Forging is a process to use special pressing machine to apply force on the metal material, the metal materiel will achieve desire mechanical proprieties, shape and dimensions.  Through forging process, it will eliminate the proprieties during the casting stage, optimized the micro structure, so it will be has better mechanical proprieties than the same grade casting material.  Forging parts very suitably for high loading and severe working conditions.

Base on the temperature, the forging divided into 3 categories:

Cold Forging- 

The term "cold forging" refers to various plastic forming processes such as cold stamping, cold extrusion, and cold upsetting. Cold forging is the shaping process that occurs below the recrystallization temperature of the material. It is forging that is performed below the recrystallization temperature. In production, it is a common practice to refer to the forging of materials that are not heated as cold forging. Cold forging materials are typically aluminum and some of its alloys, copper and some of its alloys, low-carbon steel, medium-carbon steel, and low-alloy structural steel at room temperature, where the deformation resistance is relatively low and the plasticity is good. Cold forged parts have good surface quality and high dimensional accuracy, which can replace some cutting processes. Cold forging can make the metal stronger and improve the strength of the parts.

Warm Forging-

Warm Forging to control the temperature between ambient temperature and crystallization temperature. Warm forging purpose is to obtain precision and high quality parts, not like the cold forging requires high press forging , nor like the hot forging require high temperature which will cause oxide scale and dimension changes after cool. In summary, warm forging suitable for small and complicate parts.

Hot Forging- 

Hot forging will heating the forging material above crystallization temperature. Because of the high temperature , so it has higher requires to the tooling material- heat resistance,hardness, impact toughness, thermal fatigue and wear resistance ,also easy for the machining.

As per the shape forming method, it also can be classified as :

Free Forging-（also know as open die forging , smith forging）

Free Forging is to use the impact or press force to make the metal formation to different direction on the anvil face , no limitation to achieve desire shape and mechanical property. The tools and equipment used in free forging are simple, with good versatility and low cost. Compared with casting blank, free forging eliminates shrinkage, porosity, porosity and other defects, so that the blank has higher mechanical properties. The forging is simple in shape and flexible in operation. Therefore, it has special significance in the manufacture of heavy machinery and important parts. Free forging relies on manual operation to control the shape and size of the forging, so the forging accuracy is low, the processing allowance is large, the labor intensity is large, and the productivity is not high, so it is mainly used in single piece and small batch production.

Die Forging-Close Die Forging

Die forging refers to the forging method by which the blank is formed by the die on the special die forging equipment. The forgings produced by this method have precise dimensions, small machining allowance, complex structure and high productivity. Die forging features:

1-Since there is a die to guide the flow of metal, the shape of the forging can be more complex.

2-The forging flow line inside the forging is distributed according to the forging profile, thus improving the mechanical properties and service life of the parts.

3-Simple operation, easy to realize mechanization, high productivity.

According to the equipment used, forging can be divided into hammer forging, crank press forging, flat forging, friction press forging, etc. Hammer forging uses a forging hammer, which is usually an air forging hammer. For complex forged parts, the initial forming is usually done in a preforming die cavity, and then forged in the forging die cavity. Forging dies can be classified according to their structure: those with a burr groove to accommodate excess metal are called open-die forging; those without a burr groove are called closed-die forging. Forging from raw billets is called single-die forging. For complex forged parts, multi-die forging involves preforming in several steps within the same forging die. Precision forging is a development of forging and can be used to produce complex-shaped, high-precision parts such as cone gears, blades, and aerospace parts.