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The initial recrystallization temperature of steel is about 727 °C, but 800 °C is generally used as the dividing line, and hot forging is higher than 800 °C; Between 300~800 °C is called warm forging or semi-hot forging, and forging at room temperature is called cold forging. 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, and warm forging and cold forging can effectively save materials.

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 open forging, die forging, ring grinding, and special forging.

1) Open forging

It refers to the processing method of forging with a simple general-purpose tool, or directly applying external force to the blank between the upper and lower anvils of the forging equipment to deform the blank and obtain the required geometric shape and internal quality. Forgings produced by the open forging method are called open forgings. Free forging is mainly based on the production of forgings with small batches, and forging hammers, hydraulic presses and other forging equipment are used to form and process the blanks to obtain qualified forgings. The basic processes of open forging include upsetting, drawing, punching, cutting, bending, twisting, staggering and forging. Open forging adopts the hot forging method.

2) Die forging

Die forging is divided into open die forging and closed die forging. The metal billet is deformed under pressure in the forging die chamber with a certain shape to obtain forgings, and 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 molding. 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 attributed to die forging, which can be divided into heavy metal extrusion and light metal extrusion.

Closed die forging and closed upsetting belong to the two advanced processes of die forging, because there is no flash, the utilization rate of the material is high. It is possible to finish complex forgings in one or several processes. Since there is no flash, the force area of the forging is reduced, and the required load is reduced. However, it should be noted that the blank should not be completely restricted, so the volume of the billet should be strictly controlled, the relative position of the forging die should be controlled and the forging should be measured, and efforts should be made to reduce the wear of the forging die.

3) Rolling rings

Ring rolling refers to the production of ring parts of different diameters through special equipment ring rolling machines, and is also used to produce wheel parts such as automobile wheels 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 the production of some special shapes of parts. For example, roll forging can be used as an effective preform process to significantly reduce the subsequent forming pressure; Wedge cross-rolling can produce steel balls, transmission shafts and other parts; Radial forging can produce large forgings such as gun barrels and step shafts.

5) Forging dies

According to the movement mode of the forging die, forging can be divided into swing rolling, swing rotary forging, roll forging, wedge cross-rolling, ring rolling and oblique rolling. Pendulum rolling, pendulum rotary forging and ring rolling can also be processed with precision forging. In order to improve the utilization of materials, roll forging and cross-rolling can be used as a pre-process processing of slender materials. Rotary forging, like open forging, is also partially formed, and it has the advantage that it can be formed even with a small forging force compared to the size of the forging. In this forging method, including open forging, the material expands from the vicinity of the die surface to the free surface during processing, so it is difficult to ensure accuracy, so the direction of movement of the forging die and the rotary forging process are controlled by a computer, and a product with complex shape and high precision can be obtained with a low forging force, such as forging parts such as steam turbine blades that produce a wide variety and large size.

The mold movement and degree of freedom of forging equipment are inconsistent, which can be divided into the following four forms:

In order to achieve high accuracy, care should be taken to prevent overload at the bottom dead center and to control the speed and mold position. Because these will affect the forging tolerance, shape accuracy and forging die life. In addition, in order to maintain accuracy, attention should also be paid to adjusting the slider guide rail clearance, ensuring stiffness, adjusting the bottom dead center and using auxiliary transmission devices.

There are also ways to move vertically and horizontally (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. Depending on the above methods, the required forging force, process, material utilization, output, dimensional tolerance, and lubrication and cooling methods are different, and these factors are also factors that affect the level of automation.

The forging materials are mainly carbon steel and alloy steel of various compositions, followed by aluminum, magnesium, copper, titanium, etc. and their alloys. 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. The 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 with improving product quality and reducing costs.

Generally, small and medium-sized forgings use round or square bars as blanks. The grain structure and mechanical properties of the bar stock 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 reasonably controlled, forgings with excellent performance can be forged without large forging deformation.

Ingot casting is only used for large forgings. Ingot is a cast-like structure with large columnar crystals and a loose center. Therefore, it is necessary to break the columnar crystals into fine grains through large plastic deformation, and compact them loosely to obtain excellent metal structure and mechanical properties.

The powder metallurgy prefabricated billet formed by pressing and sintering can be made into powder forgings by flashless die forging in the hot state. The forging powder is close to the density of general die forgings, has good mechanical properties, and has high precision, which can reduce the subsequent cutting processing. Powder forgings have a uniform internal structure and no segregation, which can be used to manufacture workpieces such as small gears. However, the price of powder is much higher than the price of general bars, and its application in production is limited to a certain extent.

Die forgings of the desired shape and properties can be obtained by applying static pressure to the liquid metal poured in the mold chamber to solidify, crystallize, flow, plastically deform and form under the action of pressure. Liquid metal die forging is a forming method between die casting and die forging, especially suitable for complex thin-walled parts that are difficult to form by general die forging.

In addition to the usual materials and aluminum, magnesium, copper, titanium, etc. and their alloys, iron-based superalloys, nickel-based superalloys, cobalt-based superalloys of deformed alloys are also completed by forging or rolling, but these alloys are relatively difficult to forge due to their relatively narrow plastic area, and the heating temperature of different materials, the opening temperature and the final forging temperature have strict requirements.

Different forging methods have different processes, among which the process flow of hot die forging is the longest, and the general sequence is: forging blank blanking→ forging billet heating→ roll forging preparation billet→die forging forming→ trimming→ punching→ correction → intermediate inspection, inspection of the size and surface defects of forgings→ heat treatment of forgings, in order to eliminate forging stress, improve metal cutting performance→ cleaning, mainly to remove surface oxide scale→ correction → inspection, general forgings to go through appearance and hardness inspection, important forgings also to go through chemical composition analysis, mechanical properties, Residual stress and other inspections and non-destructive testing.

05

Compared with castings, the microstructure and mechanical properties of metals can be improved after forging. After the casting structure is deformed by hot processing by the forging method, due to the deformation and recrystallization of the metal, the original coarse dendrite and columnar grains become the equiaxed recrystallization structure with finer grains and uniform size, so that the original segregation, looseness, porosity, slag inclusion and other compaction and welding in the steel ingot, its structure becomes more compact, and the plasticity and mechanical properties of the metal are improved.

The mechanical properties of castings are lower than those of forgings of the same material. In addition, forging can ensure the continuity of the metal fiber structure, make the fiber structure of the forging consistent with the shape of the forging, and the metal streamline is complete, which can ensure that the parts have good mechanical properties and long service life, and the forgings produced by precision die forging, cold extrusion, warm extrusion and other processes are incomparable to castings.

A forging is an object in which the metal is subjected to pressure to shape the desired shape or the appropriate compressive force through plastic deformation. This force is typically achieved through the use of a hammer or pressure. The forging process builds a delicate grain structure and improves the physical properties of the metal. In the real-world use of the part, a correct design will allow the particles to 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, injection, suction or other casting methods, and after cooling, it is sanded, cleaned and post-treated, etc., and the object with a certain shape, size and performance is obtained.