ENGINEERING MATERIAL | CAST IRON

 CAST IRON

The cast iron is obtained 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 contents in cast iron varies from 1.7 to 4.5%. It may be present either as free carbon(or graphite ) or combined carbon (or cementite).

Since the cart iron is a brittle material, therefore, it cannot be used in those parts which are subjected to shocks. The properties of  cast iron which makes it a valuable material for engineering purposed are its low cost, good casting characteristics, high compressive strength, wear resistance and excellent machinability. The compressive strength of cast iron is much greater than tensile strength. 

The cast iron also contains small amounts of impurities such as silicon, sulphur, manganese and phosphorus. The effect of these impurities on cast iron are as follow :

1. Silicon - It maybe present in cast iron upto 4%. It provides the formation of free graphite which makes the iron soft and easily machinable.

2. Sulphur - It makes the cast iron hard and brittle. It must be kept well below 0.1%  for most foundry purposes.

3. Manganese - It makes the cast iron white and hard. It is often kept below 0.75%.

4.  Phosphorus - It aids fusibility and fluidity in cast iron having 3 to 3.5%  carbon. The grey colour allowed to exceed 1%.

The importance type of cast iron are as follow :

(a) Grey cast iron - It is an ordinary commercial iron having 3 to 3.5% carbon. The grey colour is due the fact that carbon is present in the form of * free graphite. It has a low tensile strength, high compressive strength and no ductility. It can be easily machined.

(b) White cast iron - It is a particular variety of cast iron having 1.75 to 2.3% carbon. The value colour is due to the fact that the carbon is in the form of carbide (known as cementite). Which is the hardest constituent of iron. The value cast iron has a  high tensile strength and a low compressive strength.

(c) 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.

(d) Mottled cast iron - It is a product in between grey and white cast iron in compressive color and general properties. 

(e) Malleable cast iron - It is obtained form white cast iron by a suitable heat treatment process. According to India standard specification, the malleable cast iron may be either whiteheart, blackheart or pearlitic and are designated by the alphabets WM, BM and PM respectively. These designations are followed by a figure indicating the minimum tensile strength is MPa or N/mm².

(F) Nodular or spheroidal graphite cast iron - It is also called ductile cast iron or high strength cast iron. This type of cast iron is obtained by adding small amounts of magnesium (0.1 to  0.8%) to the molten grey iron  just after tapping.

(G) Alloy cast iron - It is produced by adding alloying elements like nickel, chromium, molybdenum, copper and vanadium in sufficient quantities. The alloy cast iron has special properties like increased strength, high wear resistance, corrosion resistance or heat resistance.

This article was taken by R.S. kurmi & J.k. Gupta