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Hot forging and cold forging are two different metal forming processes that provide similar results. Forging is the process of deforming metal into a predetermined shape using certain tools and equipment - deformation is done using heat forge, cold forge or even warm forging processes. Ultimately, manufacturers consider many criteria before choosing the type of forging that is best suited for a particular application.
With forging, where the arrangement of the grain structure imparts directional properties to the part, the grains are aligned so that they can resist the highest stresses the part will encounter. In contrast, casting and machining generally have less control over the arrangement of the grain structure.
Forging is defined as the forming or deformation of a metal in its solid state. Many forgings are done through an upsetting process in which a hammer or ram is moved horizontally to press against the end of a rod or rod to widen and change the shape of the end. Parts typically pass through successive stations before reaching their final shape. High-strength bolts "cold head" in this way. Engine valves are also formed by upsetting.
In drop hammer forging, the part is hammered into the shape of the finished part in a die, much like a blacksmith's open die hammer forging, in which the metal is hammered into the desired shape against an anvil shape. There is a difference between open die forging and closed die forging.
In open-die forging, the metal is never fully constrained by the die. In a closed die or press die, the metal for the forging process is confined between the die halves. Repeated hammering of the die forces the metal into the shape of the die, and the two halves of the die eventually meet. The energy of the hammer can be provided by steam or pneumatic, mechanical or hydraulic.
In true drop hammer forging, gravity alone pushes the hammer down, but many systems use power assist in combination with gravity. The hammer delivers a series of relatively high-speed, low-force blows to close the die.
In pressure forging, high pressure replaces high speed and the die halves are closed in a single stroke, usually provided by a power screw or hydraulic cylinder. Hammer forging is typically used to produce smaller part volumes, while press forging is typically used for high volume production and automation.
The slow application of press forging tends to machine the inside of a part better than hammering, and is often applied to large, high-quality parts (such as titanium aircraft bulkheads). Other specialized forging methods vary by these basic themes: For example, bearing races and large ring gears are made through a process called rolling ring forging, forging process can produce seamless round parts.
When a piece of metal is heat forged, it must be heated significantly. The average forging temperatures required for hot forging of different metals are: steel up to 1150°C; 360 to 520°C for aluminium alloys; 700 to 800°C (copper alloys).
During heat forge, a billet or billet is heated inductively, or in a forging furnace, or oven, to a temperature above the metal's recrystallization point. This extreme heat is necessary to avoid strain hardening of the metal during deformation. Since the metal is in a plastic state, it can be made into rather complex shapes. Metals remain malleable and malleable.
To forge certain metals, such as superalloys, a type of heat forging called isothermal forging is used. Here, the die is heated to about the temperature of the billet to avoid cooling of the surface of the part during the forging process. Forging is also sometimes carried out in a controlled atmosphere to minimize scale formation.
Traditionally, manufacturers have opted for heat forge to make parts because it allows the material to deform in its plastic state and the metal is easier to machine. Heat forging is also recommended for metal deformation with a high form rate, which is a measure of how much deformation a metal can withstand without developing defects. Other considerations for heat forge include:
The production of discrete parts; the low to medium precision; the low stress or low work hardening; the homogeneous grain structure; the increased ductility; the elimination of chemical incompatibility and porosity.
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