Advantages and disadvantages

Forging results in metal that is stronger than cast or machined metal parts. This stems from the grain flow caused through forging. As the metal is pounded the grains deform to follow the shape of the part, thus the grains are unbroken throughout the part. Some modern parts take advantage of this for a high strength-to-weight ratio.
Many metals are forged cold, but iron and its alloys are almost always forged hot. This is for two reasons: first, if work hardening were allowed to progress, hard materials such as iron and steel would become extremely difficult to work with; secondly, steel can be strengthened by other means than cold-working, thus it is more economical to hot forge and then heat treat. Alloys that are amenable to precipitation hardening, such as most alloys of aluminium and titanium, can also be hot forged then hardened. Other materials must be strengthened by the forging process itself.

Hot forging

Hot forging is defined as working a metal above its recrystallization temperature. The main advantage of hot forging is that as the metal is deformed the strain-hardening effects are negated by the recrystallization process. Other advantages include.

• Decrease in yield strength, therefore it is easier to work and takes less energy (force)
• Increase in ductility
• Elevated temperatures increase diffusion which can remove or reduce chemical inhomogeneities
• Pores may reduce in size or close completely during deformation
• In steel, the weak, ductile, FCC (face-centered-cubic) austenite is deformed instead of the strong BCC (body-centered-cubic) ferrite at lower temperatures

The disadvantages of hot forging are:

• Undesirable reactions between the metal and the surrounding atmosphere (scaling, or rapid oxidation, of the workpiece)
• Less precise tolerances due to thermal contraction and warping from uneven cooling
• Grain structure may vary throughout the metal for various reasons
• Requires a heating unit of some kind such as a gas or diesel furnace or an induction heater, which can be very expensive