Process principle
Forging is a processing method that uses forging machinery to apply pressure to metal blanks to cause them to undergo plastic deformation in order to obtain forgings with certain mechanical properties, shapes and sizes. It is one of the two major components of forging (forging and stamping). Forging can eliminate defects such as loose cast metal produced during the smelting process and optimize the microstructure. At the same time, because the complete metal flow lines are preserved, the mechanical properties of forgings are generally better than those of castings of the same material. For important parts in related machinery with high loads and severe working conditions, forgings are mostly used, except for simpler shapes that can be rolled plates, profiles or welded parts.
The initial recrystallization temperature of steel is about 727℃, but 800℃ is generally used as the dividing line. Forging above 800℃ is called hot forging; forging between 300 and 800℃ is called warm forging or semi-hot forging, and forging at room temperature is called cold forging.
Forgings used in most industries are hot forged. Warm forging and cold forging are mainly used for forging parts of automobiles, general machinery, etc. 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.
Refers to a processing method that uses simple universal tools or directly applies external force to the blank between the upper and lower anvils of the forging equipment to deform the blank to obtain the required geometric shape and internal quality of the forging. Forgings produced by free forging are called free forgings. Free forging is mainly used to produce forgings with small batches. Forging equipment such as forging hammers and hydraulic presses are used to form the blanks to obtain qualified forgings. The basic processes of free forging include upsetting, drawing, punching, cutting, bending, twisting, shifting and forging. Free forging is all hot forging.
Advantages: Free forging has the characteristics of flexible process, high versatility of equipment and tools used, and low cost. Since free forging is gradually formed and the required deformation force is small, it is the only method to produce large forgings (more than 300 tons).
Disadvantages: Low productivity, low forging precision, high labor intensity, mostly used for single-piece and small-batch production of forgings with simple shapes and low precision requirements.
2. Die forging.
Die forging is divided into open die forging and closed die forging. The metal billet is deformed under pressure in a forging die with a certain shape to obtain a forging. Die forging is generally used to produce parts with small weight and large batches. 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 ferrous metal die forging, non-ferrous metal die forging and powder product forming. As the name suggests, the materials are ferrous metals such as carbon steel, non-ferrous metals such as copper and aluminum, and powder metallurgy materials. 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 are two advanced processes of die forging. Since there is no flash, the material utilization rate is high. The finishing of complex forgings can be completed in one or several processes. Since there is no flash, the force area of the forging is reduced, and the required load is also reduced. However, care should be taken not to completely restrict the blank; to this end, the volume of the blank should be strictly controlled, the relative position of the forging die should be controlled, the forging should be measured, and efforts should be made to reduce the wear of the forging die.
3. Ring rolling.
Ring rolling refers to the production of ring parts of different diameters by special equipment ring rolling machine. It is also used to produce wheel-shaped parts such as automobile hubs and train wheels.
4. Special forging.
Special forging includes roll forging, wedge cross rolling, radial forging, liquid die forging and other forging methods, which are more suitable for producing parts with certain special shapes. For example, roll forging can be used as an effective pre-forming process to greatly reduce the subsequent forming pressure; wedge cross rolling can produce parts such as steel balls and transmission shafts; radial forging can produce large barrels, step shafts and other forgings.
Forging die
According to the movement mode of the forging die, forging can be divided into pendulum rolling, pendulum rotary forging, roll forging, wedge cross rolling, ring rolling and oblique rolling. Pendulum rolling, pendulum 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 front process of slender materials. Like free forging, rotary forging is also partially formed. Its advantage is that it can be formed even when the forging force is small compared with the size of the forging. In this forging method, including free forging, the material expands from the vicinity of the die surface to the free surface during processing. Therefore, it is difficult to ensure accuracy. Therefore, by controlling the movement direction of the forging die and the rotary forging process with a computer, products with complex shapes and high precision can be obtained with lower forging forces, such as forgings such as turbine blades with many varieties and large sizes.
The die movement and degree of freedom of forging equipment are inconsistent. According to the deformation restriction characteristics of the bottom dead point, forging equipment can be divided into the following four forms:
1. Forging force limitation form: hydraulic press with hydraulic pressure directly driving the slider.
2. Quasi-stroke limitation method: hydraulic press with hydraulic pressure driving the crank-connecting rod mechanism.
3. Stroke limitation method: mechanical press with crank, connecting rod and wedge mechanism driving the slider.
4. Energy limitation method: spiral and friction press using spiral mechanism.
In order to obtain high precision, attention should be paid to preventing overload at the bottom dead center, controlling speed and die position, because these will affect the tolerance, shape accuracy and die life of the forging. In addition, in order to maintain accuracy, attention should also be paid to adjusting the clearance of the slider guide rail, ensuring rigidity, adjusting the bottom dead center and using auxiliary transmission devices.
Slider
There are also vertical and horizontal movement modes of the slider (used for forging of slender parts, lubrication and cooling, and high-speed production of parts forging). The use of compensation devices can increase movement in other directions. The above methods are different, and the required forging force, process, material utilization, output, dimensional tolerance and lubrication and cooling methods are different. These factors also affect the level of automation.
Different forging methods have different processes. Among them, the process of hot die forging is the longest. The general order is: blanking; heating of forging billet; roll forging preparation; die forging; trimming; punching; correction; intermediate inspection, inspection of forging size and surface defects; heat treatment of forgings to eliminate forging stress and improve metal cutting performance; cleaning, mainly to remove surface oxide scale; correction; inspection, general forgings must undergo appearance and hardness inspection, and important forgings must also undergo chemical composition analysis, mechanical properties, residual stress and other inspections and non-destructive testing.
