Forging is one of the two major components of forging (forging and stamping) by using a forging machine to apply pressure to a metal blank to plastically deform it to obtain a forging with certain mechanical properties, a certain shape and size. Forging can eliminate defects such as as-cast looseness caused by metal in the smelting process, optimize the microstructure, and at the same time, the mechanical properties of the forgings are generally superior to those of the same materials due to the preservation of the complete metal flow lines. For important parts of the relevant machinery with high load and severe working conditions, forgings are often used except for the available rolled sheets, profiles or welded parts.
The initial recrystallization temperature of steel is about 727 ° C, but 800 ° C is generally used as the dividing line. Above 800 ° C is hot forging; between 300 and 800 ° C is called warm forging or semi-hot forging, forging at room temperature. It is called cold forging. Forgings used in most industries are hot forging, warm forging and cold forging are mainly used for forging of parts such as automobiles and general machinery. Warm forging and cold forging can effectively save materials.
2. Forging category
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, rolling ring, and special forging.
1) Free forging
Refers to a method of processing a forging that uses a simple universal tool or an external force directly applied to the blank between the upper and lower anvil of the forging equipment to deform the blank to obtain the desired geometry and internal quality. Forgings produced by the free forging method are called free forgings. Free forging is mainly for the production of forgings with small quantities, and forgings are processed by forging equipment such as forging hammers and hydraulic machines to obtain qualified forgings. The basic processes of free forging include upsetting, drawing, punching, cutting, bending, twisting, misalignment and forging. Free forging is a hot forging method.
2) Die forging
Die forging is divided into open die forging and closed die forging. The metal blank is pressed and deformed in a forging die having a certain shape to obtain a forged piece, and the die forging is generally used for producing a part having a small weight and a large batch size. Die forging can be divided into hot forging, warm forging and cold forging. Warm forging and cold forging are the future development directions 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 implies, the materials are ferrous metals such as carbon steel, non-ferrous metals such as copper and aluminum, and powder metallurgy materials.
Extrusion should be attributed to die forging and can be divided into heavy metal extrusion and light metal extrusion.
Closed die forging and closed upset forging are two advanced processes for die forging. Since there is no flash, the material utilization rate is high. Finishing of complex forgings is possible with one or several processes. Since there is no flash, the area of force applied to the forging is reduced and the required load is also reduced. However, care should be taken not to completely limit the blank. To this end, the volume of the blank is strictly controlled, the relative position of the forging die is controlled, and the forging is measured to reduce the wear of the forging die.
3) Rolling ring
Rolling ring refers to the production of ring-shaped parts of different diameters by special equipment grinding machine, and also used to produce wheel-shaped parts such as automobile wheels and train wheels.
4) Special forging
Special forging includes roll forging, cross wedge rolling, radial forging, liquid die forging, etc. These methods are more suitable for the production of parts with special shapes. For example, roll forging can be used as an effective preforming process to significantly reduce subsequent forming pressures; cross wedge rolling can produce steel balls, drive shafts, etc.; radial forging can produce large forgings such as barrels and step shafts.
5) Forging die
According to the movement mode of the forging die, forging can be divided into pendulum, pendulum swivel, roll forging, cross wedge rolling, boring ring and cross rolling. Swing, swivel and shackle can also be used for precision forging. In order to improve the utilization of materials, roll forging and cross rolling can be used as a front-end process for slender materials. The same rotary forging as free forging is also partially formed, which has the advantage that it can be formed in the case of a smaller forging force than the forging size. This type of forging, including free forging, expands from the vicinity of the mold surface to the free surface during processing. Therefore, it is difficult to ensure accuracy. Therefore, the movement direction of the forging die and the swaging process can be controlled by computer. The forging force obtains products with complex shapes and high precision, such as forgings such as steam turbine blades with large variety and large size.
The die motion and degree of freedom of the forging equipment are inconsistent. According to the characteristics of the bottom dead center deformation, the forging equipment can be divided into the following four forms:
Limiting the form of forging force: hydraulic press that directly drives the slider.
Quasi-stroke limit mode: hydraulic press that drives the crank-link mechanism.
Stroke Limiting Mode: Mechanical presses that drive the slider with cranks, connecting rods and wedge mechanisms.
Energy limiting method: a screw and friction press using a screw mechanism.
In order to achieve high accuracy, care should be taken to prevent overload at the bottom dead center, control speed and mold position. Because these will have an impact on forging tolerances, shape accuracy and forging die life. In addition, in order to maintain accuracy, you should also pay attention to adjust the slider rail clearance, ensure the stiffness, adjust the bottom dead center and use the auxiliary transmission and other measures.
There are also slider vertical and horizontal movements (for forging of the elongated parts, lubrication cooling and high-speed production of parts forging), the use of compensation devices can increase the movement in other directions. Different methods are used, the required forging force, process, material utilization, production, dimensional tolerance and lubrication cooling method are different. 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 and the like and alloys thereof. The raw 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. Correct 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 a great relationship to improve product quality and reduce cost.
Generally, small and medium-sized forgings use round or square bars as blanks. 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 properly controlled, high-quality forgings can be forged without requiring large forging deformation.
Ingots are only used for large forgings. The ingot is an as-cast structure with large columnar crystals and a loose center. Therefore, it is necessary to break the columnar crystal into fine crystal grains by large plastic deformation, and loosely compact, in order to obtain excellent metal structure and mechanical properties.
The powder metallurgy preform which is pressed and sintered can be made into a powder forging by hot forging without flashing. Forging powder is close to the density of general die forgings, has good mechanical properties, and has high precision, which can reduce subsequent cutting. The powder forgings have a uniform internal structure and are not segregated and can be used to manufacture workpieces such as small gears. However, the price of powder is much higher than the price of ordinary bars, and its application in production is limited.
By applying static pressure to the liquid metal poured in the mold to solidify, crystallize, flow, plastically deform and form under pressure, a die forging of desired shape and properties can be obtained. Liquid metal die forging is a forming method between die casting and die forging, and is particularly suitable for complex thin-walled parts which are difficult to form by general die forging.
Forging materials in addition to the usual materials, such as various components of carbon steel and alloy steel, followed by aluminum, magnesium, copper, titanium and other alloys, iron-based superalloys, nickel-based superalloys, cobalt-based superalloys The deformed alloys are also finished by forging or rolling, except that the alloys are relatively narrow in plasticity, so the forging difficulty is relatively large, and the heating temperature, opening and forging temperature and final forging temperature of different materials have strict requirements.
4. The process
Different forging methods have different processes, wherein the hot forging process has the longest process, and the general sequence is: forging blanking; forging billet heating; roll forging stock; die forging; trimming; punching; Intermediate inspection, inspection of the dimensions and surface defects of forgings; forging heat treatment, to eliminate forging stress, improve metal cutting performance; cleaning, mainly to remove surface oxide scale; correction; inspection, general forgings to pass the appearance and hardness inspection, important forgings It is subject to chemical composition analysis, mechanical properties, residual stress and other tests and non-destructive testing.
5. Forging features
Compared with castings, metals can improve their microstructure and mechanical properties after forging. After the hot-working deformation of the cast structure by the forging method, the original coarse dendrites and columnar grains become the equiaxed recrystallized structure with fine grains and uniform size due to the deformation and recrystallization of the metal, so that the original segregation in the steel ingot, The compaction and welding of loose, stomata and slag inclusions make the structure more compact and improve the plasticity and mechanical properties of the metal.
The mechanical properties of castings are lower than those of forgings of the same material. In addition, the forging process can ensure the continuity of the metal fiber structure, so that the fiber structure of the forging is consistent with the shape of the forging, and the metal streamline is complete, which can ensure the good mechanical properties and long service life of the part by precision die forging and cold extrusion. Forgings produced by processes such as warm extrusion are incomparable to castings.
A forging is an object to which a metal is pressed to shape a desired shape or a suitable compressive force by plastic deformation. This power is typically achieved by using a hammer or pressure. The forging process creates a refined grain structure and improves the physical properties of the metal. In the practical use of components, a correct design enables the particles to flow in the direction of the main pressure. The casting is a metal molded object obtained by various casting methods, that is, the smelted liquid metal is poured into a pre-prepared mold by pouring, injecting, inhaling or other casting method, and after cooling, after falling sand, cleaning and then Processing, etc., the resulting article with a certain shape, size and performance.
Source: China Welded Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)