristics,high compressive strength and excellent machinabilty.
The cast iron contains small quantities of impurities such as silicon,sulphur,manganese and phosphorus. Effect of these impurities are as follows.
Silicon
Cast iron consists silicon upto 4%. It provide the formation of free graphite which makes the iron soft and easy to machining.
Sulphur
This content makes the cast iron brittle and hard.It must be kept below to 0.1% for better foundry purposes.
Manganese.
It also makes the cast iron hard and white. It is often kept below 0.75%
Phosphorus.
It aids fusibility and fluidity in cast iron , but induces brittleness. It is up to 1% in the cast iron.
Important Types of Cast Iron.
1:Grey Cast Iron.
As its name indicates, grey cast iron contains carbon 3 to 3.5%. It is ordinary commercial iron. It has low tensile strength, high compressive strength and no ductility. It can be easily machined.
2: White Cast Iron.
This type of cast iron contain carbon up to 1.75 to 2.3%. The white colour is due to the fact that the carbon is in the form of carbide(Known as cementite.), which the hardest constituent of iron.The white cast iron has a high tensile but low compressive strength.
3: Chilled Cast Iron.
It is a white cast iron produced by quick cooling of molten iron. The quick cooling is generally called chilling and the iron so produced is known as chilled cast iron.
4: Molted Cast Iron
It is a product in between grey and white cast iron in composition,colour and general properties.
5: Malleable Cast Iron
It is obtained from white cast iron from a suitable heat treatment process.(Annealing)
6: Spheroidal or nodular cast iron
It is also called ductile cast iron or high strength cast iron. This type is obtained by adding small amounts of magnesium (0.1 to 0.8%).
7: Alloy Cast Iron
It is obtained by adding alloys like nickel,chromium,copper and vanadium in sufficient quantities.The alloy cast iron has special properties like increased strength , high wear resistance,corrosion and heat resistance.
