Hot Forging

What Is Hot Casting?

Hot forging requires the metal to be heated above its recrystallization temperature. This allows for the flow stress and energy required to form the metal to lower, effectively increasing the rate of production (or strain rate). Hot forging aids in making the metal easier to shape as well as less likely to fracture.

Iron, along with its alloys, are almost always hot forged for two main reasons: #1) If work hardening progresses, hard materials (such as steel and iron) will become more difficult to work with, and #2) It is a more economical option to hot forge metals such as steel and then follow with heat treatment processes as metals such as steel can be strengthened through other processes (and not necessarily just cold working processes).

Average temperatures for hot forging includes: Aluminum (Al) Alloys – 360° (680°F) to 520°C (968°F); Copper (Cu) Alloys – 700°C (1 292°F) – 800°C (1 472°F); Steel – up to 1 150°C (2 102°F)

How are Hot Forgings Made?

During hot forging, the temperature reaches above the recrystallization point of the formed metal. As the step of plastically deforming the metal above the recrystallization temperature, these high temperatures are required in order to avoid strain hardening during deformation. This process typically involves heating the metal (above its recrystallization point) and then comminuting it into a mold that can also be heated as needed. Because the metal is hot, it is easy to “move” and enables manufacturers to make more complex shapes than cold forging.

For superalloys, which have low malleability, processes such as isothermal forging (deformation in a controlled atmosphere) are used to avoid oxidation. Isothermal forging, also known as hot forging, is a thermal processing process that keeps a workpiece at its maximum temperature throughout the forming process.

Maintaining this temperature is done by heating the mold – it will be at an elevated or slightly lower temperature of the workpiece. The force applied by the mold forms the workpiece, and because the mold is also at an elevated temperature, the cooling of the workpiece between the mold working interfaces is eliminated. This in turn leads to an improvement in the flow properties of the metal (work piece).

Advantages of Hot Forgings

Increased ductility
Complex shapes
High precision
Cost benefit
Enhanced stiffness
Size: 1 in to 30 in
Weight: Ounces to more than 100 pounds

Material	Characteristics	Application
Stainless Steel	Corrosion-resistant	Used in steam turbines, pressure vessels, and other applications in petrochemical, medical, food processing industries. Used at temperatures up to 1800 F under low stress and to 1250 F under high-stress.
Low Carbon and Low Alloy Steel	Easily processed Good mechanical properties Low material cost	Widely used at temperature lower than 900 F.
HSLA/Microalloy Steel	Good mechanical properties Low material cost Simple thermomechanical treatment	Mainly used at temperature lower than 400 F for structural and engine applications in the aircraft and transportation industries.
Aluminum	Good strength-to-weight ratio Readily forged	Mainly used at temperature lower than 400 F for structural and engine applications in the aircraft and transportation industries.
Nickel-Base Superalloy	Oxidation resistance Creep-rupture strength	Used at temperature between 1200 and 1800 F. Used for structural shapes, turbine components, fittings, and valves.
Titanium	High strength Low density Excellent corrosion resistance	About 40% lighter in weight compared to steel parts. Used primarily in the temperature services to 1000 F. Used for aircraft engine components and structures, ship components, and valves and fitting in transportation and chemical industries.