There are two methods to increase the strength of a metal, strain hardening and case hardening.

Case hardening involves diffusing one element into the surface of a metal.
Materials such as steel are alloys, containing a main crystal structure of an element with another element diffused into the interstitial spaces of the crystal lattice. By producing steel with a low carbon composition, it is easier to machine parts. By diffusing more carbon into the surface of the part after manufacture, it can be strengthened. To case harden, the surface of the material is heated under an atmosphere of the element you wish to diffuse. This creates a shell of hardened, high carbon composition steel, or whatever alloy mix your working with.

Strain hardening involves deforming the material to a reduced size in order to strengthen it.
Metals have a crystal lattice, which is like a bunch of oranges stacked in the supermarket. Defects, called dislocations, would be a line of missing oranges or a plane of missing oranges, causing an inconsistency in the lattice. If a metal is squeezed through cold rollers these dislocations become squeezed together, or in other words, the dislocation density increases. A material with a high dislocation density tends to not want to bend or stretch because of the forces that the dislocations impart on one another. This therefore hardens the material.