Forging steel is a central part of the metal working industry and has been used for centuries to produce high tolerance quality products. The initial form of metal forging was a blacksmith with an anvil and a heated forge. Using a hammer, heated metal was placed on the anvil to be beaten and hammered into swords, cookware, battle shields, and other metal products. Over the centuries, forging steel has progressed through the addition of technological advancements and specialized equipment.

Open Die Forging
Open die forging involves deforming steel by placing it between dies that do not enclose the steel. The shape of the billet is changed by hammering or stamping it by a series of repetitions with each blow to the steel billet changing its shape. The workpiece is shaped between the top ram and a die that is placed on the bottom anvil. It is an imprecise forging method used for shaping simple forms. Once the process is completed, the forged piece requires a significant amount of machining.

Closed Die Forging
Closed die forging, known as impression die forging, is used to produce small or medium components. It is a plastic deformation process that forces carbon steel between halves of a die. The process makes it possible to form intricate parts with complex geometries, the key to which is the shaping and forming of the die, a complicated machining process.
The bottom portion of the die for closed die forging is attached to an anvil. The hammer portion of the die drops repeatedly down on the die cavity to force the billet into the desired shape. Excess material, referred to as flash, from the multiple blows of the hammer, is squeezed out of the die and cools quickly, which prevents more flash from forming. Additionally, the flash acts as a barrier preventing other soft material from leaving the die causing the steel to fill the die.
Industrial closed die forging has the workpiece move through a series of dies with the first of the series used to distribute the metal and forming a rough shape of the workpiece. It is a fullering, edging, or bending impression. The die cavities after the first impression are referred to as blocking cavities, each of which resembles the configuration of the final product.
The forging load has to be precision set to ensure that the final product will have the correct form. If the load is too low, the carbon steel will not take the shape of the die. If the load is too high, the process will overstress the steel and lead to cracking and incorrect deformation.

Cold Forging
Cold forging deforms steel below its recrystallization temperature or near room temperature. The low temperature of the steel makes the forging process more difficult and requires more energy and force. The steel billet, used in the process, is brittle and subject to cracking during forging. Cold forging pancakes the grains of the steel and elongates them making them stronger and more resilient.
The basis of cold forging is impacting the workpiece to plastically deform it, under compressive forces, where the workpiece is located between a die and a punch. It is a displacement process that takes the workpiece and forces it into a desired shape. Some of the techniques of cold forging are extrusion, coining, upsetting, and swaging, each of which can take place in the same stroke or separate strokes.
●Extrusion – With cold extrusion, pressure is applied to a steel billet to force it through a specially designed die. A punch or press is used to apply the pressure to cause the billet to take the shape and form of the die. The process is divided into forward extrusion, backward extrusion, compound extrusion, and radial extrusion.
●Coining – In coining, steel is subjected to pressure to deform the workpiece to change its surface. It is used when very fine features or high relief is necessary for a part.
●Upsetting – The upsetting process involves horizontally pressing against the end portion of bar stock to form an end or shape.
●Swaging – Swaging is used to change the size and shape of a workpiece. It is similar to extrusion in that it forces the workpiece through a die. With swaging, no material is removed from the workpiece but has its shape and size changed.

Roll Forging
Roll forging or roll forming uses rollers to shape and form steel. Cylindrical or semi-cylindrical rollers with grooves for shaping are used to deform round or flat bar stock. As the bar stock passes between the rollers, its thickness is reduced as its length increases. Roll forming can be completed using cold or hot bar stock with hot bar stock being the preferred choice. For the heated process, the bar stock is heated sufficiently to make it malleable and ductile such that it can easily be formed. The grooves in the rollers have the precise shape geometry of the completed part and are what forges the workpiece to the correct dimensions.
A unique form of roll forging is ring forging, which involves ring rollers that decrease the dimensions of a ring of steel by squeezing it. The process is designed to remove the need for welding rings and forms perfectly shaped rings.

Drop Forging
Drop forging uses impression dies and a heavy hammer to compress steel billets into designed shapes. A drop hammer that contains the upper die is a mechanical device that is powered by a pneumatic or hydraulic cylinder. The lower half of the die, as is found in open die forging, is attached to the anvil directly below the drop hammer. The steel billet is heated to a temperature that makes it malleable and placed in the lower die on the anvil.
The drop hammer, under great force, pounds the steel billet until it completely fills the lower die cavity. As the drop hammer strikes the die, flash is pushed outside the impression created by the die. A draft angle is added as part of the die to allow the release of the completed part.

Hot Forging
Hot forging is one of the most common methods for forging steel and harkens back to the days of the blacksmith with his hammer and anvil. The use of hot forging reduces the amount of force that is necessary to shape and mold the workpiece. Heated steel has better flow, which makes it ideal for open and closed die forging. Additionally, the heating process anneals a workpiece and relieves its stress making it ready for further processing.
With hot forging, the workpiece is heated to a point above its recrystallization temperature. The process of recrystallization deforms grains of the crystal structure, which are replaced by stress free grains that grow until the original grains are gone. It is used in steel forging to eliminate the effects of strain hardening. The recrystallization temperature is one third to one half the melting point of the steel.
Hot forging makes it possible to precisely adjust the fine microstructure of the steel. The strength and durability of the metal can be clearly defined and is used in manufacturing processes with high loads, both static and dynamic, that place demands on products.

Heated Die Forging
Heated die forging is a form of hot forging and is used to achieve closer tolerances to reduce machining and the cost of steel. The use of a heated die reduces the number of preforming and blocking processes, which further lowers the cost of processing and tooling. Ram speeds can be slower to reduce the necessary pressure to form the workpiece.
Using cold dies leads to uneven plastic flow of the heated steel, referred to as die chilling. To avoid die chilling, dies are heated using furnaces or other means to temperatures of 400o F to 500o F (205o C to 260°C), a process that depends on the type of equipment being used. The ideal method is to heat the die to the temperature of the workpiece, which is referred to as heated or hot die forging.
Die heating is an essential part of modern forging with electric infrared heating being the most efficient. It is more effective than gas or electric furnace die heating, direct flame heating, electric calrod radiant heating, and gas radiant heating. Infrared technology uniformly heats a die to prevent hot or cold spots. Since electric infrared heaters are mobile, they are able to heat the die in place to provide continuous heat.