Forging is a processing method that uses forging machinery to exert pressure on metal billets to produce plastic deformation to obtain forgings with certain mechanical properties, shapes and sizes. Forging (forging and stamping) is one of the two major components. Forging can eliminate as cast porosity and other defects produced in the smelting process of metal, optimize the microstructure. At the same time, due to the preservation of complete metal streamline, the mechanical properties of forgings are generally superior to castings of the same material. For important parts with high load and severe working conditions in related machinery, forgings are mostly used, except for simple rolled plates, profiles or weldments.
The initial recrystallization temperature of steel is about 727 ℃, but 800 ℃ is generally used as the dividing line, and hot forging is used above 800 ℃; It is called warm forging or semi hot forging at 300~800 ℃, and cold forging at room temperature.
Forgings used in most industries are hot forging. Warm forging and cold forging are mainly used for forging parts such as automobiles and general machinery. Warm forging and cold forging can effectively save materials.
Process classification
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, ring rolling and special forging.
1. Free forging.
It refers to the processing method of forging which uses simple universal tools or directly applies external force to the blank between the upper and lower anvils of forging equipment to deform the blank and obtain the required geometric shape and internal quality. Forgings produced by free forging method are called free forgings. Free forging is mainly used to produce forgings with small batch. Forging hammer, hydraulic press and other forging equipment are used to shape the blank and obtain qualified forgings. The basic processes of free forging include upsetting, elongation, punching, cutting, bending, twisting, staggering and forging. Free forging adopts hot forging.
Advantages: free forging is characterized by flexible process, large universality of equipment and tools, low cost, etc. Since free forging is formed step by step and requires less deformation force, it is the only way to produce large forgings (above 300T).
Disadvantages: low productivity, low forging precision, and high labor intensity. It is mainly used for single piece and small batch production of forgings with simple shape and low precision requirements.
2. Die forging.
Die forging is also divided into open die forging and closed die forging. The metal blank is compressed and deformed in the forging die chamber with a certain shape to obtain forgings. Die forging is generally used to produce parts with small weight and large batch. Die forging can be divided into hot 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 materials, die forging can also be divided into ferrous metal die forging, nonferrous metal die forging and powder product forming. As the name implies, the materials are ferrous metals such as carbon steel, nonferrous metals such as copper and aluminum, 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 upsetting are two advanced processes of die forging. Because there is no flash, the utilization rate of materials is high. It is possible to finish complex forgings with one or several processes. Since there is no flash, the force bearing area of the forging is reduced, and the required load is also reduced. However, it should be noted that the blank cannot be completely restricted. For this reason, the volume of blank should be strictly controlled, the relative position of forging die should be controlled and the forging should be measured to try to reduce the wear of forging die.
3. Grinding ring.
Grinding ring refers to the production of annular parts with different diameters through special equipment ring mill, and also used to produce wheel shaped parts such as car hubs 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 producing parts with special shapes. For example, roll forging can be used as an effective pre forming process to greatly reduce the subsequent forming pressure; Cross wedge rolling can produce steel ball, transmission shaft and other parts; Radial forging can produce large gun barrel, step shaft and other forgings.
forging die
According to the motion mode of forging die, forging can be divided into rotary forging, rotary forging, roll forging, cross wedge rolling, ring rolling and cross rolling. Rotary forging, rotary rotary forging and ring rolling can also be processed by precision forging. In order to improve the utilization rate of materials, roll forging and cross rolling can be used as the previous working procedure of slender materials. Like free forging, rotary forging is also locally formed. Its advantage is that it can be formed when the forging force is small compared with the forging size. In this forging method, including free forging, the materials expand from the die face to the free surface during processing, so it is difficult to ensure the accuracy. Therefore, the computer control of the motion direction of the forging die and the rotary forging process can be used to obtain products with complex shape and high accuracy with low forging force, such as turbine blades with many varieties and large size.
The die movement and degree of freedom of the forging equipment are inconsistent. According to the deformation limitation characteristics of the lower dead center, the forging equipment can be divided into the following four forms:
1. Form of limited forging force: hydraulic press that directly drives the sliding block by oil pressure.
2. Quasi stroke limit mode: oil press that drives crank linkage.
3. Stroke limit mode: mechanical press with slider driven by crank, connecting rod and wedge mechanism.
4. Energy limiting mode: screw and friction press with screw mechanism.
In order to obtain high accuracy, attention should be paid to preventing overload at the bottom dead center, and controlling the speed and mold position. Because these will affect the forging tolerance, shape accuracy and die life. In addition, in order to maintain the accuracy, attention should also be paid to adjusting the slider guide clearance, ensuring the rigidity, adjusting the lower dead center and using auxiliary transmission devices and other measures.
slider
There are also two ways of sliding block vertical and horizontal movement (used for forging slender parts, lubrication and cooling, and forging parts for high-speed production), and the use of compensation devices can increase movement in other directions. The above methods are different. The required forging force, process, material utilization, output, dimensional tolerance and lubrication and cooling methods are different. These factors also affect the automation level.
Different forging methods have different processes, among which the process of hot die forging is the longest, and the general sequence is: blanking of forging blank; Forging billet heating; Roll forging preparation; Die forging forming; Trimming; Punching; Correction; Intermediate inspection, to inspect the size and surface defects of forgings; Forging heat treatment to eliminate forging stress and improve metal cutting performance; Cleaning, mainly to remove surface oxide scale; Correction; Inspection: general forgings shall be subject to appearance and hardness inspection, and important forgings shall also be subject to chemical composition analysis, mechanical property, residual stress inspection and non-destructive testing.
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