Forging is a kind of processing method that uses forging machinery to exert pressure on metal blanks to produce plastic deformation to obtain forgings with certain mechanical properties, shape and size. Forging (forging and stamping) is one of the two major components. Forging can eliminate the defects such as as as as-cast looseness and optimize the micro-structure of metals in the smelting process. At the same time, the mechanical properties of forgings are generally better than those of castings of the same material because of the preservation of complete metal streamlines. Forgings are often used for important parts with high load and severe working conditions in related machinery, except for simple rolled plates, profiles or weldments.

It can be understood that forging is a method of forming iron blocks under the action of force by heating or not heating. Commonly known as: strike iron.

Deformation Temperature of Forging Materials

The starting recrystallization temperature of steel is divided into 800 C, hot forging is higher than 800 C, warm forging or Semi-Hot forging is called between 300 800 C, and cold forging is called 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 save material effectively.

Types of Forging

According to the forging temperature, it can be divided into hot forging, warm forging and cold forging.

According to the forging temperature, it can be divided into hot forging, warm forging and cold forging.

1、Free forging. It refers to the processing method of forgings with simple universal tools or direct external force between the upper and lower anvils of forging equipment to deform the blanks and obtain the required geometric shape and internal quality. The forgings produced by free forging method are called free forgings. Free forging is mainly used to produce forgings with small batches. Forging equipments such as hammer and hydraulic press are used to process the blanks, and qualified forgings are obtained. The basic processes of free forging include upsetting, drawing, punching, cutting, bending, torsion, offset and forging. Free forging is hot forging.

2、Die forging. Die forging is divided into open die forging and closed die forging. Metal blanks are compressed and deformed in a certain shape of the forging die chamber to obtain forgings. Die forging is generally used to produce parts with small weight and large batches.

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 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 implies, the materials are ferrous metals such as carbon steel, non-ferrous metals such as copper and aluminum, and powder metallurgical materials. Extrusion belongs to die forging, which can be divided into heavy metal extrusion and light metal extrusion. It should be noted that the blank can not be completely limited. Therefore, the volume of the blank should be strictly controlled, the relative position of the forging die should be controlled, and the forging parts should be measured so as to reduce the wear of the forging die.

3、Ring grinding. Ring rolling refers to the production of ring parts with different diameters by special equipment ring rolling machine. It is also used to produce wheel parts such as automobile hub and train wheel.

4、Special forging. Special forging includes roll forging, cross wedge rolling, radial forging, liquid forging and other forging methods, which are more suitable for the production of parts with special shapes.

For example, roll forging can be used as an effective pre-forming process, greatly reducing the subsequent forming pressure; cross wedge rolling can produce steel balls, transmission shafts and other parts; radial forging can produce large barrels, step shafts and other forgings.

forging die

According to the movement 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 forging and rotary forging are also available for precision forging of the first 400 MN (40,000 tons) heavy aviation die forging hydraulic press ring in China. In order to improve the utilization rate of materials, roll forging and cross rolling can be used as the preceding process of slender materials. Rotary forging, like free forging, is also locally formed. Its advantage is that compared with the size of forgings, it can also be formed with less forging force. This forging method, including free forging, expands the material from near the die surface to the free surface during processing, so it is difficult to guarantee the accuracy. Therefore, the movement direction of the forging die and the rotary forging process can be controlled by computer, and the products with complex shape and high precision can be obtained with lower forging force, such as the production of many varieties and sizes. Large steam turbine blades and other forgings.

The die movement of forging equipment is not consistent with the degree of freedom. According to the characteristics of deformation limitation at lower dead point, forging equipment can be divided into the following four forms:

1、Limited forging force form: hydraulic press which directly drives slider.

2、Quasi-stroke limit mode: hydraulic press with hydraulic drive crank-connecting rod mechanism.

3、Stroke limit mode: mechanical press with slider driven by crank, connecting rod and wedge mechanism.

4、Energy limitation: screw and friction press with screw mechanism. In order to obtain high accuracy, attention should be paid to preventing overload at lower dead point, controlling speed and die position in hot test of heavy aviation die forging hydraulic press. Because these will have an impact on forging tolerance, shape accuracy and die life. In addition, in order to maintain accuracy, attention should also be paid to adjusting the clearance of slider guide rail, ensuring stiffness, adjusting the lower dead point and utilizing subsidized transmission device.

Forging slider

Forging slider can be divided into vertical and horizontal motion (for forging slender parts, lubricating cooling and forging parts in high-speed production). The motion in other directions can be increased by using compensation device. The forging force, process, material utilization rate, output, dimension tolerance and lubrication and cooling mode are different for the first large disc product to be forged successfully. These factors also affect the level of automation.

Materials for forging

Forging materials are mainly carbon steel and alloy steel with various components, followed by aluminium, magnesium, copper, titanium and their alloys. The original state of the material is bar, ingot, metal powder and liquid metal. The ratio of cross-sectional area of metal before deformation to that after deformation is called 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 are very important to improve product quality and reduce cost. Round or square bars are commonly used as blanks for medium and small forgings. The grain structure and mechanical properties of the bar are uniform, 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, the forgings with good properties can be forged without large forging deformation. Ingot is only used for large forgings. The ingot is as-cast structure with large columnar crystal and loose center. Therefore, it is necessary to break columnar grains into fine grains through large plastic deformation and compact them loosely to obtain excellent metal structure and mechanical properties. Powder metallurgy preform, which is compacted and sintered, can be made into powder forgings by hot forging without flash. Forging powder is close to the density of general die forgings, has good mechanical properties and high accuracy, which can reduce the subsequent cutting process. Powder forging has uniform internal structure and no segregation. It can be used to manufacture small gears and other workpieces. However, the price of powder is much higher than that of bars, which limits its application in production. By applying static pressure to the liquid metal poured into the die chamber to solidify, crystallize, flow, plastic deformation and forming under the action of pressure, the required shape and properties of the die forgings can be obtained. Liquid metal die forging is a forming method between die casting and die forging, especially for complex thin-walled parts which are difficult to form in general die forging. In addition to the usual materials, such as carbon steel and alloy steel of various compositions, followed by aluminum, magnesium, copper, titanium and their alloys, the wrought alloys of iron-based superalloys, nickel-based superalloys and cobalt-based superalloys are also forged or rolled, except that the plastic zones of these alloys are relatively narrow. Therefore, forging is relatively difficult. There are strict requirements for heating temperature, open forging temperature and final forging temperature of different materials.

Forging process flow

Different forging methods have different processes, among which hot die forging has the longest technological process, and the general sequence is: blank blanking; forging billet heating; roll forging billet; die forging forming; trimming; punching; rectification; intermediate inspection to check the size and surface defects of forgings; forging heat treatment to eliminate forging stress and improve metal cutting performance; cleaning, mainly removing surface oxygen. Chemicals; Correction; Inspection, general forgings to pass the appearance and hardness inspection, important forgings also through chemical composition analysis, mechanical properties, residual stress testing and non-destructive testing.

Forging characteristics

Compared with castings, the structure and mechanical properties of metals can be improved by forging. Due to the deformation and recrystallization of metals, the coarse dendrites and columnar grains become equiaxed recrystallization structure with fine grains and uniform size after hot working deformation by forging method. The original segregation, porosity, porosity and slag inclusion in ingot are compacted and welded, and the structure becomes more compact and improved. Plasticity and mechanical properties of metals. The mechanical properties of castings are lower than those of forgings of the same material. In addition, forging process can ensure the continuity of metal fiber structure, make the fiber structure of forgings consistent with the shape of forgings, complete metal streamline, and ensure that parts have good mechanical properties and long service life. Forgings produced by precision die forging, cold extrusion, warm extrusion and other processes are incomparable to castings. Forgings are those in which the metal is subjected to pressure to shape the desired shape or suitable compressive force through plastic deformation. This force is typically achieved by the use of hammers or pressures. The forging process builds fine grain structure and improves the physical properties of metals. In the practical use of components, a correct design can make particles flow in the direction of main pressure. Casting is a metal forming object obtained by various casting methods, that is, the melted liquid metal is injected into the pre-prepared casting mold by pouring, injection, inhalation or other casting methods. After cooling, the object with certain shape, size and performance is obtained through sand drop, cleaning and post-treatment.

Forging Level Analysis

Forging level in China is developed on the basis of introducing, digesting and absorbing foreign technology. After years of technological development and transformation, the technology level of leading enterprises in the industry, including process design, forging technology, heat treatment technology, machining technology and product testing, has been greatly improved.

（1）Advanced manufacturers of process design generally adopt computer simulation technology, computer-aided process planning and virtual technology to improve the level of process design and product manufacturing capacity. DATAFOR, GEMARC/AUTOFORGE, DEFORM, LARSTRAN/SHAPE and THERMOCAL are introduced and applied to realize the process control of computer design and hot working.

（2）Most hydraulic press with forging technology of 40 MN or more are equipped with 100-400 T.M main forging manipulator and 20-40 T.M assistant manipulator. A considerable number of manipulators are controlled by computer, which realizes the comprehensive control of forging process. The forging accuracy can be controlled at ≤3 mm. Laser dimension measuring device is used for on-line measurement of forgings.

（3）Heat treatment technology focuses on improving product quality, heat treatment efficiency, energy saving and environmental protection. For example, the heating process of heating furnace and heat treatment furnace is controlled by computer, and the burner is controlled to realize automatic adjustment of combustion, temperature, ignition and heating parameter management; waste heat utilization and heat treatment furnace are equipped with regenerative combustion chamber; polymer quenching oil tank with low pollution ability and effective control of cooling is adopted. Water-based quenching medium gradually replaces traditional quenching oil.

（4）The proportion of CNC machine tools in the machine-processing technology industry has gradually increased. Some enterprises in the industry have machining centers. According to different types of products, they are equipped with special processing machinery, such as five-coordinate machining centers, blade processing machines, roll mills, roll lathes and so on.

（5）Some domestic enterprises have equipped with the latest testing instruments and testing technology, adopted modern automatic ultrasonic flaw detection system with computer-controlled data processing, and adopted various special automatic ultrasonic flaw detection systems to complete the certification of various quality systems. The key production technology of high-speed and heavy-duty gear forgings has been conquered continuously, and industrialized production has been realized on this basis. On the basis of introducing advanced production technology and key equipment from abroad, China has been able to design and manufacture its own production equipment for high-speed and heavy-duty gear forgings, which is close to the international advanced level. The upgrading of technology and equipment level has strongly promoted the development of domestic forging industry.

Importance of Forging

Forging production is one of the main processing methods to provide blanks for mechanical parts in mechanical manufacturing industry. By forging, not only the shape of mechanical parts can be obtained, but also the internal structure of metals can be improved, and the mechanical and physical properties of metals can be improved. Generally, forging is used to manufacture important mechanical parts with high stress and high requirement. For example, the shaft, rotor, impeller, blade, retaining ring, column of large hydraulic press, high pressure cylinder, roll of rolling mill, crankshaft of internal combustion engine, connecting rod, gear, bearing and artillery of national defense industry are all produced by forging. [7] Therefore, forging production is widely used in metallurgy, mining, automobile, tractor, harvesting machinery, petroleum, chemical industry, aviation, aerospace, weapons and other industrial sectors, and it also plays an important role in daily life. In a sense, the annual output of forgings, the proportion of die forgings in the total output of forgings, and the size and ownership of forging equipment reflect the industrial level of a country to a certain extent.