Cast Iron: Cast iron is basically an alloy of iron and carbon and is obtained by re-melting pig iron with coke, limestone and steel scrap in a furnace known as cupola. The carbon content in cast iron varies from 1.7% to 6.67%. It also contains small amounts of silicon, manganese, phosphorus and sulphur in form of impurities elements.
General properties of cast iron: Cast iron is very brittle and weak in tension and therefore it cannot be used for making bolts and machine parts which are liable to tension. Since the cast iron is a brittle material and therefore, it cannot be used in those parts of machines which are subjected to shocks. It has low cost, good casting characteristics, high compressive strength, high wear resistance and excellent machinability. These properties make it a valuable material for engineering purposes. Its tensile strength varies from 100 to 200 MPa, compressive strength from 400 to 1000 MPa and shear strength is 120 MPa. The compressive strength of cast iron is much greater than the tensile strength. The carbon in cast iron is present either of the following two forms:
1. Free carbon or graphite.
2. Combined carbon or cementite.
The cast iron is classified into seven major kinds as follows:
(a) Grey cast iron, (b) White cast iron, (c) Mottled cast iron (d) Malleable cast iron, (e)
Nodular cast iron, (f) Meehanite cast iron. (g) Alloy cast iron and The chemical composition, extraction, properties and general applications of these types of cast iron are discussed as under.
Grey cast iron: Grey cast iron is grey in color which is due to the carbon being principally in the form of graphite (C in free form in iron). It contains:
C = 2.5 to 3.8%.
Si = 1.1 to 2.8 %
Mn = 0.4 to 1.0%
P = less than 0.15%
S = less than 0.1%
Fe = Remaining
It is produced in cupola furnace by refining or pig iron.
(i) When fractured it gives grey color.
(ii) It can be easily cast.
(iii) It is marked by presence of flakes of graphite in a matrix of ferrite and pearlite or austenite; graphite flakes occupy 10% of metal volume.
(iv) It can be easily machined and possesses machinability better than steel.
(v) It possesses lowest melting of ferrous alloys.
(vi) It possesses high vibration damping capacity.
(vii) It has high resistance to wear.
(viii) It possesses high fluidity and hence can be cast into complex shapes and thin sections.
(ix) It possesses high compressive strength.
(x) It has a low tensile strength.
(xi) It has very low ductility and low impact strength as compared with steel.
Applications : The grey iron castings are mainly used for machine tool bodies, automotive cylinder blocks, pipes and pipe fittings and agricultural implements. The other applications involved are
(i) Machine tool structures such as bed, frames, column etc.
(ii) Household appliances etc.
(iii) Gas or water pipes for under ground purposes.
(iv) Man holes covers.
(v) Piston rings.
(vi) Rolling mill and general machinery parts.
(vii) Cylinder blocks and heads for I.C. engines.
(viii) Frames of electric motor.
(ix) Ingot mould. And
(x) General machinery parts.
(xi) Sanitary wares.
(xii) Tunnel segment.