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What is forging technology

What is forging?

Forging has a long history in China. It continues with the production mode of manual workshop. Probably in the early 20th century. It gradually appeared in railway, ordnance, shipbuilding and other industries in the mode of mechanical industrialization. The main sign of this transformation is the use of powerful forging machines.

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Forging is widely used in automobile manufacturing. With the progress of science and technology and the continuous improvement of workpiece accuracy requirements, precision forging technology with the advantages of high efficiency, low cost, low energy consumption and high quality has been more and more widely used. According to the different deformation temperatures during metal plastic forming, precision cold forging can be divided into cold forging, temperature forming, sub hot forging, hot precision forging, etc. the produced auto parts include: automobile clutch engagement ring gear, input shaft parts of automobile transmission, bearing ring, automobile constant velocity universal joint sliding sleeve series products, automobile differential gear, automobile front axle, etc.

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Definition and classification of forging

Definition of forging

Forging is a processing method that uses forging machinery to exert pressure on metal blank to produce plastic deformation to obtain forgings with certain mechanical properties, certain shape and size. It is one of the two components of forging (forging and stamping).
Forging can eliminate the defects such as as as cast porosity in the smelting process and optimize the microstructure. At the same time, because the complete metal streamline is preserved, the mechanical properties of forgings are generally better than those of castings of the same material. For important parts with high load and severe working conditions in relevant machinery, forgings are mostly used, except for rolled plates, profiles or weldments with simple shape.

Classification of forging

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

  • Free forging: refers to the processing method of forging by using simple universal tools or directly applying external force to the blank between the upper and lower anvil of forging equipment to deform the blank and obtain the required geometry and internal quality.
  • Die forging: refers to the forging obtained by pressing and deforming the metal blank in the forging die chamber with a certain shape. 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.
  • Ring grinding: it refers to the production of ring parts with different diameters by special equipment ring grinding machine. It is also used to produce wheel parts such as automobile hub and train wheel.
  • 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 some special shape parts. 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 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 higher than 800 ℃; It is called warm forging or semi hot forging at 300 ~ 800 ℃, and it is called cold forging at room temperature. Forgings used in most industries are hot forging. Warm forging and cold forging are mainly used for forging automobile, general machinery and other parts. Warm forging and cold forging can effectively save materials.
According to the movement mode of forging die, forging can be divided into swing rolling, swing rotary forging, roll forging, cross wedge rolling, ring rolling and cross rolling.

Forging materials

The forging materials are mainly carbon steel and alloy steel of various components, followed by aluminum, magnesium, copper, titanium and their alloys, iron-based superalloys, nickel based superalloys and cobalt based superalloys. The deformation alloys are also completed by forging or rolling. However, due to the relatively narrow plastic zone of these alloys, it will be relatively difficult to forge, and the heating temperature of different materials, There are strict requirements for open forging temperature and final forging temperature.
The original state of the material is bar, ingot, metal powder and liquid metal. The ratio of the cross-sectional area of metal before deformation to the cross-sectional area after deformation is called forging ratio.
Correct selection of forging ratio, reasonable heating temperature and holding time, reasonable initial and final forging temperature, reasonable deformation and deformation speed are closely related to improving product quality and reducing cost.

Common forging methods and their advantages and disadvantages

Free forging

Free forging refers to the processing method of forging by using simple universal tools or directly applying external force to the blank between the upper and lower anvil of forging equipment to deform the blank and obtain the required geometry and internal quality. Forgings produced by free forging method are called free forgings.

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Free forging mainly produces forgings with small batch. Forging hammer, hydraulic press and other forging equipment are used to form and process the blank to obtain qualified forgings. The basic processes of free forging include upsetting, drawing, punching, cutting, bending, torsion, dislocation and forging. Free forging adopts hot forging.
Free forging process includes basic process, auxiliary process and finishing process.
The basic processes of free forging are upsetting, drawing, punching, bending, cutting, torsion, dislocation and forging, while the three processes most commonly used in actual production are upsetting, drawing and punching.
Auxiliary process: pre deformation process, such as jaw pressing, ingot edge pressing, shoulder cutting, etc.
Finishing process: a process to reduce surface defects of forgings, such as removing unevenness and shaping of forging surface.
Advantage:

  • (1) Forging flexibility is large, which can produce small parts less than 100kg and heavy parts up to 300t;
  • (2) The tools used are simple general tools;
  • (3) Forging forming is to deform the blank gradually in different regions. Therefore, the tonnage of forging equipment required for forging the same forging is much smaller than that of model forging;
  • (4) Low precision requirements for equipment;
  • (5) Short production cycle.

Disadvantages and limitations:

  • (1) The production efficiency is much lower than that of model forging;
  • (2) The forging has the advantages of simple shape, low dimensional accuracy and rough surface; Workers have high labor intensity and high technical level;
  • (3) It is not easy to realize mechanization and automation.

Die forging

Die forging refers to the forging method of forging by forming the blank with a die on the special die forging equipment. The forging produced by this method has accurate size, small machining allowance, complex structure and high productivity.
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According to different classification of equipment used: Die Forging on hammer, die forging on crank press, die forging on flat forging machine and die forging on friction press, etc.
The most commonly used equipment for die forging on hammer is steam air die forging hammer, anvil free hammer and high-speed hammer.
Forging die bore: according to its different functions, it can be divided into die forging die bore and blank making die bore.

Die forging chamber

  • (1) Pre forging die chamber: the function of the pre forging die chamber is to deform the blank to be close to the shape and size of the forging, so that the metal can easily fill the die chamber and obtain the required size of the forging during final forging. For forgings with simple shape or small batch, pre forging die chamber can not be set. The fillet and inclination of the pre forging die bore are much larger than that of the final forging die bore, and there is no flash groove.
  • (2) Final forging die chamber: the function of the final forging die chamber is to make the blank finally deform to the shape and size required by the forging. Therefore, its shape should be the same as that of the forging; However, due to shrinkage when the forging is cooled, the size of the final forging die chamber shall be enlarged by a shrinkage amount compared with the size of the forging. Shrinkage of steel forgings is 1.5%. In addition, there are flash grooves around the die bore to increase the resistance of metal flowing out of the die bore, promote the metal to fill the die bore and accommodate excess metal at the same time.

Blank making die bore

For the forging with complex shape, in order to make the blank shape basically conform to the forging shape, so that the metal can be reasonably distributed and well filled in the die chamber, the blank must be made in the blank making die chamber in advance.

  • (1) Draw long die bore: it is used to reduce the cross-sectional area of a part of the blank to increase the length of the part. The drawing die bore is divided into open type and closed type.
  • (2) Rolling die chamber: it is used to reduce the cross-sectional area of one part of the blank to increase the cross-sectional area of another part, so as to distribute the metal according to the shape of the forging. Rolling die bore is divided into open and closed.
  • (3) Bending die chamber: for bending rod die forgings, it is necessary to bend the blank with bending die chamber.
  • (4) Cut off die bore: it forms a pair of knife edges on the corners of the upper die and the lower die to cut off metal.

Advantage:

  • High production efficiency. During die forging, the deformation of metal is carried out in the die chamber, so the required shape can be obtained quickly;
  • It can forge forgings with complex shapes, make the distribution of metal streamline more reasonable, and improve the service life of parts;
  • Die forgings have accurate dimensions, good surface quality and small machining allowance;
  • Save metal materials and reduce cutting workload;
  • Under the condition of sufficient batch, the cost of parts can be reduced.

Disadvantages and limitations:

  • The weight of die forgings is limited by the capacity of general die forging equipment, mostly below 7okg;
  • The manufacturing cycle of forging die is long and the cost is high;
  • The investment cost of die forging equipment is higher than that of free forging.

Roll forging

Roll forging refers to a forging process in which a pair of opposite rotating fan-shaped dies are used to make the blank produce plastic deformation, so as to obtain the required forging or forging blank.
Roll forging deformation is a complex three-dimensional deformation. Most of the deformed materials flow along the length direction to increase the blank length, and a few materials flow laterally to increase the blank width. In the process of roll forging, the cross-sectional area of billet root decreases continuously. Roll forging is applicable to the deformation processes such as shaft drawing, slab rolling and material distribution along the length direction.
Roll forging can be used to produce connecting rods, twist drills, wrenches, spikes, hoes, pickaxes and turbine blades. Roll forging process uses the principle of rolling forming to gradually deform the blank.
Compared with ordinary die forging, roll forging has the advantages of simple equipment structure, stable production, low vibration and noise, easy automation and high production efficiency.

Die forging of tire

Tire die forging is a forging method that uses free forging square method to blank and then finally form in the tire die. It is a forging method between free forging and die forging. There are few die forging equipment, most of which are free forging hammers, which are widely used in small and medium-sized enterprises.
There are many kinds of tire dies used in tire die forging. The common ones in production are: type drop, buckle die, sleeve die, cushion die, clamping die, etc.
Closed cylinder die is mostly used for forging rotary forgings. For example, gears with bosses on both ends are sometimes used for forging non rotating forgings. Closed cylinder die forging belongs to no flash forging.
For tire die forgings with complex shape, it is necessary to add two half dies (i.e. add a parting surface) in the barrel die to make a combined barrel die, and the blank is formed in the die chamber composed of two half dies.
The closing film is usually composed of upper and lower molds. In order to match the upper and lower dies and avoid dislocation of forgings, guide posts and guide pins are often used for positioning. Die clamping is mostly used to produce non rotating forgings with complex shape, such as connecting rod, fork forgings, etc.
Compared with free forging, die forging has the following advantages:

  • (1) Because the blank is formed in the die bore, the forging size is more accurate, the surface is more smooth, and the distribution of streamline structure is more reasonable, so the quality is higher;
  • (2) Tire die forging can produce forgings with complex shapes; Because the shape of the forging is controlled by the die bore, the blank forming is faster and the productivity is 1 ~ 5 times higher than that of free forging;
  • (3) There are few remaining blocks, so the machining allowance is small, which can not only save metal materials, but also reduce machining hours.

Disadvantages and limitations:

  • (1) Large tonnage forging hammer is required;
  • (2) Only small forgings can be produced;
  • (3) The service life of tire mold is low;
  • (4) When working, the tire mold is generally moved by manpower, so the labor intensity is high;
  • (5) Tire die forging is used to produce medium and small batch forgings.

Forging defects and analysis

The raw materials for forging are ingot, rolled stock, extruded stock and forging stock. Rolling stock, extrusion stock and forging stock are semi-finished products processed by rolling, extrusion and forging of ingots. 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 leads to defects in the forging. The following briefly introduces some common defects in forgings.

Forging defects caused by raw material defects usually include:

Surface crack: surface crack mostly occurs on rolled bar and forged bar, generally in 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 elongate along the deformation direction, expose to the surface and develop deep inside during rolling. As another example, if the surface of the blank is scratched during rolling, it will cause stress concentration during cooling, which may crack along the scratch, etc. If such cracks are not removed before forging, they may expand during forging and cause forging cracks.
Folding: the reason for folding is that when the metal blank is in the rolling process, due to the incorrect sizing of the groove on the roll, or the burr generated by the wear surface of the groove is involved during rolling, a crease with a certain inclination angle with the material surface is formed. For steel, there is iron oxide inclusion in the crease and decarburization around it. If the folding is not removed before forging, it may cause folding or cracking of forgings.
Scab: scab is a layer of peelable film on the local area of the rolled surface.
The formation of scab is due to the splashing of liquid steel during casting and condensation on the surface of ingot. It is pressed into a film during rolling and attached to the surface of rolled material. After the forging is cleaned by pickling, the film will peel off and become the surface defect of the forging.
Lamellar fracture: the characteristic of lamellar fracture is that its fracture or section is very similar to the broken slate and bark.
Lamellar fracture mostly occurs in alloy steel (chromium nickel steel, chromium nickel tungsten steel, etc.), and it is also found in carbon steel. This defect is caused by the non-metallic inclusions, dendrite segregation and porosity in the steel, which are elongated along the rolling direction in the process of forging and rolling, so that the steel is lamellar. If there are too many impurities, there is a risk of delamination and fracture in forging. The more serious the lamellar 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 fine reflective line showing crystal brightness on the longitudinal fracture, most of which run through the whole fracture, and most of which are generated in the axial part.
The bright line is mainly caused by alloy segregation. Slight bright lines have little effect on mechanical properties, and serious bright lines will significantly reduce the plasticity and toughness of the material.
Non metallic inclusions: non metallic inclusions are mainly formed by chemical reactions between components or between metal and furnace gas and container during the cooling process of molten steel melted or cast. In addition, during metal melting and casting, inclusions can also be formed due to the refractory falling into the liquid steel, which are collectively referred to as slag inclusion. On the cross section of the forging, non-metallic inclusions can be distributed in point, sheet, chain or block shape. Serious inclusions are easy to cause forging cracking or reduce the service performance of the material.
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 evenly distributed during billet opening and rolling. Carbide segregation will reduce the forging deformation properties of steel and easily cause forging cracking. Forgings are prone to local overheating, overburning and quenching cracking 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. The fracture features can be divided into two categories: first, it is flat sheet, and the color ranges from silver gray and light yellow to brown and dark brown; Second, it is a small, dense and flashing point.
Aluminum alloy oxide film is formed when the open melt liquid level interacts with water vapor or other metal oxides in the atmosphere during the casting process. It is rolled into the liquid metal during the casting process.
The oxide film in forgings and die forgings has no obvious influence on the longitudinal mechanical properties, but has a great influence 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 spots varies, and the length is 1 ~ 20mm or longer. White spots are common in alloy steels such as nickel chromium steel and nickel chromium molybdenum steel, and also found in ordinary carbon steel. They are hidden defects. White spots are produced under the joint action of hydrogen, microstructure stress during phase transformation and thermal stress. They are more likely to occur when the steel contains more hydrogen and cools (or heat treatment after forging) too quickly after hot pressure processing.
Forgings forged with steel with white spots are easy to crack during heat treatment (quenching), and sometimes even fall into blocks. White spot reduces the plasticity of steel and the strength of parts. It is a stress concentration point. Like a sharp cutter, it is easy to become fatigue crack and lead to fatigue failure under the action of alternating load. Therefore, white spots are absolutely not allowed in forging raw materials.
Coarse grained ring: coarse grained ring is often a defect on extruded aluminum or magnesium alloy bars.
The extruded bars of aluminum and magnesium alloys supplied after heat treatment often have coarse-grained rings on the outer layer of their circular section. The thickness of coarse-grained ring increases gradually from the beginning to the end of extrusion. If the lubrication condition during extrusion is good, the coarse-grained ring can be reduced or avoided after heat treatment. On the contrary, the thickness of the ring will increase.
The cause of coarse crystal ring is related to many factors. However, the main factor is the friction between the metal and the extrusion barrel in the extrusion process. This friction results in the crushing degree of the outer layer grains in the cross section of the extruded bar is much greater than that in the center of the bar. However, due to the influence of the cylinder wall, the temperature in this area is low, and it cannot be completely recrystallized during extrusion. During quenching and heating, the non recrystallized grains recrystallize and grow up, swallowing the recrystallized grains, so a coarse-grained ring is formed on the surface layer.
The blank with coarse grain ring is easy to crack during forging. If the coarse grain is left on the surface of the forging, the performance of the part will be reduced.
Pipe shrinkage residue: pipe shrinkage residue is generally caused by the concentrated shrinkage cavity generated at the riser of ingot, which is not removed completely and remains in the steel during blank opening and rolling.
Dense inclusions, porosity or segregation will generally appear in the area near the shrinkage residue. A gap with irregular folds in the transverse magnification. It is easy to cause cracking of forgings during forging or heat treatment.

Defects caused by improper material preparation and its influence on forgings

Defects caused by improper material preparation include the following:
Skew cutting: skew cutting refers to that the inclination of the blank end face relative to the longitudinal axis exceeds the specified allowable value due to the failure to compress the bar when loading and unloading on the sawing machine or punch. Severe skew may cause folding during forging.
The end of the blank is bent and burred: when loading and unloading the cutting machine or punch, the blank has been bent before being cut due to the excessive gap between the blade of the scissors or the cutting die, or the blade is not sharp. As a result, part of the metal is squeezed into the gap of the blade or die to form an end drooping burr.
The blank with burr is easy to cause 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 shear. As a result, part of the end metal is pulled off and the end face is concave. Such blank is easy to fold and crack during forging.
End crack: when cold shearing large section alloy steel and high carbon steel bars, end cracks are often found 3 ~ 4h after shearing. It is mainly because the unit pressure of the blade is too large, so that the blank with circular section is flattened into an ellipse. At this time, a great 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 action 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 crack: the gas cutting crack is generally located at the end of the blank, which is caused by the lack of preheating of raw materials before gas cutting and the tissue stress and thermal stress during gas cutting.
For the billet with gas cutting crack, the crack will further expand during forging. Therefore, it shall be removed in advance before forging.
Convex core cracking: during lathe blanking, convex core is often left in the center of the bar end face. In the forging process, due to the small section and fast cooling of the convex core, its plasticity is low, but the section of the blank matrix is large, slow cooling and high plasticity. Therefore, the abrupt junction of the section becomes the part of stress concentration, and the plasticity of the two parts is quite different, so it is easy to cause cracking around the convex core under the action of hammering force.

Defects often caused by improper heating process

Defects caused by improper heating can be divided into:

  • (1) Defects caused by the change of the histochemical state of the outer layer of the blank due to the influence of the medium, such as oxidation, decarburization, carburization, sulfurization, copper infiltration, etc;
  • (2) Defects caused by abnormal changes in internal organizational structure, such as overheating, overburning and lack of heat penetration;
  • (3) Due to the uneven distribution of temperature in the billet, the billet cracking caused by excessive internal stress (such as temperature stress and microstructure stress).

Here are some common defects:
Decarburization: decarburization refers to the phenomenon that the carbon on the surface of the metal is oxidized at high temperature, so that the carbon content on the surface is significantly lower than that inside.
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 properties of 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 oil furnace is heated, when the position of the blank is close to the oil furnace nozzle or in the area where the two nozzles cross inject fuel oil, the oil and air are not mixed well, so the combustion is incomplete, resulting in the formation of a reducing carburizing atmosphere on the surface of the blank, resulting in the effect of surface carburization.
Carburization deteriorates the machinability of forgings and is easy to cut.
Overheating: overheating refers to the phenomenon of coarse grain caused by excessive heating temperature of metal blank, too long residence time within the specified forging and heat treatment temperature range, or too high temperature rise due to thermal effect.
Widmanstatten structure often appears in carbon steel (hypoeutectoid or hypereutectoid steel) after overheating. After overheating, martensitic steel often has intracrystalline texture, and tool and die steel is often characterized by primary carbide angulation to determine the superheated structure. After overheating, titanium alloy appears obvious β phase boundary is flat, straight and slender widmanstatten structure. The fracture of alloy steel after overheating will appear stone fracture or strip fracture. The mechanical properties, especially the impact toughness, will be reduced due to the coarse grain of superheated structure.
Generally, after normal heat treatment (normalizing and quenching), the microstructure of overheated structural steel can be improved and the properties can be restored. This overheating is often called unstable overheating; The serious overheating of alloy structural steel can not be completely eliminated after general normalizing (including high-temperature normalizing), annealing or quenching. This overheating is often called stable overheating.
Overburning: overburning refers to that when the heating temperature of the metal blank is too high or the residence time in the high-temperature heating zone is too long, the oxygen and other oxidizing gases in the furnace penetrate into the gap between the metal grains and oxidize with iron, sulfur and carbon to form a eutectic of fusible oxides, destroy the connection between the grains and sharply reduce the plasticity of the material. For the metal with serious overburning, it will crack with a slight blow when removing the coarseness, and a transverse crack will appear at the overburned part when pulling out.
There is no strict temperature boundary between overburning and overheating. It is generally characterized by oxidation and melting of grains. For carbon steel, the grain boundary melts during overburning and fishbone ledeburite appears at the grain boundary due to melting during Overburning of severe oxygen chemical die steel (high speed steel, Cr12 section steel, etc.). Grain boundary melting triangle zone and remelting ball appear when aluminum alloy is overburned. After the forgings are burned, they are often unable to be saved and 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 in the low-temperature stage, the billets will produce great thermal stress due to large internal and external temperature difference. In addition, at this time, the plasticity of the blank is poor due to low temperature. If the value of thermal stress exceeds the strength limit of the blank, there will be radial heating cracks from the center to the periphery, cracking the whole section.
Copper embrittlement: Copper embrittlement is cracked on the forging surface. At high magnification, light yellow copper (or copper solid solution) is distributed along the grain boundary.
When the billet is heated, such as the residual copper oxide chips in the furnace, the oxidized steel is reduced to free copper at high temperature, and the molten steel atoms expand along the austenite grain boundary, weakening the relationship between grains. In addition, when the copper content in the steel is high [> 2% (mass fraction)], if heated in an oxidizing atmosphere, a copper rich layer is formed under the iron oxide scale, which also causes steel embrittlement.

Defects often caused by improper forging process

Defects caused by improper forging process usually include the following:
Large grain: large grain is usually caused by too high initial forging temperature and insufficient deformation degree, or too high final forging temperature, or the deformation degree falls into the critical deformation zone. The deformation degree of aluminum alloy is too large to form texture; When the deformation temperature of superalloy is too low, coarse grains may also be caused when mixed deformation structure is formed. The large grain size will reduce the plasticity and toughness of the forging and the fatigue performance.
Grain unevenness: grain unevenness refers to that the grain in some parts of the forging is particularly coarse, but some parts are small. The main reason for the uneven grain is that the uneven deformation of the blank makes the degree of grain breakage different, or the deformation degree of the local area falls into the critical deformation zone, or the local work hardening of the superalloy, or the local grain is coarse during quenching and heating. Heat resistant steels and superalloys are particularly sensitive to grain inhomogeneity. The non-uniform grain will significantly reduce the durability and fatigue properties of forgings.
Cold hardening phenomenon: during forging deformation, due to low temperature or too fast deformation speed, as well as too fast cooling after forging, the softening caused by recrystallization may not keep up with the strengthening (hardening) caused by deformation, so that the cold deformation structure is still partially retained in the forging after hot forging. The existence of this structure improves the strength and hardness of forgings, but reduces the plasticity and toughness. Severe cold hardening may cause forging crack.
Crack: forging crack is usually caused by large tensile stress, shear stress or additional tensile stress during forging. The position where the crack occurs is usually the position with the maximum stress and the thinnest thickness of the blank. If there are microcracks on the surface and inside of the blank, or there are structural 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, exceeding the allowable plastic pointer of the material, cracks may occur in the processes of coarsening, lengthening, punching, reaming, bending and extrusion.
Cracking: forging cracking is a shallow turtle like crack on the surface of the forging. In forging forming, such defects are most likely to occur on the surface under tensile stress (e.g. unfilled convex part or bent part).
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, there are copper precipitation, coarse surface grains, decarburization or surfaces heated for many times on the steel surface;
  • (3) The sulfur content of the fuel is too high, and the sulfur seeps into the steel surface.

Flash crack: forging flash crack is a crack on the parting surface during die forging and trimming. The causes of flash cracks may be as follows:

  • ① In die forging operation, the metal flows strongly due to heavy blow, resulting in reinforcement penetration.
  • ② The trimming temperature of magnesium alloy die forgings is too low; The trimming temperature of copper alloy die forgings is too high.

Parting surface crack: the forging parting surface crack refers to the crack along the forging parting surface. There are many non-metallic inclusions in raw materials, which flow and concentrate to the parting surface during die forging, or the residual of pipe shrinkage will normally form die surface cracks after being squeezed into flash during die forging.
Folding: forging folding is formed by the confluence of oxidized surface metal during metal deformation. It can be formed by the confluence of two (or more) metal convection; It can also be formed by the rapid flow of a large amount of metal, bringing the surface metal of adjacent parts to flow, and the two converge; It can also be formed due to bending and reflux of deformed metal; It can also be formed by local deformation of part of the metal and being pressed into another part of the metal. Folding is related to the shape of raw materials and blanks, die design, arrangement of forming process, lubrication and actual operation of forging.
Forging folding not only reduces the bearing area of parts, but also often becomes a fatigue source due to the stress concentration here.
Through flow: forging through flow is a form of improper distribution of streamline. In the flow through zone, the flow lines originally distributed at a certain angle converge to form the flow through zone, and the grain size inside and outside the flow through zone may be quite different. The cause of flow through is similar to that of folding. It is formed by the confluence of two metals or one metal with the other, but the metal in the flow through part is still a whole.
Forging through flow reduces the mechanical properties of forgings, especially when the grains on both sides of the through flow belt are very different.
Unsmooth distribution of forging flow line: unsmooth distribution of forging flow line refers to the flow line disorder such as flow line cutting, reflux and eddy current on the low magnification of forging. If the die design is improper or the forging method is unreasonable, the flow line of preform is disordered; The uneven flow of metal caused by improper operation of workers and die wear can make the streamline distribution of forgings not smooth. Unsmooth flow line will reduce various mechanical properties, so there are requirements for flow line distribution for important forgings.
Casting structure residue: forging casting structure residue mainly occurs in forgings with ingots as blanks. The as cast structure mainly remains in the difficult deformation area of the forging. Insufficient forging ratio and improper forging method are the main causes of casting structure residue.
The residual structure of forging and casting will reduce the properties of forgings, especially the impact toughness and fatigue properties.
The carbide segregation level does not meet the requirements: the forging carbide segregation level does not meet the requirements, which mainly occurs in the ledeburite tool and die steel. The main reason is that the carbide in the forging is unevenly distributed, distributed in bulk, concentrated or network. The main reason for this defect is the poor carbide segregation level of raw materials, coupled with insufficient forging ratio or improper forging method during forging modification. Forgings with this defect are prone to local overheating and quenching crack during heat treatment and quenching. The cutting tools and molds are easy to collapse when used.
Banded structure: forging banded structure is a kind of structure in which ferrite and pearlite, ferrite and austenite, ferrite and bainite, ferrite and martensite are banded in forgings. They mostly appear in Hypoeutectic steel, austenitic steel and semi martensitic steel. This structure is the banded structure produced during forging deformation when two phases coexist, which can reduce the transverse plastic index of the material, especially the impact toughness. When forging or working parts, it is often easy to crack along the ferrite belt or the junction of two phases.
Insufficient local filling: insufficient local filling of forging mainly occurs in ribs, convex corners, corners and rounded corners, and the size does not meet the drawing requirements. The causes may be:

  • ① Low forging temperature and poor metal fluidity;
  • ② Insufficient equipment tonnage or hammering force;
  • ③ The design of blank making die is unreasonable, and the blank volume or section size is unqualified;
  • ④ Oxide scale or welding deformed metal is accumulated in the die bore.

Undervoltage: forging undervoltage refers to the general increase of the dimension perpendicular to the parting surface, which may be caused by:

  • ① Low forging temperature.
  • ② The equipment tonnage is insufficient, the hammering force is insufficient or the hammering times are insufficient.

Stagger: forging stagger refers to the displacement of the upper half of the forging along the parting surface relative to the lower half. The causes may be:

  • ① The gap between the sliding block (hammer head) and the guide rail is too large;
  • ② The design of forging die is unreasonable, and there is a lack of lock or guide post to eliminate dislocation force;
  • ③ Poor mold installation.

Axis bending: the axis of forging is bent and there is an error with the geometric position of the plane. The possible causes are:

  • ① Not paying attention when the forging is out of the die;
  • ② Uneven stress during trimming;
  • ③ When the forging is cooled, the cooling rate 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 too fast cooling speed, or large structural stress may be caused by 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 speed is too slow, the carbide will precipitate in a network along the grain boundary. For example, when bearing steel is slowly cooled at 870 ~ 770 ℃, carbides precipitate along the grain boundary.
Forging network carbide is easy to cause quenching crack during heat treatment. In addition, it also deteriorates the service performance of parts.

Defects often caused by improper post forging heat treatment process

Defects caused by improper post forging heat treatment process usually include:
Excessive or insufficient hardness: the reasons for insufficient hardness of forgings caused by improper post forging heat treatment process are:

  • ① Too low quenching temperature;
  • ② Quenching heating time is too short;
  • ③ Tempering temperature is too high;
  • ④ Severe decarburization of forging surface caused by repeated heating;
  • ⑤ Unqualified chemical composition of steel, etc.

The reasons for the high hardness of forgings caused by improper post forging heat treatment process are:

  • ① Too fast cooling after normalizing;
  • ② Normalizing or tempering heating time is too short;
  • ③ Unqualified chemical composition of steel, etc.

Uneven hardness: the main reason for uneven hardness caused by forging is improper heat treatment process, such as excessive one-time charging or too short holding time; Or partial decarburization of forgings caused by heating.

Defects often caused by improper cleaning process of forgings

The defects generated during the cleaning of forging 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 high depth of acid and the long residence time of the forging in the pickling tank, or by the unclean cleaning of the forging surface and the residual acid on the forging surface.
Corrosion crack: if there is 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 microstructure 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 forgings and warm forgings are more and more used in the automotive industry, and the product shape is closer and closer to the final shape. Precision forging will develop accordingly with the progress of technology and related technologies in the future. In addition, for 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.
Net shape forming is defined as follows:

  • (1) Compared with traditional plastic forming, smaller subsequent machining can be obtained, which can meet the dimensional and tolerance requirements of parts.
  • (2) The forming process can meet the dimensional and tolerance requirements of parts without subsequent machining.
  • (3) Within the size and tolerance of the parts, the forging may not need the forming process of subsequent machining.

Metal plastic processing is developing towards three goals:

  • (1) Product precision (net shape part development).
  • (2) Process rationalization (taking the minimum investment cost and production cost as the principle of process integration and application).
  • (3) Automation and labor saving.

Forging technology of super long cylinder forgings

The so-called super long cylinder is the cylinder whose length exceeds the design length of the conventional mandrel. The length of long cylinder forging is 1.5 times of the length of conventional mandrel, and its forging process is as follows:

Process Description

1) Undercut.
Due to the different technical requirements and methods of smelting and pouring of various steel ingots, the size of the countersunk area formed is also different. Generally, the removal of this part of materials shall be controlled at 5% – 10% of the total mass of steel ingots, and shall be selected according to the importance of forgings. The cylinder has ultrasonic flaw detection requirements, which is an important part, and the undercut is taken as 7%.
2) Fire consumption.
For large forgings, the fire consumption per fire is generally 1.5% – 2%. Because the surface area of hollow forgings is always larger than that of solid forgings with the same diameter, more oxide scale is produced. Each fire consumption of this process is taken as 2%.
3) Forging ratio.
When selecting forging ratio, material type, forging quality, blank size, equipment capacity and forging man hour shall be considered according to technical requirements.
Excessive forging ratio will not only increase the forging workload, but also cause the anisotropy of metal fiber structure and mechanical properties.
Forging ratio is an important index to measure the internal quality of forgings, so it must be selected reasonably.

Key points of operation

1) Blanking.
The blank size and blanking size is the prerequisite to ensure the overall size of the cylinder. Whether the blank size and the blank position are correct or not will directly affect the finished size and appearance quality of the cylinder.
Therefore, all dimensions must be strictly controlled during blank opening. The round shall be drawn before billet opening, and the blanking shall be carried out according to the process size on the premise of measuring the quasi total length and ensuring the cutting amount at the bottom of the ingot.
2) Upsetting.
Upsetting is a means to ensure the forging ratio of the whole forging. Therefore, it must be upsetted to size according to the process. If cracks are found during upsetting, they shall be burned, stripped and cleaned in time.
3) Punch.
During punching, the punch shall be placed in the center of the blank to ensure that the blank in the center area of the blank is fully removed. In addition, punching eccentricity will bring difficulties to the subsequent process.
4) Mandrel extension.
The lengthening of super long cylinder is generally completed by forging section by section. For some cylinders with a small length beyond the mandrel, the method of turning head forging can be adopted. This method is relatively simple, but the risk is large. For the cylinder that exceeds the mandrel too much, the section by section forging method is generally adopted.
In the whole forging process, always pay attention to the straightness of the whole cylinder to avoid bending. Otherwise, it will be difficult to exit and straighten at the same time.

Source: Network Arrangement – China Flanges Manufacturer: www.epowermetals.com

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@epowermetals.com

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