Forging knowledge: what is forging?

What is forging?

Forging is a processing method that uses forging machinery to exert pressure on metal blank to make it plastic deformation, so as to obtain forgings with certain mechanical properties, certain shape and size. Forging and stamping are both plastic processing properties, collectively referred to as forging.
Forging is a common forming method in mechanical manufacturing. Forging can eliminate the as cast looseness and welding holes of metal, and the mechanical properties of forgings are generally superior to those of castings of the same material. For important parts with high load and severe working conditions in machinery, forgings are mostly used, except for simple rolled plates, profiles or weldments.

Forging knowledge: what is forging?

Characteristics of forgings

Compared with castings, the microstructure and mechanical properties of metals can be improved after forging. Due to the deformation and recrystallization of the metal, the original coarse dendrites and columnar grains are transformed into equiaxed recrystallized structures with finer grains and uniform size after the casting structure is deformed by hot working by forging method. The original segregation, looseness, porosity, slag inclusion, etc. in the ingot are compacted and welded, and the structure becomes more compact, which improves the plasticity and mechanical properties of the metal material.
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, keep the fiber structure of the forging consistent with the shape of the forging, and ensure that the metal streamline is complete. It can ensure that the 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.
Forging is an article in which metal is pressed to shape the required shape or suitable compression force through plastic deformation. This force is typically achieved by using a hammer or pressure. The forging process creates a refined granular structure and improves the physical properties of the metal. In the practical use of components, a correct design can make the particle flow in the direction of the main pressure. Castings are metal shaped objects obtained by various casting methods, that is, the molten liquid metal is poured into the prepared mold by pouring, injection, suction or other casting methods, and the objects with certain shape, size and performance are obtained after cooling, sand falling, cleaning and post-treatment.

Classification of forging

According to different production tools, forging technology can be divided into free forging, module forging, roller ring forging and special forging.

  • Free forging: refers to the processing method of forging which uses simple universal tools or directly exerts external force on the blank between the upper and lower anvils of forging equipment to deform the blank and obtain the required geometric shape and internal quality.
  • Die forging: refers to the forging obtained by the compression deformation of the metal blank in the forging die chamber with a certain shape. 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.
  • Grinding ring: It refers to the ring parts with different diameters produced by special equipment grinding ring machine, and also used to produce wheel shaped parts such as car hubs and train wheels.
  • Special forging: including roll forging, cross wedge rolling, radial forging, liquid die 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 to greatly reduce the subsequent forming pressure; Cross wedge rolling can produce steel ball, transmission shaft and other parts; Radial forging can produce large gun barrel, step shaft and other forgings.

According to forging temperature, forging technology can be divided into hot forging, warm forging and cold forging.
The initial recrystallization temperature of steel is about 727 ℃, but 800 ℃ is generally used as the dividing line, and hot forging is used above 800 ℃; It is called warm forging or semi hot forging at 300~800 ℃, and cold forging 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 effectively save materials.
According to the motion mode of forging die, forging can be divided into rotary forging, rotary forging, roll forging, cross wedge rolling, ring rolling and cross rolling.

Forging materials

Forging materials are mainly carbon steel and alloy steel of various compositions, followed by aluminum, magnesium, copper, titanium and their alloys, iron base superalloys, nickel base superalloys, and cobalt base superalloys. Deformed alloys are also forged or rolled. However, due to their relatively narrow plastic zones, these alloys are relatively difficult to forge. There are strict requirements for the heating temperature, opening and final forging temperatures of different materials.
The original state of materials includes bar stock, ingot, metal powder and liquid metal. The ratio of cross sectional area of metal before deformation to that after deformation is called 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 have a great relationship to improve product quality and reduce cost.

Forging process

Different forging methods have different processes, among which the process of hot die forging is the longest, and the general sequence is: blanking of forging blank; Forging billet heating; Roll forging preparation; Die forging forming; Trimming; Punching; Correction; Intermediate inspection, to inspect the size and surface defects of forgings; Forging heat treatment to eliminate forging stress and improve metal cutting performance; Cleaning, mainly to remove surface oxide scale; Correction; Inspection: general forgings shall be subject to appearance and hardness inspection, and important forgings shall also be subject to chemical composition analysis, mechanical property, residual stress inspection and non-destructive testing.

Forging defects and analysis

The raw materials for forging are ingots, rolled materials, extruded materials and forging blanks. The rolled products, extruded products and forged billets are semi-finished products formed by rolling, extruding and forging of ingots respectively. In general, the appearance of internal defects or surface defects of ingots is sometimes inevitable. In addition, the improper forging process in the forging process eventually led to defects in the forgings. The following is a brief introduction to some common defects in forgings.

Forging defects caused by defects of raw materials usually include:

Surface crack: the surface crack occurs on the rolled bar and forged bar, generally in a linear shape, consistent with the main deformation direction of rolling or forging. There are many reasons for this defect, for example, the subcutaneous bubbles in the ingot stretch along the deformation direction while rolling, while exposing to the surface and developing toward the interior. For example, if the surface of the billet is scratched during rolling, the stress concentration will be caused during cooling, which may lead to cracks along the scratch, etc. If such cracks are not removed before forging, they may expand during forging and cause forging cracks.
Folding: Folding is caused by the incorrect sizing of groove on the roll or the burr generated on the wear surface of groove being involved during rolling, which forms a folding seam with a certain angle to the material surface. For steel, there are iron oxide inclusions in the crease and decarburization around. Folding, if not removed before forging, may cause folding or cracking of forgings.
Scab: Scab is a layer of peelable film on local area of rolled material surface.
The formation of scab is due to the splashing of molten steel during casting, which condenses on the surface of the ingot. When rolling, it is pressed into a film and attached to the surface of the rolled material, which is called scab. After the forging is cleaned by pickling, the film will peel off and become the surface defect of the forging.
Layered fracture: The characteristic of layered fracture is that its fracture or section is very similar to the broken slate and bark.
Layered fracture occurs mostly in alloy steel (chromium nickel steel, chromium nickel tungsten steel, etc.), and also in carbon steel. This defect is caused by non-metallic inclusions, dendrite segregation, porosity and other defects in the steel, which are stretched along the rolling direction during forging and rolling, making the steel sheet. If there are too many impurities, there is a danger of delamination and fracture in forging. The more serious the layered fracture, the worse the plasticity and toughness of the steel, especially the low transverse mechanical properties, so the steel with obvious lamellar defects is unqualified
Bright line (bright area): The bright line is a thin reflective line with crystal brightness on the longitudinal fracture, most of which run through the entire fracture, and most of which are produced in the axial part.
The bright line is mainly caused by alloy segregation. Slight bright lines have little effect on mechanical properties, while severe bright lines will significantly reduce the plasticity and toughness of materials.
Non metallic inclusions: non-metallic inclusions are mainly formed during the cooling process of molten steel smelting or casting due to chemical reactions between components or between metal and furnace gas or containers. In addition, during metal smelting and casting, as the refractory falls into the molten steel, inclusions can also be formed, which are collectively referred to as slag inclusions. On the cross section of forgings, non-metallic inclusions can be distributed in the form of dots, sheets, chains or lumps. Serious inclusions are easy to cause cracks in forgings or reduce the service performance of materials.
Carbide segregation: Carbide segregation often occurs in alloy steels with high carbon content. It is characterized by more carbide accumulation in local areas. It is mainly caused by ledeburite eutectic carbides and secondary network carbides in steel, which are not broken and uniformly distributed during bloom and rolling. Carbide segregation will reduce the forging deformation performance of steel and easily cause the forging cracking. Forgings are prone to local overheating, over burning and quenching cracks during heat treatment and quenching.
Aluminum alloy oxide film: aluminum alloy oxide film is generally located on the web of die forgings and near the parting surface. It is a tiny crack in the macrostructure and a whorl in the macrostructure. The characteristics of the fracture can be divided into two categories: first, it is flat and flaky, with the color ranging from silver gray, light yellow to brown and dark brown; Second, small, dense and shiny dots.
The oxide film of aluminum alloy is formed when the open molten liquid surface interacts with water vapor or other metal oxides in the atmosphere during the casting process and is rolled into the interior of liquid metal during the casting process.
The oxide film in forgings and die forgings has no obvious effect on the longitudinal mechanical properties, but has a greater impact on the mechanical properties in the height direction. It reduces the strength properties in the height direction, especially the elongation, impact toughness and corrosion resistance in the height direction.
White spots: white spots are mainly characterized by round or oval silver white spots on the longitudinal fracture of the billet, and small cracks on the transverse fracture. The size of white dots varies from 1 to 20 mm or longer. White spots are common in nickel chromium steel, nickel chromium molybdenum steel and other alloy steels, and are also found in ordinary carbon steel. They are hidden defects. White spots are produced under the joint action of hydrogen, structural stress during phase transformation and thermal stress. They are more likely to occur when there is more hydrogen in the steel and the cooling (or heat treatment after forging) after hot pressure processing is too fast.
Forgings forged from steel with white spots are easy to crack during heat treatment (quenching), and sometimes even fall into pieces. White spots reduce the plasticity of steel and the strength of parts. They are stress concentration points. Like sharp cutters, they can easily become fatigue cracks under the action of alternating loads, leading to fatigue damage. Therefore, white spots are absolutely not allowed in forging raw materials.
Coarse crystal ring: Coarse crystal ring is usually a defect on aluminum alloy or magnesium alloy extruded bars.
The extruded bars of aluminum and magnesium alloys supplied after heat treatment often have coarse crystal rings on the outer layer of their circular sections. The thickness of coarse crystal ring increases gradually from the beginning to the end of extrusion. If the lubrication condition during extrusion is good, coarse crystal rings can be reduced or avoided after heat treatment. On the contrary, the ring thickness will increase.
The cause of coarse crystal ring is related to many factors. But the main factor is the friction between the metal and the extrusion cylinder during the extrusion process. This kind of friction results in the crushing degree of the outer layer grains of the extruded bar cross section is much greater than that of the grains at the center of the bar. However, due to the influence of the cylinder wall, the temperature in this area is low, and it is not completely recrystallized during extrusion. During quenching and heating, the recrystallized grains recrystallize and grow up to swallow the recrystallized grains, so a coarse crystal ring is formed on the surface.
The billet with coarse crystal ring is easy to crack during forging. If the coarse crystal ring remains on the surface of the forging, the performance of the part will be reduced.
Pipe shrinkage residue: The pipe shrinkage residue is generally caused by the concentrated shrinkage cavity generated in the riser of the steel ingot which is not removed completely and remains in the steel during bloom and rolling.
Dense inclusions, looseness or segregation will generally occur in the area near the shrinkage tube residue. A gap with irregular folds in the transverse macrostructure. Forgings are easy to crack during forging or heat treatment.

Defects Caused by Improper Material Preparation and Their Effects on Forgings

The defects caused by improper material preparation are as follows:
Inclined cutting: the inclined amount of the blank end surface relative to the longitudinal axis exceeds the specified allowable value because the bar is not compressed when the sawing machine or the punch is loading and unloading. Severe shearing inclination may cause folding during forging.
The end of the blank is bent and has burrs: when loading and unloading materials on the shearing machine or punch, the gap between the scissors or cutting die edges is too large or the cutting edge is not sharp, so that the blank has been bent before being cut. As a result, part of the metal is squeezed into the gap between the blade or die, forming the end drooping burrs.
The billets with burrs are prone to local overheating and overburning during heating, and folding and cracking during forging.
Blank end face depression: when loading and unloading on the shear machine, due to the small gap between the scissors, the upper and lower cracks of the metal section do not coincide, resulting in secondary shearing. As a result, part of the end metal is pulled off, and the end face is concave. Such blanks are easy to fold and crack during forging.
End crack: when cold shearing large section alloy steel and high carbon steel bars, cracks are often found at the end 3~4h after shearing. It is mainly because the unit pressure of the blade is too large, so that the round section of the blank is compressed into an ellipse, at this time, a large internal stress is generated in the material. The flattened end face strives to restore its original shape, and cracks often appear within a few hours after cutting under the effect of internal stress. Shear cracks are also easy to occur when the material hardness is too high, the hardness is uneven and the material segregation is serious.
For billets with end cracks, the cracks will further expand during forging.
Gas cutting cracks: gas cutting cracks are generally located at the end of the blank, which are caused by the organization stress and thermal stress generated during gas cutting because the raw materials are not preheated before gas cutting.
For billets with gas cutting cracks, the cracks will further expand during forging. Therefore, it shall be removed before forging.
Cracking of convex core: when the lathe is blanking, there is often a convex core left at the center of the end face of the bar. In the process of forging, the convex core has low plasticity due to its small cross section and fast cooling, but the blank matrix has large cross section, slow cooling and high plasticity. Therefore, the abrupt junction of the cross section becomes a stress concentration part, and the plastic difference between the two parts is large, so under the impact of hammer force, cracks are easily caused around the convex core.

Defects often caused by improper heating process

Defects caused by improper heating can be divided into:

  • (1) Defects caused by the change of the microstructure and chemical state of the outer layer of the billet due to the influence of the medium, such as oxidation, decarburization, carburization, sulfurizing, copper infiltration, etc;
  • (2) Defects caused by abnormal changes in internal organizational structure, such as overheating, overburn, and lack of heat penetration;
  • (3) Due to the uneven distribution of temperature in the billet, the internal stress (such as temperature stress, organizational stress) is too large, which results in billet cracking.

Here are some common defects:
Decarburization: Decarburization refers to the phenomenon that the surface carbon of metal is oxidized at high temperature, which makes the carbon content in the surface significantly lower than that in the interior.
The depth of decarburization layer is related to the composition of steel, the composition of furnace gas, temperature and holding time at this temperature. Decarburization is easy to occur when heated in oxidizing atmosphere, high carbon steel is easy to decarburize, and steel with high silicon content is also easy to decarburize.
Decarburization reduces the strength and fatigue performance of the parts, and weakens the wear resistance.
Carburization: Carburization often occurs on the surface or part of the surface of forgings heated by oil furnace. Sometimes the thickness of the carburized layer reaches 1.5~1.6mm, the carbon content of the carburized layer reaches about 1% (mass fraction), and the carbon content of local points even exceeds 2% (mass fraction), resulting in ledeburite structure.
This is mainly because when the billet is heated in the oil furnace, when it is close to the nozzle of the oil furnace or in the area where two nozzles cross to spray fuel oil, the oil and air are not mixed well, so the combustion is incomplete. As a result, a reductive carburizing atmosphere is formed on the surface of the billet, resulting in the effect of surface carburization.
Carburization deteriorates the machinability of forgings and makes them easy to cut.
Overheating: overheating refers to the phenomenon that the heating temperature of the metal blank is too high, or the residence time is too long within the specified forging and heat treatment temperature range, or the grain is coarse due to excessive temperature rise due to thermal effect.
A carbon steel (hypoeutectoid or hypereutectoid steel) tends to have widmanstatten after overheating. When the martensitic steel is overheated, the intragranular texture often appears, and the overheated structure of the tool and die steel is usually judged by the feature of primary carbide angulation. After titanium alloy is overheated, obvious β Phase grain boundary and straight and slender widmanstatten structure. After overheating, the fracture of alloy steel will appear stone fracture or strip fracture. Superheated structure, due to coarse grains, will lead to the reduction of mechanical properties, especially impact toughness.
Generally, after normal heat treatment (normalizing and quenching), the structure of overheated structural steel can be improved and its properties can be restored. This overheating is often called unstable overheating; However, the severe overheating of alloy structural steel can not be completely eliminated after general normalizing (including high-temperature normalizing), annealing or quenching treatment, which is often called stable overheating.
Oversintering: Oversintering refers to that the heating temperature of the metal billet is too high or the time spent in the high-temperature heating zone is too long, and the oxygen and other oxidizing gases in the furnace penetrate into the gap between the metal grains and oxidize with iron, sulfur, carbon, etc. to form a eutectic of fusible oxides, which destroys the relationship between the grains and sharply reduces the plasticity of the material. The metal with severe overburning will crack with a slight blow when removing the rough part, and will have transverse cracks at the overburning part when pulling the long part.
There is no strict temperature boundary between overburn and overheating. Overburning is generally judged by the oxidation and melting of grains. For carbon steel, when the grain boundary is melted during overburning, and when the mold steel (high-speed steel, Cr12 type steel, etc.) for severe oxidation is overburned, the grain boundary will appear fishbone ledeburite due to melting. Grain boundary melting triangle zone and remelting ball appear when aluminum alloy is overburned. Forgings are often irretrievable after burning, so they have to be scrapped.
Heating crack: when heating large ingots with large section size and high alloy steel and superalloy billets with poor thermal conductivity, if the heating speed is too fast at the low temperature stage, the billets will generate great thermal stress due to large internal and external temperature differences. In addition, at this time, the billet has poor plasticity due to low temperature. If the value of thermal stress exceeds the strength limit of the billet, there will be a radiating heating crack from the center to the periphery, causing the entire section to crack.
Copper brittleness: The copper brittleness is cracked on the surface of the forging. At high magnification, yellowish copper (or copper solid solution) is distributed along the grain boundary.
When the billet is heated, if there are copper oxide scraps left in the furnace, the oxidized steel will be reduced to free copper at high temperature, and the molten steel atoms will expand along the austenite grain boundary, weakening the connection between grains. In addition, when the copper content in steel is high [>2% (mass fraction)], if heated in an oxidizing atmosphere, a copper rich layer is formed under the scale of iron oxide, which also causes steel brittleness.

Defects often caused by improper forging process

Defects caused by improper forging process usually include the following:
Large grains: large grains are usually caused by too high initial forging temperature and insufficient deformation degree, or too high final forging temperature, or deformation degree falling into the critical deformation zone. The deformation degree of aluminum alloy is too large, forming texture; When the deformation temperature of superalloy is too low, coarse grains may also be formed when mixed deformation structures are formed. The coarser grain will reduce the plasticity and toughness of forgings, and the fatigue property will be significantly reduced.
Uneven grain: Uneven grain refers to that the grain in some parts of the forging is especially coarse, while that in some parts is small. The main reason for the uneven grain is that the uneven deformation of the billet makes the grain broken differently, or the deformation degree of the local area falls into the critical deformation area, or the high-temperature alloy is locally work hardened, or the local grain is coarse during quenching and heating. Heat resistant steels and superalloys are particularly sensitive to grain inhomogeneity. Uneven grain will significantly reduce the endurance and fatigue properties of forgings.
Cold hardening phenomenon: due to low temperature or too fast deformation speed during forging deformation, as well as too fast cooling after forging, the softening caused by recrystallization may not catch up with the strengthening (hardening) caused by deformation, so that the cold deformation structure remains partially in the forging after hot forging. The existence of this structure improves the strength and hardness of the forging, but reduces the plasticity and toughness. Severe cold hardening may cause forging cracks.
Cracks: forging cracks are usually caused by large tensile stress, shear stress or additional tensile stress during forging. The place where the crack occurs is usually the place where the billet has the maximum stress and the thinnest thickness. If there are microcracks on the surface and inside of the blank, or there are organizational defects in the blank, or the plasticity of the material is reduced due to improper hot working temperature, or the deformation speed is too fast and the deformation degree is too large, which exceeds the allowable plastic pointer of the material, cracks may occur in such processes as roughening, elongation, punching, reaming, bending and extrusion.
Cracking: forging cracking is a shallow turtle shaped crack on the surface of the forging. This kind of defect is most likely to occur on the surface subject to tensile stress during forging forming (for example, the unfilled protruding part or the part subject to bending).
The internal causes of cracking may be multifaceted:

  • (1) There are too many fusible elements such as Cu and Sn in the material;
  • (2) When heated at high temperature for a long time, the steel surface has copper precipitation, surface grain coarseness, decarburization, or the surface that has been heated for many times;
  • (3) The sulfur content of the fuel is too high, and the sulfur seeps into the steel surface.

Flap crack: forging flash crack is a crack on parting surface during die forging and trimming. The causes of flash cracks may be: ① The strong metal flow during the die forging operation resulted in the phenomenon of threading. ② The cutting temperature of magnesium alloy die forgings is too low; The trimming temperature of copper alloy die forgings is too high.
Crack on parting surface: The crack on the forging parting surface refers to the crack generated along the parting surface of the forging. There are many non-metallic inclusions in the raw materials. During die forging, the flow to the parting surface and the concentrated or shrunk pipe residues are squeezed into the flash and then the normal component cracks on the die surface.
Folding: Forged folding is formed when the oxidized surface metals converge during metal deformation. It can be formed by the confluence of two (or more) metal convection; It can also be formed by the rapid mass flow of a stream of metal to bring the surface metal of adjacent parts to flow, and the two converge; It can also be formed due to bending and backflow of deformed metal; It can also be formed by partial deformation of some metals and being pressed into other metals. Folding is related to the shape of raw materials and blanks, the design of dies, the arrangement of forming processes, lubrication and the actual operation of forging.
Forging folding not only reduces the load-bearing area of the parts, but also becomes the fatigue source due to the stress concentration here.
Through flow: forging through flow is a form of improper streamline distribution. In the cross flow area, the flow lines originally distributed at a certain angle converge to form the cross flow, which may cause a large difference in the grain size inside and outside the cross flow area. The cause of the perforation is similar to folding, which is formed by the confluence of two metals or one metal with another, but the metal in the perforation part is still a whole.
The mechanical properties of forgings are reduced by forging through flow, especially when the grain difference on both sides of the through flow belt is large.
Unsmooth streamline distribution of forgings: Unsmooth streamline distribution of forged forgings refers to streamline disorder such as streamline cutting, backflow and eddy current at low magnification of forgings. If the die design is improper or the forging method is unreasonable, the flow line of the prefabricated blank is disordered; Uneven metal flow caused by improper operation of workers and wear of dies can lead to uneven distribution of forging streamline. Unsmooth streamline will reduce various mechanical properties. Therefore, streamline distribution is required for important forgings.
Casting structure residue: forging and casting structure residue mainly occurs in forgings with ingots as billets. As cast microstructure mainly remains in the difficult deformation area of forgings. Insufficient forging ratio and improper forging method are the main reasons for residual casting structure.
The residual forging and casting structure will reduce the properties of forgings, especially the impact toughness and fatigue properties.
Inconformity of carbide segregation level: Inconformity of forged carbide segregation level mainly occurs in ledeburite tool and die steel. It is mainly due to the uneven distribution of carbides in forgings, which are concentrated in large blocks or distributed in networks. The main reasons for this defect are the poor level of carbide segregation of raw materials, and the insufficient forging ratio or improper forging method during forging modification. Forgings with this defect are prone to local overheating and quenching cracks during heat treatment and quenching. The finished cutting tools and moulds are easy to break when used.
Banded structure: forged banded structure is a kind of structure in which ferrite and pearlite, ferrite and austenite, ferrite and bainite, and ferrite and martensite are banded in forgings. They are mostly found in hypoeutectic steel, austenitic steel and semi martensitic steel. This kind of structure is a banded structure produced during forging deformation under the condition of coexistence of two phases, which can reduce the transverse plastic index of materials, especially the impact toughness. It is easy to crack along the ferrite belt or the junction of two phases when forging or parts are working.
Local insufficient filling: the local insufficient filling of forging mainly occurs in rib, convex corner, corner and fillet, and the size does not meet the drawing requirements. The reasons may be: ① low forging temperature and poor metal fluidity; ② Insufficient equipment tonnage or hammering force; ③ The design of the blank making die is unreasonable, and the volume or section size of the blank is unqualified; ④ The oxide scale or welding deformed metal is accumulated in the mold chamber.
Under pressure: forging under pressure means that the size perpendicular to the parting surface generally increases, which may be caused by: ① low forging temperature. ② Insufficient equipment tonnage, hammering force or hammering times.
Stagger: forging stagger is the displacement of the forging along the upper half of the parting surface relative to the lower half. The possible causes are: ① The clearance between the slider (hammer head) and the guide rail is too large; ② The design of forging die is unreasonable, and there is no lock or guide pillar to eliminate the displacement force; ③ Poor mold installation.
Axis bending: the axis of forged forging is bent, and there is an error with the geometric position of the plane. The possible causes are: ① The forging is not careful when it is out of the die; ② Uneven stress during trimming; ③ When the forging is cooled, the cooling speed of each part is different; ④ Improper cleaning and heat treatment.

Defects often caused by improper cooling process after forging

Defects caused by improper cooling after forging usually include the following:
Cooling crack: during the cooling process after forging, large thermal stress will be generated in the forging due to the excessive cooling speed, and large organizational stress may also be caused by the structural transformation. If these stresses exceed the strength limit of the forging, the forging will produce smooth and slender cooling cracks.
Network carbide: When forging steel with high carbon content, if the stop forging temperature is high and the cooling rate is too slow, the carbide will be precipitated along the grain boundary as a network. For example, when the bearing steel is slowly cooled at 870~770 ℃, the carbide will precipitate along the grain boundary.
Forging network carbide is easy to cause quenching cracks during heat treatment. In addition, it also deteriorates the service performance of the parts.

Defects often caused by improper post forging heat treatment process

Defects caused by improper post forging heat treatment process usually include:
Too high or insufficient hardness: the reasons for insufficient hardness of forgings due to improper post forging heat treatment process are: ① the quenching temperature is too low; ② The quenching heating time is too short; ③ The tempering temperature is too high; ④ Severe decarburization of forging surface caused by repeated heating; ⑤ The chemical composition of steel is unqualified.
The forging hardness is too high due to improper post forging heat treatment process, which is caused by: ① too fast cooling after normalizing; ② Normalizing or tempering heating time is too short; ③ The chemical composition of steel is unqualified.
Uneven hardness: the main reason for uneven hardness caused by forging is improper heat treatment process regulations, such as too much charging at one time or too short holding time; Or local decarburization of forgings caused by heating.

Defects often caused by improper cleaning process of forgings

The defects produced during the cleaning of forged forgings usually include the following:
Excessive pickling: excessive pickling of forging will make the forging surface loose and porous. This defect is mainly caused by the excessive depth of acid and the long time that the forgings stay in the pickling tank, or because the forging surface is not cleaned cleanly and the acid remains on the forging surface.
Corrosion crack: if there is a large residual stress after forging of forged martensitic stainless steel forgings, it is easy to produce small network corrosion cracks on the surface of forgings during pickling. If the structure is coarse, the crack formation will be accelerated.

Application of precision forging in automobile industry

In recent years, the rapid development of precision forging technology has promoted the progress of automobile manufacturing industry. Cold forging and warm forging are more and more used in the automobile industry, and the product shape is closer to the final shape. Precision forging will be developed with the progress of future technology and related technologies. In addition, based on the purpose of reducing production cost, reducing product weight, simplifying part design and manufacturing, and improving product added value, the field of metal plastic forming is actively developing towards high-precision net shape forming technology.
The definition of net shape forming is as follows:
(1) Compared with the traditional plastic forming, the subsequent machining can be smaller, which can meet the dimensional and tolerance requirements of the parts.
(2) The forming process that can meet the size and tolerance requirements of the parts without subsequent machining at the local important positions of the formed parts.
(3) Within the size and tolerance range of the parts, the forging can be formed without subsequent machining.
Metal plastic processing is developing towards three goals:

  • (1) Precision products (development of net shaped parts);
  • (2) Process rationalization (taking the minimum investment cost and production cost as the principle of process integration and application);
  • (3) Automation and labor saving.

Development overview of forged flange industry

Flange, also known as flange plate or flange, is mainly used for connection of tubular parts. Flanges are very common in the application of mechanical parts, and are widely used in petrochemical pipelines, metal pressure vessels, upper and lower water pipelines of buildings, municipal water supply pipelines, ships, electric power and other industries.
According to the different raw materials used, flanges can be divided into carbon steel flanges, stainless steel flanges and alloy steel flanges; According to different manufacturing processes, flanges can be divided into forged flanges and cast flanges. Forged flanges are mainly produced by free forging or die forging process; Cast flanges are manufactured by casting process.
At present, China’s forged flange industry has made great progress in equipment level, forging technology and processing technology, and the quality and performance of products have been greatly improved. Due to the low labor cost, the forged flanges produced in China have a strong competitive advantage in the world, and the export volume has reached a high level in recent years. Germany, Japan and other industrial developed countries have few domestic flange manufacturers due to their high labor costs. The required flange products are mainly imported from developing countries such as China, India and Brazil.
Process design: Advanced manufacturers generally adopt computer simulation technology of hot working, computer aided process design and virtual technology, which improves the level of process design and product manufacturing capability. The simulation programs such as DATAFOR, GEMARC/AUTOFORGE, DEFORM, LARSTRAN/SHAPE and THERMOCAL are introduced and applied to realize the process control of computer design and hot working.

  • Flange Forging technology: Most hydraulic presses of 40MN and above are equipped with 100-400t. m main forging operator and 20-40t M auxiliary operating machine, and a considerable number of operating machines are controlled by computers, which realizes the comprehensive control of forging process, so that the forging accuracy can be controlled within ± 3mm. The online measurement of forgings adopts laser dimension measurement device.
  • Heat treatment technology: focus on improving product quality, improving heat treatment efficiency, saving energy, protecting the environment, etc. For example, the heating process of heating furnace and heat treatment furnace is controlled by computer, and the burner is controlled to realize automatic combustion regulation, furnace temperature regulation, automatic ignition and heating parameter management; Waste heat utilization, heat treatment furnace equipped with regenerative combustion chamber, etc; The polymer quenching oil tank with low pollution capacity and effective cooling control is adopted, and various water-based quenching mediums gradually replace the traditional quenching oil.
  • Machining technology: the proportion of numerical control machine tools in the industry has gradually increased. Some enterprises in the industry have set up processing centers and are equipped with proprietary processing machinery according to different types of products, such as five coordinate machining centers, blade processing machines, roll grinders, roll lathes, etc.
  • Quality assurance measures: some domestic enterprises have been equipped with the latest detection instruments and testing technology, modern automatic ultrasonic flaw detection system with computer controlled data processing, various special automatic ultrasonic flaw detection systems, and completed the certification of various quality systems. The key production technology of high-speed and heavy load gear forgings has been continuously conquered, and on this basis, the industrial production has been realized. On the basis of introducing foreign advanced production technology and key equipment, China has been able to design and manufacture production equipment for high-speed and heavy load gear forgings by itself. These equipment are close to the international advanced level, and the improvement of technology and equipment level has effectively promoted the development of the domestic forging industry.


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