Forging is a processing method that uses forging machinery to apply pressure to metal billets to produce plastic deformation in order to obtain forgings with certain mechanical properties, certain shapes and sizes, one of the two major components of forging (forging and stamping). Through forging can eliminate defects such as cast looseness produced by the metal in the smelting process, optimize the microstructure, while the mechanical properties of forgings are generally better than castings of the same material due to the preservation of the complete metal flow line. Related machinery in the high load, working conditions of the important parts, in addition to the shape of the simpler available rolled plate, profile or welded parts, more forgings.

1. Deformation temperature

The beginning recrystallization temperature of steel is about 727 ℃, but generally used 800 ℃ as a division line, higher than 800 ℃ is hot forging; between 300 ~ 800 ℃ is called warm forging or semi-hot forging, forging at room temperature is called cold forging. Forgings used in most industries are hot forging, warm forging and cold forging are mainly used for automotive, general machinery and other parts of the forging, warm forging and cold forging can be effective in saving materials.

2. Forging categories

As mentioned above, according to the forging temperature, it can be divided into hot forging, warm forging and cold forging. According to the forming mechanism, forging can be divided into free forging, die forging, lapping ring and special forging.

1) Free forging

It refers to the processing method of forging parts with simple general-purpose tools, or in the forging equipment between the upper and lower anvil to apply external force directly to the billet, so that the billet deformation and obtain the required geometric shape and internal quality of forgings. The forgings produced by the free forging method are called free forgings. Free forging is mainly used to produce forgings in small batches, using forging hammers, hydraulic presses and other forging equipment to form and process the billet to obtain qualified forgings. The basic process of free forging includes upsetting, drawing, punching, cutting, bending, twisting, misshifting and forging. Free forging is all hot forging.

2）Die Forging

Die forging is also divided into open die forging and closed die forging. Metal billet in a certain shape forging die chamber pressure deformation and obtain forgings, die forging is generally used for the production of small weight, large batch of parts. Die forging can be divided into hot die forging, warm forging and cold forging. Warm forging and cold forging is the future development direction of die forging, but also represents the level of forging technology.

According to the material points, die forging can also be divided into ferrous metal die forging, non-ferrous metal die forging and powder products forming. As the name suggests, it is the material is carbon steel and other ferrous metals, copper and aluminum and other non-ferrous metals and powder metallurgy materials, respectively.

Extrusion should be classified as die forging, which can be divided into heavy metal extrusion and light metal extrusion.

Closed die forging and closed upsetting belong to the two advanced processes of die forging, as there is no flying edge, the utilization rate of the material is high. It is possible to finish complex forgings in one process or several processes. Since there is no flying edge, the force area of the forging is reduced, and the required load is also reduced. However, care should be taken not to make the billet completely restricted, and for this reason the volume of the billet should be strictly controlled, the relative position of the forging die should be controlled and the forgings should be measured, and efforts should be made to reduce the wear of the forging die.

3）Ring lapping

Ring lapping refers to the production of ring-shaped parts of different diameters by means of special equipment ring lapping machines, which are also used to produce wheel-shaped parts such as automobile wheels and train wheels.

4）Special forging

Special forging including roll forging, wedge cross-rolling, radial forging, liquid die forging and other forging methods, these methods are more suitable for the production of certain special-shaped parts. For example, roll forging can be used as an effective pre-forming process, significantly reducing the subsequent forming pressure; wedge cross-rolling can produce steel balls, drive shafts and other parts; radial forging can produce large barrels, step shafts and other forgings.

5）Die forging

According to the movement of forging die, forging can be divided into pendulum rolling, pendulum swivel forging, roll forging, wedge cross-rolling, ring rolling and inclined rolling. Pendulum rolling, pendulum swivel forging and ring rolling can also be processed by fine forging. In order to improve the utilization of material, roll forging and cross rolling can be used as a pre-processing of slender material. Like free forging, rotary forging is also partial forming, which has the advantage of being formed with less forging force compared to the forging size. In this forging method including free forging, the material expands from near the die surface to the free surface during processing, therefore, it is difficult to ensure the accuracy. Therefore, the direction of the forging die movement and the rotary forging process are controlled by computer, so that products with complex shapes and high accuracy can be obtained with lower forging forces, for example, the production of forgings such as turbine blades with many varieties and large sizes.

Forging equipment die movement and degrees of freedom are inconsistent, according to the lower dead point deformation restriction characteristics, forging equipment can be divided into the following four forms.

Forging force limiting form: hydraulic presses with direct drive of the ram by oil pressure.

Quasi-stroke limiting form: hydraulic presses with oil pressure driving the crank linkage mechanism.

Stroke limiting method: Mechanical presses with crank, linkage and wedge mechanism driving the ram.

Energy limiting method: Screw and friction presses utilizing a screw mechanism.

In order to obtain high accuracy care should be taken to prevent overloading at the lower dead center and to control the speed and die position. This is because these can have an effect on forging tolerances, shape accuracy and die life. In addition, in order to maintain the accuracy, attention should also be paid to adjust the slide guide clearance, ensure the stiffness, adjust the lower dead point and the use of subsidized transmission device and other measures.

There are also vertical and horizontal movement of the slide (for forging of slender parts, lubrication and cooling and high-speed production of parts forging) way of differentiation, the use of compensation devices can increase the movement of other directions. The above-mentioned methods are different, and the required forging force, process, material utilization, output, dimensional tolerances and lubrication and cooling methods are different, and these factors are also factors that affect the level of automation.

3. Forging materials

Forging materials are mainly carbon steel and alloy steel of various compositions, followed by aluminum, magnesium, copper, titanium, etc. and their alloys. The original state of the material is bar, ingot, metal powder and liquid metal. The ratio of the cross-sectional area of the metal before deformation to the cross-sectional area after deformation is called the forging ratio. The correct choice of forging ratio, reasonable heating temperature and holding time, reasonable starting forging temperature and final forging temperature, reasonable deformation and deformation speed is very relevant to improve product quality and reduce costs.

Generally, small and medium-sized forgings are made of round or square bars as billets. The grain organization and mechanical properties of the bar are uniform and good, the shape and size are accurate, the surface quality is good, and it is easy to organize mass production. As long as the heating temperature and deformation conditions are reasonably controlled, it does not require large forging deformation to forge forgings with excellent performance.