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Material Use

The material used must have a good "forgeability". By forgeability we mean the capability of the material to undergo deformation by forging without cracking. A material that can be shaped with low forces and without cracking is said to have a good forgeability. Usually forgeability competes with other desirable properties of the material and of the geometry of the workpiece, such as strength, corrosion resistance, toughness, fatigue resistance, heat resistance, size and section thickness. Therefore, the material is often selected on a compromise basis.

It is important to point out that the combined effects of temperature and deformation change the properties of the starting material. While it is often desirable to pick the starting properties based on compatibilities with the process, it is necessary to know or to be able to predict how the process will alter them.

Grain Alteration and Impurities Reorientation

Upon solidification of metals, especially in large sections,coarse dentritic grains form and a certain amount of segregation of impurities occurs. Hence, as-solidified metals typically have a nonuniform grain structure with a rather large grain size.

Reheating the metal without prior deformation will simply promote grain growth and a concurrent decrease in properties. However, when deforming the metal sufficiently at temperatures above the recrystallization temperature, the distorted structure is rapidly replaced by new strain-free grains. Grain growth can then occur. The metal can be cooled to "freeze in" the current structure or be further deformed and recrystillized. In general, the final structure will be a fine, randomly oriented, spherical-shaped grain structure. Such structure results in a net increase in strength, ductility, and toughness.

Forging, hot and cold, also affects the orientation of inclusions or impurity particles in the metal. With normal melting and cooling, the impurities locate along the grain boundary interfaces. It can assist a crack in its propagation along the metal. When a piece of metal is plastically deformed, the impurity material often distorts and flows along with the metal. Those impurities, often nonmetallic, don't recrystallize with the base metal and often produces an aligned fiber structure. Such a structure clearly have directional properties, being stronger in one direction than in another. An impurity originally oriented so as to aid crack movement through the metal can be reoriented into a "crack-arrestor" configuration, perpendicular to the direction of crack propagation.

Most commonly forged materials are steels, copper, forging brass, naval brass, bronze, and copper alloys. As for steels, their forgeability decreases as their carbon and alloy content increases.

Following is a table of materials and their 'forgability' ranking. The ranking indicates the material suitability for the forging process.

A value of zero means that the corresponding material is never used with this process, a ranking of 100 means that it is excellent for use with this process.

Source: DeGarmo, Black, Kohser, Materials and Processes in Manufacturing.