p2w QSUof the carbon atom, causing the solubility of carbon to be practically zero in austenite iron.
However, in alpha iron the holes are comparable to the size of carbon atom, allowing an
interstitial solubility of about 2 per cent carbon austenite iron. Thus the number of available
spaces and the relative size of carbon atom limits the amount of latter to dissolve in the solid
solutions and form the useful kinds of carbon alloys.
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Iron is by for the most important of the metals used in engineering construction. It is available
in abundance, but does not occur freely in nature. The ores of iron are classed according to the
iron mineral which is predominant. The iron content of the main ores are as follows:
Magnetite (Fe 3 O 4 ) — 70–75%,
Haematite (Fe 2 O 3 ) — 70%,
Limonite (2Fe 3 O 3 .3H 2 O) — 60%, it is hydrated haematite,
Iron pyrite (FeS 3 ) — 47%, and
Siderite (FeCO 3 ) — 40%
The fundamental chemical principles in the
extraction of iron from the ores is very simple.
Heating the ores in the presence of a reducing
agent will result in the formation of CO or CO 2 ,
librated as a gas, and metallic iron. Iron owes
its greatest utility to the fact that it alloys freely
with other elements, and its inherent properties
are markedly altered and improved for varying
conditions of service.
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The iron ore is dressed by crushing it to about
50 mm cubes. The impurities are knocked off
and the ore is then calcined to drive off moisture.
The calcined ore is smelted in blast furnance
(Fig. 13.1). The iron is deoxidised and a part of
sulphur is also removed. Then limestone, which
acts as flux*, is added to finally remove the
sulphur. The molten metal is tapped from the
furnance and is cast in the form of pigs.
g
Pig iron is classified as Bessemer pig, foundry
pig, forge pig, and mottled pig.
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