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Bessemer pig derives its name because of its use in the manufacture of steel by Bessemer
process using haematite ore. Impurities such as sulphur, phosphorus and copper are not
desirable in the Bessemer pig. Foundry pig, also known as grey pig, contains sufficient quantity
of free carbon and is produced when the furnace is provided with sufficient fuel. When fuel
provided is insufficient and if sufficient sulphur is present in the ore forge pig is produced. This
is also known as white pig. Mottled pig is in between the grey and white varieties. They exhibit
mottled fracture.
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Pig iron contains 3–4% carbon, 0.5–3.5% silicon, 0.5–2% manganese, 0.02–0.1% sulphur and
0.03–1% Phosphorus.
Pig iron is hard and brittle with fusion temperature of 1200°C and melts easily. It can be
hardened but cannot be tempered and magnetised. Its compressive strength is high but is weak
in tension and shear. Pig iron does not rust and cannot be riveted or welded.
Pig iron is most suitable for making columns, base plates, door brackets, etc.
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Pig iron is remelted with limestone (flux) and
coke and refined in Cupola furnace (Fig. 13.2). It
is then poured into moulds of desired size and
shape. The product is known as cast iron
containing about 2–4% of carbon in two forms,
i.e., as the compound cementite—in a state of
chemical combination; and as free carbon—in a
state of mechanical mixture. Carbon in the first
form is called combined carbon, and graphite in
the latter form. The quality of cast iron thus
depends upon the state in which carbon exists in
it. The word cast iron is a misnomer as steel with
carbon content less than 2% can also be cast. The
striking difference between steel and cast iron is
that the form or is plastic and forgeable while the
latter is not. However, some of the modern cast
iron develop a fair degree or plasticity and
toughness.
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