The forging of steel from ancient times has been considered an arduous physical labour and at the same time an art. Working with steel heated to a high temperature of nearly 1000 deg. centigrade, to give it a high degree of deformation in a very short period before it loses its plasticity, demands a great deal of effort coupled with a high degree of skill.
When a steel is forged, its micro structure and mechanical properties undergo changes. These depend on the following factors :
- The temperature of the forging process
- The degree of reduction of the cross-sectional area
- The method of forging employed
- The chemical composition of the base metal
During forging, the structure of the metal changes in two diametrically opposite directions, and becomes fibrous. The grains of steel are drawn out in the direction in which the metal flows, and changes from a coarse-grained to a fine-grained structure. Forging, as compared to rolling, results in a greater degree of disorientation of the fibre of the metal resulting in superior mechanical properties.
The formation of new grain structure is called recrystallisation. This occurs at fairly high temperatures for steel. Also, higher the forging temperature, greater will be the growth of these grains. Consequently, steel should be completed at temperatures which foster recrystallisation and should be completed at temperatures at which grain growth no longer occurs. This will ensure fine-grained forgings, possessing good mechanical properties. It can therefore be concluded that the structure of a steel and the mechanical properties of a forging will depend on the degree to which its grains have been worked as a result of their deformation and recrystallisation.
The mechanical properties of forgings also depend on the method of their production. Forgings should be produced without cutting across the fibre of the metal. The strength and toughness of a forging is improved if the fibre of the metal is continuous along its entire length.