Forging is one of the two components of forging (forging and stamping), which uses forging machinery to exert pressure on metal blank and make it produce plastic deformation to obtain forgings with certain mechanical properties, shapes and sizes. Through forging, defects such as loose as cast can be eliminated in the process of metal smelting, and the microstructure can be optimized. At the same time, the mechanical properties of forgings are generally better than those of castings of the same materials because the complete metal streamline is preserved. Related machinery in high load, severe working conditions of the important parts, in addition to the shape of the simple available rolled plate, profile or welding parts, the use of forgings.

 

 

　　

1. Deformation temperature
The initial recrystallization temperature of steel is about 727℃, but 800℃ is generally used as the dividing line, higher than 800℃ is hot forging; Between 300 ℃ and 800℃ is called warm or semi-hot forging, forging at room temperature is called cold forging. The forgings used in most industries are hot forging. Warm forging and cold forging are mainly used in forging of automobile, general machinery and other parts. Warm forging and cold forging can save material effectively.

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

1) Free forging. It refers to a forging process that uses simple universal tools or directly exerts external forces on the blank between the upper and lower anvil iron of forging equipment to deform the blank and obtain the desired geometry and internal quality. The forgings produced by free forging method are called free forging. Free forging mainly produces forgings in small batches. Forging equipment such as forging hammer and hydraulic press is used to form and process blanks to obtain qualified forgings. The basic processes of free forging include upsetting, drawing, punching, cutting, bending, twisting, shifting and forging. Free forging adopts hot forging mode.

2) Die forging. Die forging is divided into open die forging and closed die forging. The metal blank is obtained by compression deformation in the forging die bore with a certain shape. Die forging is generally used to produce parts with small weight and large batch. Die forging can be divided into hot die forging, warm forging and cold forging. Warm forging and cold forging are the future development direction of die forging and also represent the level of forging technology.

According to the material, die forging can also be divided into black metal die forging, non-ferrous metal die forging and powder product forming. As the name suggests, the materials are carbon steel and other ferrous metals, copper and aluminum and other non-ferrous metals and powder metallurgy materials.

Extrusion belongs to die forging, which can be divided into heavy metal extrusion and light metal extrusion.
Closed die forging and closed heading forging are two advanced processes of die forging. Because there is no flying edge, the utilization rate of materials is high. It is possible to finish complex forgings with one or several processes. Since there is no flying edge, the forgings have less force area and less load required. However, it should be noted that the blank can not be completely restricted, so it is necessary to strictly control the volume of the blank, control the relative position of the forging die and measure the forging die, and try to reduce the wear of the forging die.

3) Roller ring. Roller ring refers to the production of ring parts with different diameters by special equipment roller ring machine, which is also used to produce wheel shaped parts such as car hub and train wheels.

4) Special forging. Special forging includes roll forging, cross wedge rolling, radial forging, liquid die forging and other forging methods, which are more suitable for the production of some special shape parts. For example, roll forging can be used as an effective preforming process to significantly reduce subsequent forming pressure; Cross wedge rolling can produce steel ball, transmission shaft and other parts; Radial forging can produce large - scale forgings such as gun barrel and step shaft.

5) Forging die
According to the movement mode of forging die, forging can be divided into swing forging, swing forging, roll forging, cross wedge rolling, ring rolling and diagonal rolling. Swing forging, swing forging and ring forging can also be finished by precision forging. In order to improve the utilization rate of materials, roll forging and cross rolling can be used as the first process of slender materials. Like free forging, rotary forging is also partial forming, its advantage is that compared with the forging size, forging force can be achieved under the condition of small formation. Including free forging, this forging mode, processing materials from near the die surface to the free surface extension, therefore, it is difficult to ensure accuracy, so, the direction of movement of the forging die and rotary forging sequence with computer control, can be used to obtain low forging force complex shape, high precision products, such as the production of many varieties, large size of steam turbine blades and other forgings.

The die movement of forging equipment is inconsistent with the degree of freedom. According to the deformation restriction characteristics of the bottom dead center, forging equipment can be divided into the following four forms:
Form of limited forging force: oil press directly driving the slider.
Quasi-stroke limiting mode: hydraulic drive crank-link mechanism hydraulic press.
Stroke limiting mode: Mechanical press with crank, connecting rod and wedge mechanism driving the slide block.
Energy limitation mode: Screw and friction press using screw mechanism.

In order to obtain high accuracy, attention should be paid to prevent overload at the bottom dead point, control speed and mold position. Because these will have an impact on forging tolerance, shape accuracy and forging die life. In addition, in order to maintain accuracy, attention should also be paid to adjusting the clearance of slide guide rail, ensuring stiffness, adjusting the bottom dead point and using the auxiliary transmission device and other measures.

There is also vertical and horizontal movement of the slider (used for forging slender parts, lubricating cooling and forging parts of high-speed production), the use of compensation devices can increase the movement in other directions. The above methods are different, the required forging force, process, material utilization rate, yield, dimensional tolerance and lubrication cooling mode are different, these factors are also influencing the level of automation.

3. Forging materials
Forging materials are mainly carbon steel and alloy steel of various components, followed by aluminum, magnesium, copper, titanium and their alloys. Materials in their original state are bars, ingots, 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. Proper selection of forging ratio, reasonable heating temperature and holding time, reasonable initial forging temperature and final forging temperature, reasonable deformation amount and deformation speed have great influence on improving product quality and reducing cost.

Generally, small and medium-sized forgings use round or square bar material as blank. The grain structure and mechanical properties of the bar are uniform and good, the shape and size are accurate, and the surface quality is good, which is convenient for mass production. As long as the heating temperature and deformation conditions are controlled reasonably, the forging with good performance can be produced without large forging deformation.

Ingot is only used for large forgings. Ingot is a cast structure with a large columnar crystal and loose center. Therefore, the columnar crystal must be broken into fine grains through large plastic deformation, and loose compaction, in order to obtain excellent metal structure and mechanical properties.

Powder forging can be made by pressing and firing the preform of powder metallurgy in hot state by die forging without flying edge. The forging powder is close to the density of common die forging parts, has good mechanical properties, and high precision, can reduce the subsequent cutting. Powder forgings have uniform internal structure and no segregation, which can be used to manufacture small gear and other workpieces. But the price of powder is much higher than that of general bar, and its application in production is limited.

By applying static pressure to the liquid metal poured into the die bore, it solidifies, crystallizes, flows, deforms and forms under the action of pressure, the die forging parts of desired shape and performance can be obtained. Liquid metal die forging is a forming method between die casting and die forging. It is especially suitable for complex thin wall parts which are difficult to be formed by common die forging.

In addition to the usual materials for forging, such as carbon steel and alloy steel of various components, followed by aluminum, magnesium, copper, titanium and other alloys, the deformation alloy of iron superalloy, nickel superalloy and cobalt superalloy is also completed by forging or rolling. However, because of the relatively narrow plastic zone of these alloys, the forging difficulty will be relatively large. The heating temperature of different materials, open forging temperature and final forging temperature have strict requirements.

4. Technological process
Different forging methods have different processes, among which hot die forging process is the longest, the general sequence is: forging blank blanking; Forging blank heating; Roll forging preparation blank; Die forging forming; Cutting edge; Punching; Correct; Intermediate inspection, inspection forgings size and surface defects; Heat treatment of forgings to eliminate forging stress and improve metal cutting performance; Cleaning, mainly to remove the surface oxide skin; Correct; Check, general forgings to go through the appearance and hardness inspection, important forgings after chemical composition analysis, mechanical properties, residual stress and other tests and non-destructive testing.

5, forging features
Compared with castings, the structure and mechanical properties of metal after forging can be improved. Due to metal deformation and recrystallization, the original coarse dendrites and columnar grains are transformed into equiaxed recrystallized grains with finer grains and uniform sizes. The original segregation, porosity, porosity and slag inclusion in the ingot are compacted and welded, and the structure becomes tighter, which improves the plastic and mechanical properties of the metal.

The mechanical properties of castings are lower than those of forgings of the same material. In addition, forging processing can ensure the continuity of metal fiber tissue, so that the fiber tissue of the forging and the shape of the forging remain consistent, the metal flow line is complete, can ensure that the parts have good mechanical properties and long service life. The forging produced by precision forging, cold extrusion, warm extrusion and other processes are incomparable to castings

Forgings are objects in which metal is pressed to shape a desired shape or suitable compression force through plastic deformation. This force is typically achieved through the use of a hammer or pressure. The forging process builds a fine granular structure and improves the physical properties of the metal. In the real use of parts, a correct design can make the particle flow in the direction of the main pressure. Castings are metal forming objects obtained by various casting methods, that is, the smelted liquid metal is injected into the pre-prepared casting mold by pouring, pressing, suction or other casting methods, and after cooling through sand dropping, cleaning and post-processing, the objects with a certain shape, size and performance are obtained.