When carbon in small quantities is added to iron, ‘Steel’ is obtained. Since the
influence of carbon on mechanical properties of iron is much larger than other alloying
elements. The atomic diameter of carbon is less than the interstices between iron atoms
and the carbon goes into solid solution of iron. As carbon dissolves in the interstices, it
distorts the original crystal lattice of iron.
This mechanical distortion of crystal lattice interferes with the external applied
strain to the crystal lattice, by mechanically blocking the dislocation of the crystal
In other words, they provide mechanical strength. Obviously adding more and
more carbon to iron (upto solubility of iron) results in more and more distortion of the
crystal lattices and hence provides increased mechanical strength.
However, solubility of more carbon influences negatively with another important property of iron called the‘ductility’ (ability of iron to undergo large plastic deformation). The a-iron or ferrite is very soft and it flows plastically.
Hence we see that when more carbon is added, enhanced mechanical strength is obtained, but ductility is reduced. Increase in carbon content is not the only way, and certainly not the desirable way to get increased strength of steels.
More amount of carbon causes problems during the welding process. We will see later,
how both mechanical strength and ductility of steel could be improved even with low
carbon content. The iron-carbon equilibrium diagram is a plot of transformation of ironwith respect to carbon content and temperature.
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