History of Steel:
Steel is an alloy of iron
and carbon and other elements. Because of its high tensile strength and low cost, it is a major
component used in buildings, infrastructure, tools,
ships, automobiles, machines, appliances, and weapons.
Iron is the base metal of steel.
Iron is able to take on two crystalline forms (allotropic forms), body centered cubic (BCC)
and face centered cubic (FCC), depending on its temperature. In the
body-centred cubic arrangement, there is an iron atom
in the centre of each cube, and in the face-centred cubic, there is one at the center of each of the
six faces of the cube. It is the interaction of the allotropes of iron with the alloying elements,
primarily carbon, that gives steel and cast iron their range of unique
properties.
In pure iron, the crystal structure
has relatively little resistance to the iron atoms slipping past one another,
and so pure iron is quite ductile, or soft and easily
formed. In steel, small amounts of carbon, other elements, and inclusions
within the iron act as hardening agents that prevent the movement of dislocations that are common in the crystal lattices of iron atoms.
The carbon in typical steel alloys
may contribute up to 2.14% of its weight. Varying the amount of carbon and many
other alloying elements, as well as controlling their chemical and physical
makeup in the final steel (either as solute elements, or as precipitated
phases), slows the movement of those dislocations that make pure iron ductile,
and thus controls and enhances its qualities. These qualities include such
things as the hardness,
quenching behavior, need for annealing, tempering behavior,
yield strength,
and tensile strength
of the resulting steel. The increase in steel's strength compared to pure iron
is possible only by reducing iron's ductility.