Forging is a manufacturing process in which metal is shaped by using compressive forces. These forces are usually applied by a hammer or a die. There are three temperatures that forging can be classified in: Cold forging (or cold heading), warm forging and hot forging. Cold forging occurs at room temperature and sometimes occurs by a process called impression die forging, or closed die forging. In some closed die forging techniques, the metal is placed into a die connected to an anvil. A hammer then hits the metal and forces it into the die, causing the metal to take on that shape. In another technique, the metal is heated and placed in a die with a top and bottom impression. As the top and bottom impression come together, they compress the metal into the shape of the die. Cold forging is normally used to forge standard or carbon alloy steels.
In warm and hot forging, the metal is heated in a forge and then shaped into a final product. Warm and hot forging mostly use a process called open die forging, where there is a flat die and the metal is compressed and deformed at high temperatures into the desired shape. Open die forging can be used to make such parts as bars, blanks, rings, hollows or spindles, or any custom designed metal components. As the piece of metal is shaped after being heated, its internal grain deforms and follows the shape of the part. Consequentially, the grain is continuous, giving the piece improved strength. Forged parts are mostly used in machinery that requires high strength. Iron and steel are almost exclusively hot forged so that secondary machining operations on the piece are possible.
Forging vs. Casting
When an object is cast, it is heated to a temperature above its melting point and then poured into a mold. Then the object is allowed to cool down, solidifying it into the shape of the original mold. Think of casting in terms of making Jell-O. When you make Jell-O, you mix hot water with the gelatin mix, and you pour this hot mix into a mold. When you allow this mold to cool, it takes on more of a solid shape and assumes the shape of the mold. When an object is forged, it is heated into a pliable state and then forced into the desired shape. Forgings have finer grain structure and thus higher tensile strength. Forgings are stronger and more durable than castings and thus have a longer life span. Forging materials and dies tend to be more expensive than the molds needed to cast objects. Castings are used for objects that are either too large or physically unsuitable for forging.
Advantages of Cold Forging (aka Cold Heading)
Because no material is shaved off when forging alloys at room temperature, the amount of wasted alloy can be reduced to almost nothing. Also, since the alloy is shaped at room temperature, there is no need to heat the metal and thus saves the manufacturer money on heating costs.
Cold Heading Methods
There are several methods that can be used during cold heading. These vary based on the type of metal used and the intended final shape. The three most common methods are forward extrusion (insert hyperlink), backward extrusion (insert hyperlink) and upset (insert hyperlink). Each is extremely important and having a unique purpose.
Forward Extrusion
This method reduces the diameter of the metal by forcing the metal to flow through a cavity with a smaller diameter than the piece. While the metal is forced through the cavity, extreme weight is used to compress the metal onto itself. Different metals have different compression yields. Softer metals like aluminum, tin and gold provide very favorable yield. Harder metals like steel and iron don’t have much compression, and thus don’t come out as thin when pushed through the cold heading machinery.
Backward Extrusion
During backward extrusion, holes are made and the material is allowed to flow around the cold heading item, which is used to create a penetrating punch. The penetrating punch pushes pieces of the metal through the created holes without cutting the metal out. A manufacturer might use a combination of the techniques in order to craft the metal into the desired shape. For example, a manufacturer might use backward extrusion to create a thin, shaped piece of metal. This can be used to make wiring or other circular material.
Upset
Upset forging deforms the metal under great pressure into high strength components, which can vary in size. This method is ideal for crafting pieces that only require one end to be forged, such as nails and bolts.
Tools Required for Cold Heading
Generally, in order to perform cold heading, manufacturers will use:
- Cold Header
- Part Former
- Punches
- Dies
Cold header and part formers both use horizontal reciprocating rams and stationary bolsters. A die is a type of mold that is the shape of the desired shape of the metal. A punch is a press that is sometimes powered by hydraulics that compresses the metal into the desired shape.
