Explain how the properties of iron can be changed by the addition of carbon?
Carbon gives iron hardness by distorting its crystal lattice.
In a perfect crystal, the metallic bonds are so strong that a lot of force must be applied to make it deform permanently.
Most crystals are not perfect, however, and iron is no exception. There are often vacancies in the atomic lattice.
These vacancies alter the arrangement of other parts of the lattice and introduce strain into the crystal.
An external force can more easily shear the crystal along a lattice plane one unit at a time until the dislocation reaches the boundary of the crystal.
That's why pure iron is relatively "soft".
Steel is an interstitial alloy of iron and carbon that contains between 0.002% and 2.1 % (m/m) of carbon.
The carbon atoms are slightly too large to fit perfectly into the interstices.
They introduce imperfections in the crystal layer.
It becomes harder for the layers to slide past each other, and the iron becomes harder.
Steel piano wire is the world's strongest wire. One wire, 2 mm in diameter, can hold a 1500 kg car.