The pressure of the rolls and the impact of the forging hammer on a piece of metal increase the strength of the metal from 2 to 5 times, according to the composition, the degree to which the metal is heated, and the pressure applied. The composition of a metal determines its ductility and the degree to which its form can be changed by working, while the heat and pressure to which it is subjected determine the depth to which the rolling or hammering is effective. The rolling of a cold metal makes the surface very hard. The strength of a metal is increased by rolling or hammering because (1) blow holes (including microscopic gas cavities throughout the metal) are pressed very small, and because (2) mechanical pressure crowds the metal crystals more closely together, breaking up planes of cleavage along which tearing would naturally take place. The second effect increases strength and hardness, but decreases ductility and the per cent of elongation before breaking. Working a metal very hot does not cause marked changes in its crystalline structure because the crystals are more or less mobile according to the heat, and are helped by the heat to assume their natural positions relative to one another.

Two ingots from the same heat, i. e., produced from the same heat in a converter or furnace, will show different elastic and tensile strengths and different degrees of elongation according to the degree of reduction each undergoes in rolling or hammering.