Forging related knowledge overview

Forging is a processing method that uses forging machinery to apply pressure to metal billets, causing them to undergo plastic deformation to obtain forgings with certain mechanical properties, shapes, and sizes. It is one of the components of forging (forging and stamping). Forging can eliminate defects such as casting porosity that occur during the smelting process of metals, optimize the microstructure, and at the same time, due to the preservation of intact metal flow lines, the mechanical properties of forgings are generally better than castings of the same material. In related machinery, important parts with high loads and severe working conditions are often forged, except for simple shaped plates, profiles, or welded parts that can be rolled.

01 Deformation temperature

The starting recrystallization temperature of steel is about 727 ℃, but 800 ℃ is commonly used as the dividing line, and those above 800 ℃ are hot forged; Forging between 300 and 800 ℃ is called warm forging or semi hot forging, while forging at room temperature is called cold forging. The forgings used in most industries are hot forging, while 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.

02 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, ring rolling, and special forging.

1) Free forging

The processing method of forging a workpiece with the required geometric shape and internal quality by using simple universal tools or applying external force directly between the upper and lower anvils of forging equipment to deform the billet. The forgings produced by the free forging method are called free forgings. Free forging is mainly used to produce small batch forgings, and forging equipment such as forging hammers and hydraulic presses are used to shape and process the blanks to obtain qualified forgings. The basic processes of free forging include upsetting, stretching, punching, cutting, bending, twisting, misalignment, and forging. Free forging adopts hot forging method.

2) Die forging

Die forging is divided into open die forging and closed die forging. Metal billets are compressed and deformed in a forging die with a certain shape to obtain forgings. Forging is generally used to produce parts with small weight and large batch sizes. Forging can be divided into hot forging, warm forging, and cold forging. Warm forging and cold forging are the future development directions of forging, and also represent the level of forging technology.

According to material classification, die forging can also be divided into black metal die forging, non-ferrous metal die forging, and powder product forming. As the name suggests, the materials are black metals such as carbon steel, non-ferrous metals such as copper and aluminum, and powder metallurgy materials.

Extrusion belongs to forging and can be divided into heavy metal extrusion and light metal extrusion.

Closed die forging and closed forging are two advanced processes of die forging. Due to the absence of burrs, the utilization rate of materials is high. It is possible to complete the precision machining of complex forgings with one or several processes. Due to the absence of burrs, the stress area of the forging is reduced, and the required load is also reduced. However, it should be noted that the blank should not be completely restricted. To achieve this, the volume of the blank should be strictly controlled, the relative position of the forging die should be controlled, and measurements of the forging should be taken to minimize the wear of the forging die.

3) Rolling ring

Ring rolling refers to the production of circular parts with different diameters using specialized equipment such as ring rolling machines, and is also used to produce wheel shaped parts such as car hubs and train wheels.

4) Special forging

Special forging methods include roll forging, cross wedge rolling, radial forging, liquid forging, etc., all of which are suitable for producing certain special shaped parts. For example, roll forging can serve as an effective pre forming process, significantly reducing the subsequent forming pressure; Wedge cross rolling can produce steel balls, transmission shafts and other parts; Radial forging can produce large forged parts such as gun barrels and step shafts.

5) Forging die

According to the movement mode of the forging die, forging can be divided into swing forging, swing rotary forging, roll forging, cross wedge rolling, ring rolling, and oblique rolling. Rotary forging, 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 preceding processes for processing slender materials. Rotary forging, like free forging, is also locally formed, and its advantage is that it can be formed with less forging force compared to the size of the forging. This forging method, including free forging, expands the material from the vicinity of the mold surface to the free surface during processing, making it difficult to ensure accuracy. Therefore, by controlling the movement direction of the forging die and the forging process with a computer, complex shaped and high-precision products can be obtained with lower forging force, such as producing a variety of large turbine blades and other forgings.

The motion and degrees of freedom of forging equipment molds are inconsistent and can be divided into the following four forms:

Limit the form of forging force

A hydraulic press that directly drives the slider with hydraulic pressure.

Quasi stroke restriction method

A hydraulic press with a crank connecting rod mechanism driven by hydraulic pressure.

Stroke restriction method

A mechanical press with a slider driven by a crank, connecting rod, and wedge mechanism.

Energy restriction method

Utilize a spiral mechanism and friction press.

In order to achieve high precision, attention should be paid to preventing overload at the bottom dead center, controlling speed and mold position. Because all of these will have an impact on forging tolerances, shape accuracy, and forging die life. In addition, in order to maintain accuracy, attention should be paid to adjusting the clearance between the slider guide rails, ensuring stiffness, adjusting the bottom dead center, and using auxiliary transmission devices.

There are also vertical and horizontal movements of the slider (used for forging slender parts, lubrication and cooling, and forging high-speed production parts), and compensation devices can be used to increase movement in other directions. The above methods are different, and the required forging force, process, material utilization rate, output, dimensional tolerances, and lubrication and cooling methods are also factors that affect the level of automation.

03 Forging materials

The forging materials mainly consist of carbon steel and alloy steel with various compositions, followed by aluminum, magnesium, copper, titanium and their alloys. The original states of materials include bar, ingot, metal powder, and liquid metal. The ratio of the cross-sectional area of a 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 and final forging temperature, reasonable deformation amount and deformation speed are closely related to improving product quality and reducing costs.

Generally, circular or square bar materials are used as blanks for small and medium-sized forgings. The grain structure and mechanical properties of the bar material are uniform and good, with accurate shape and size, good surface quality, and easy to organize for mass production. As long as the heating temperature and deformation conditions are reasonably controlled, high-quality forgings can be forged without significant forging deformation.

Ingots are only used for large forgings. Ingot is a cast structure with large columnar crystals and loose centers. Therefore, it is necessary to crush the columnar crystals into fine grains through large plastic deformation, loosen and compact them, in order to obtain excellent metal structure and mechanical properties.

Powder metallurgy preforms formed by pressing and firing can be made into powder forgings by non flash forging in the hot state. The density of forging powder is close to that of general die forgings, with good mechanical properties and high precision, which can reduce subsequent cutting processing. The internal structure of powder forgings is uniform without segregation, and can be used to manufacture small gears and other workpieces. However, the price of powder is much higher than that of general bar materials, which limits its application in production.

By applying static pressure to the liquid metal poured into the mold cavity, it can solidify, crystallize, flow, undergo plastic deformation, and form under pressure to obtain the desired shape and properties of the forging. Liquid metal 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 conventional materials such as aluminum, magnesium, copper, titanium, and their alloys, the deformation alloys of iron-based high-temperature alloys, nickel based high-temperature alloys, and cobalt based high-temperature alloys are also completed by forging or rolling. However, these alloys have relatively narrow plastic zones, making forging relatively difficult. Different materials have strict requirements for heating temperature, forging temperature, and final forging temperature.

04 Process flow

Different forging methods have different processes, among which the process flow of hot forging is the longest, generally in the order of: forging blank cutting → forging blank heating → roll forging preparation → forging forming → edge cutting → punching → correction → intermediate inspection to inspect the size and surface defects of the forging → forging heat treatment to eliminate forging stress and improve metal cutting performance → cleaning, mainly removing surface oxide skin → correction → inspection. Generally, forgings need to undergo appearance and hardness inspection, and important forgings also need to undergo chemical composition analysis, mechanical properties, residual stress testing, and non-destructive testing.

05 Characteristics of forgings

Compared with castings, metals can improve their microstructure and mechanical properties after forging processing. After hot working deformation through forging method, the casting structure transforms from coarse dendrites and columnar grains to equiaxed recrystallized structures with finer and uniformly sized grains due to metal deformation and recrystallization. This compacts and welds the segregation, looseness, porosity, slag inclusions, etc. inside the steel ingot, making the structure more compact and improving 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, forging processing can ensure the continuity of the metal fiber structure, keeping the fiber structure of the forging consistent with the shape of the forging, and ensuring the integrity of the metal streamline, which can ensure that the parts have good mechanical properties and long service life. The forgings produced by precision forging, cold extrusion, warm extrusion and other processes are incomparable to castings.

A forging is an object made of metal that is subjected to pressure and shaped into a desired shape or suitable compressive force through plastic deformation. This power is typically achieved through the use of a hammer or pressure. The forging process creates a refined particle structure and improves the physical properties of the metal. In the practical use of components, a correct design can enable particle flow in the direction of the main pressure. Casting is a metal formed object obtained by various casting methods, that is, smelted liquid metal is injected into a pre prepared mold by casting, injection, suction or other casting methods, cooled, and then subjected to sand removal, cleaning and post-treatment to obtain an object with a certain shape, size and performance.