The ease with which the forging process can be done is called forgeability. The forgeability of a material can also be defined as the capacity of a material to undergo deformation under compression without rupture. Forgeability increases with temperature up to a point at which a second phase, e.g., from ferrite to austenite in steel, appears or grain growth become excessive. The basic lattice structure of metals and their alloys seem to be an excellent index to their relative forgeability. Specific mechanical properties of materials are also influenced by forgeability. Metals with low ductility have reduced forgeability at a higher strain rate, whereas highly ductile metals are less strongly affected by increasing strain rates. Pure metals have good malleability, which makes them have good forging properties. The metals having high ductility at cold working temperature possesses good forgeability.
Cast parts made up of cast iron are strong in compression but are brittle and weak in tension. Such parts made using cast iron tend to be bulky and are used where they will not be subjected to high stresses. A few examples are machine bases, cylinder blocks, gearbox housings etc. Besides the above factors, the cost is another primary consideration in deciding whether to cast a component or forge it. An internal combustion engine connecting rod is one example where a forging will save machining time and the amount of material required.
In contrast, the cylinder block of the same engine would be costly if produced by any process other than the casting process. Another good point associated with casting is that big or small complex shapes can easily be cast. Small parts can directly be machined out from regular section materials economically. Apart machined out from the rolled steel stock definitely, possesses better mechanical properties than a conventionally cast part. Sometimes the shape and size of a part would mean removing a large amount of material by machining. It is sometimes more economical to forge the region, thereby reducing the machining time and the amount of material required.
The primary alloys used for cold or hot forging are aluminum and copper, including relatively pure metals. Carbon steels with 0.25 % carbon or less are readily hot forged or cold-headed. High carbon and high alloy steels are almost always hot forged. Magnesium possessing a hexagonal close-packed (HCP) structure has little ductility at room temperature but is readily hot forged. Aluminum alloys are developed between 725°F and 851°F or about 750°F below the solidification temperature. Aluminum alloys don’t form scale during hot forging operations; die life is thus excellent. Copper and brasses with 30% or more miniature zinc have excellent forgeability in cold forging operation. High zinc brasses can be cold-forged to a limited extent but are superior to hot forging alloys. Magnesium alloys are forged on presses at a temperature above 750°F. Magnesium must be protected from ignition or oxidation by an inert atmosphere of sulphur dioxide (SO2) at higher temperatures.