What is Forging?

What is Forging?

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

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Classification of Forging
Forging Materials
Common forging methods and their advantages and disadvantages
Free forging
Die forging
Roll forging
Foetal die forging
Forging Defects and Analysis

2. Classification of Forging
According to different production tools, forging technology can be divided into free forging, module forging, ring rolling and special forging.
Free forging: refers to the processing method of forgings with simple universal tools, or directly applying external force between the top and bottom anvils of forging equipment to deform the billet and obtain the required geometric shape and internal quality.
Die forging: refers to the metal blank in a certain shape of the forging die chamber by compression deformation to obtain forgings. 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.
Ring rolling: refers to the production of ring parts of different diameters by special equipment ring rolling machine, also used to produce wheel parts such as automobile hub, train wheel, etc.
Special forging: including roll forging, cross wedge rolling, radial forging, liquid forging and other forging methods, these methods are more suitable for the production of parts with special shapes. For example, roll forging can be used as an effective pre-forming process, greatly reducing the subsequent forming pressure; cross wedge rolling can produce steel balls, transmission shafts and other parts; radial forging can produce large barrels, step shafts and other forgings.
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. Hot forging is more than 800℃. It is called warm forging or Semi-Hot forging between 300 and 800℃, and cold forging is called cold forging when 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 save material effectively.
According to the movement mode of forging die, forging can be divided into rotary forging, rotary forging, roll forging, cross wedge rolling, ring rolling and cross rolling.

3. Forging Materials
Forging materials are mainly carbon steel and alloy steel of various compositions, followed by aluminum, magnesium, copper, titanium and their alloys, iron-based superalloys, nickel-based superalloys and cobalt-based superalloys. The deformed alloys of these alloys are also forged or rolled, but because their plastic zones are relatively narrow, the forging difficulty will be different. There are strict requirements for heating temperature, open forging temperature and final forging temperature of large and different materials.
The original state of the material is bar, ingot, metal powder and liquid metal. The ratio of cross-sectional area of metal before deformation to that after deformation is called forging ratio.
Correct selection of forging ratio, reasonable heating temperature and holding time, reasonable initial forging temperature and final forging temperature, reasonable deformation amount and deformation speed are very important to improve product quality and reduce cost.
. Common forging methods and their advantages and disadvantages
1. Free forging
Free forging refers to the processing method of forging which uses simple universal tools or directly exerts external force between the upper and lower anvils of forging equipment to deform the blank and obtain the required geometric shape and internal quality. The forgings produced by free forging method are called free forgings.

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Figure 1 Free forging
Free forging is mainly used to produce forgings with small batches. Forging equipments such as hammer and hydraulic press are used to process the blanks, and qualified forgings are obtained. The basic processes of free forging include upsetting, drawing, punching, cutting, bending, torsion, offset and forging. Free forging is hot forging.
Free forging process
It includes basic process, auxiliary process and finishing process.
The basic processes of free forging are upsetting, drawing, punching, bending, cutting, torsion, staggering and forging joints. In practice, upsetting, drawing and punching are the three most commonly used processes.
Auxiliary process: Pre-deformation process, such as clamp, ingot edge, shoulder cutting, etc.
Finishing process: the process of reducing the surface defects of forgings, such as clearing the rough surface of forgings and shaping.
Forging flexibility, can produce less than 100 kg of small parts, can also produce heavy parts up to 300 tons;
The tools used are simple and general tools.
Forging is the gradual deformation of the blank in the subregion, so the tonnage of forging equipment needed for forging the same forging is much smaller than that for model forging.
The precision requirement of the equipment is low.
The production cycle is short.
Disadvantages and limitations:
The production efficiency is much lower than that of model forging.
The forgings are simple in shape, low in dimension accuracy and rough in surface. The worker has high labor intensity and requires high technical level.
It is not easy to realize mechanization and automation.
2. Die forging
Die forging refers to the forging method of obtaining forgings by forming blanks on special die forging equipment. The forgings produced by this method are accurate in size, small in processing allowance and high in productivity with complex structure.

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According to the different types of equipment used: hammer die forging, crank press die forging, flat forging machine die forging and friction press die forging.
The most commonly used equipment for hammer forging is steam-air hammer, anvil-free hammer and high-speed hammer.
Forging die bore:
According to its different functions, it can be divided into two categories: die forging chamber and blank making chamber.

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Fig. 3 Die for die forging on hammer
(1-hammer head; 2-upper die; 3-flying groove; 4-lower die; 5-die pad; 6,7,10-fastening wedge; 8-parting surface; 9-die chamber)
1) Die Forging Chamber
(1) Pre-forging die bore:
The function of the pre-forging die chamber is to deform the blank to the shape and size close to the forging, so that when the final forging is carried out, the metal can easily fill the die chamber and obtain the required size of the forging. Pre-forging die chamber is not necessary for simple forgings or small batches. The fillet and obliquity of the pre-forging die chamber are much larger than that of the final forging die chamber, and there is no flash groove.
(2) Final forging die bore:
The function of the final forging die chamber is to make the blank finally deform to the required shape and size of the forging. Therefore, its shape should be the same as that of the forging. However, because the forging shrinks when it is cooled, the size of the final forging die chamber should be enlarged by one shrinkage compared with the size of the forging. The shrinkage of steel forgings is 1.5%. In addition, there are flash grooves around the die chamber to increase the resistance of metal flowing out of the die chamber, to make the metal fill the die chamber, and to accommodate the surplus metal.
2) Blanking Mold Chamber
For complex forgings, in order to make the shape of the blank basically conform to the shape of the forging, so that the metal can be reasonably distributed and well filled in the chamber, it is necessary to pre-produce the blank in the chamber.
(1) Drawing the 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 chamber can be divided into open type and closed type.

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Figure 4 Drawing die bore: (a) Open type; (b) Closed type
(2) Rolling die bore:
It is used to reduce the cross-sectional area of one part of the blank to increase the cross-sectional area of the other part, so that the metal is distributed according to the shape of the forging. Rolling die chamber is divided into open and closed type.

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Fig. 5 Rolling die bore: (a) open type; (b) closed type
(3) Bending die bore:
For the bending rod forgings, the blank needs to be bent by the bending chamber.

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Figure 6 Bending die bore
(4) Cut off the die bore:
It consists of a pair of cutters at the angles of the upper and lower dies, which are used to cut off the metal.

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Figure 7 Cut-off die bore
The production efficiency is high. In 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 metal streamline distribution more reasonable, and improve the service life of parts.
Die forgings have more precise size, better surface quality and less machining allowance.
Save metal material 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.
3. Roll forging
Roll forging refers to the forging process in which a pair of rotating fan-shaped dies are used to produce plastic deformation of the blank, so as to obtain the required forgings or forgings.
The deformation principle of roll forging is shown above. Roll forging deformation is a complex three-dimensional deformation. Most of the deformed materials flow along the length direction to increase the length of the billet, while a small number of materials flow along the transverse direction to increase the width of the billet. In the process of roll forging, the cross section area of the blank root decreases continuously. Roller forging is suitable for the deformation process of 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, picks and turbine blades. The roll forging process uses the rolling forming principle to gradually deform the blank.
Compared with common die forging, roll forging has the advantages of simple equipment structure, stable production, low vibration and noise, easy to realize automation and high production efficiency.
4. Foetal die forging
Foetal die forging is a kind of forging method which uses free forging method to make billet and then finally forms in the die. It is a forging method between free forging and die forging. There are few die forging equipments and most of them are used in small and medium-sized enterprises of free forging hammer.
There are many kinds of die used in tire forging. In production, there are many kinds of die, such as model wrestling, fastening die, sleeve die, cushion die, closing die, etc.
Closed cylinder die is mostly used for forging rotary forgings. For example, gears with convex ends are sometimes used for forging non-revolving forgings. Closed cylinder die forging belongs to non-flash forging.
For complex shape forgings, two half-dies (i.e. one parting surface) are added to the cylinder die to make the combined cylinder die. The blank is formed in the chamber composed of two half-dies.
The conjugate film is usually composed of upper and lower modules. In order to make the upper and lower dies coincide with each other and not cause the forgings to shift, guide pillars and pins are often used to locate the forgings. Closing die is mostly used to produce complex non-revolving forgings, such as connecting rods, fork forgings and so on.
Compared with free forging, die forging has the following advantages:
Because the billet is formed in the die chamber, the forging size is more accurate, the surface is fairly smooth, and the distribution of streamline structure is more reasonable, so the quality is higher.
Die forging can forge complex forgings, because the shape of the forgings is controlled by the die chamber, the billet forming is faster, and the productivity is 1-5 times higher than that of free forging.
There are few spare blocks, so the processing allowance is small, which can save metal materials and reduce the processing time.
Disadvantages and limitations:

  • Large tonnage forging hammer is needed.
  • Only small forgings can be produced.
  • The service life of the mould is low.
  • When working, it usually relies on manpower to move the mould, so the labor intensity is relatively high.
  • Die forging is used in the production of medium and small batches of forgings.

III. Forging Defects and Analysis
The raw materials used for forging are ingots, rolled materials, extruded materials and forged blanks. Rolling, extrusion and forging billet are semi-finished products of ingot after rolling, extrusion and forging respectively. Generally speaking, the appearance of internal or surface defects of ingots is inevitable. In addition, the improper forging process in the forging process will eventually lead to defects in the forgings. Following is a brief introduction to some common defects in forgings.
1. Forging defects due to defects in raw materials usually include:
Surface crack
Surface cracks mostly occur on rolled and forged bars, generally in a straight line shape, consistent with the main deformation direction of rolling or forging. There are many reasons for this defect. For example, the subcutaneous bubbles in ingot extend along the deformation direction while rolling, and then expose to the surface and develop deep inside. For example, in rolling, the surface of billet, such as scratched, will cause stress concentration during cooling, which may cause cracking along scratches and so on. If this kind of crack is not removed before forging, it may expand and cause cracks in forgings.
The reason for folding is that when the metal billet is rolled, the groove sizing on the roll is incorrect, or the burrs on the worn surface of the groove are involved in the rolling process, forming a fold with a certain inclination to the material surface. For steel, there is iron oxide inclusion in the crack and decarbonization around it. If folding is not removed before forging, it may cause folding or cracking of forgings.
To scar
Scar is an exfoliable film on the local area of the rolled surface.
Scar formation is caused by the splash of molten steel during casting and condensation on the surface of ingot. Scar formation is formed when the film is pressed and attached to the surface of the rolled material during rolling. After the forging is cleaned by acid pickling, the film will peel off and become a surface defect of the forging.
Layered fracture
Layered fracture is characterized by its fracture or cross section similar to broken slate and bark.
Layered fracture occurs mostly in alloy steels (chromium-nickel steel, chromium-nickel-tungsten steel, etc.) and is also found in carbon steels. This defect is due to the existence of non-metallic inclusions, dendrite segregation and pore loosening in the steel. During forging and rolling, the steel is elongated along the rolling direction, resulting in sheet-like steel. If there are too many impurities, there is a risk of delamination and rupture in forging. The more serious the lamellar fracture is, the worse the plasticity and toughness of the steel, especially the low transverse mechanical properties, so the steel with obvious lamellar defects is not qualified.
Bright line (bright area)
Bright lines are thin lines with reflective ability and crystalline brightness on the longitudinal fracture. Most of them run through the whole fracture, and most of them occur in the axis part.
Bright wires are mainly caused by alloy segregation. The slight bright line has little effect on the mechanical properties, and the severe bright line will obviously reduce the plasticity and toughness of the material.
Nonmetallic inclusions
Non-metallic inclusions are mainly formed during the cooling process of molten steel during melting or casting due to chemical reactions between components or between metals and furnace gases and containers. In addition, inclusions can also be formed when refractories fall into molten steel during metal smelting and casting, which are collectively called slag inclusions. On the cross section of forgings, non-metallic inclusions can be distributed in dots, flakes, chains or lumps. Serious inclusions can easily cause cracking of 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 the accumulation of more carbides in local areas. It is mainly caused by ledeburite eutectic carbide and secondary network carbide in steel, which are not broken and evenly distributed during billet opening and rolling. Carbide segregation will reduce the forging deformability of steel and easily cause cracking of forgings. The forgings are easy to be overheated, over-heated and quenched during heat treatment and quenching.
Aluminum alloy oxide film
Aluminum alloy oxide film is usually located on the web of die forgings and near the die splitting surface. There are two kinds of characteristics on the fracture surface: one is flat sheet, the color ranges from silver-gray, light yellow to brown and dark brown; the other is small, dense and shiny punctate.
The oxide film of aluminium alloy is formed when the exposed melt surface interacts with water vapor or other metal oxides in the atmosphere during melting and casting. It is formed in the inner part of the liquid metal involved in the process of casting.
The oxide film in forgings and die forgings has no obvious effect on the longitudinal mechanical properties, but has great influence on the mechanical properties in the direction of height. It reduces the strength properties in the direction of height, especially the elongation, impact toughness and corrosion resistance in the direction of height.
White dot
The main characteristics of white spot are round or elliptical silver-white spot on the longitudinal fracture surface of billet and fine crack on the transverse fracture surface. White spots vary in size, ranging in length from 1 to 20 mm or longer. White spots are common in alloy steels such as nickel-chromium steel and nickel-chromium-molybdenum steel. They are also found in common carbon steels, which are hidden defects inside. White spots are produced under the combined action of structural stress and thermal stress during hydrogen and phase transformation. It is easier to produce white spots when the hydrogen content in steel is high and the cooling (or heat treatment after forging) after hot pressing is too fast.
Forgings forged from steel with white spots are liable to crack during heat treatment (quenching), sometimes falling in blocks. White spot reduces the plasticity of steel and the strength of parts. It is the stress concentration point. Like a sharp cutter, it can easily become fatigue crack under the action of alternating loads and lead to fatigue failure. Therefore, white spots are absolutely not allowed in forging raw materials.
Coarse-grained ring
Coarse grained rings are often defects in extruded bars of aluminium or magnesium alloys.
The extruded bars of aluminium and magnesium alloys supplied after heat treatment often have coarse grained rings on the outer surface of their circular sections. The thickness of coarse grained ring increases gradually from the beginning to the end of extrusion. If the lubrication condition is good during extrusion, coarse grained rings can be reduced or avoided after heat treatment. On the contrary, the ring thickness will increase.
The reason for the formation of coarse-grained rings is related to many factors. But the main factor is the friction between metal and extrusion cylinder during extrusion. This kind of friction results in the crushing degree of the grain on the surface of the cross section of the extruded bar much greater than that at the center of the bar. However, due to the influence of cylinder wall, the temperature in this region is low, and the recrystallization during extrusion is not complete. The recrystallized grains during quenching and heating are recrystallized, and the recrystallized grains are grown and swallowed up, thus forming coarse grained rings on the surface.
Blanks with coarse grained rings are easy to crack during forging. If coarse grained rings are left on the surface of forgings, the performance of parts will be reduced.
Shrinkage pipe remnants
The residual shrinkage pipe is generally caused by the centralized shrinkage holes produced in the riser of the ingot which are not removed completely and remain in the steel during billet opening and rolling.
Dense inclusions, looseness or segregation usually occur in the area near the shrinkage pipe residue. Irregular wrinkled crevices at transverse low magnification. Cracking of forgings is easy to occur during forging or heat treatment.
2. Defects caused by improper material preparation and their effects on forgings
There are several defects caused by improper preparation.
Cutting oblique
Cutting obliquity is when cutting material on a saw or punch, because the bar is not compacted, the obliquity of the end face of the blank relative to the longitudinal axis exceeds the allowable value. Severe shear may fold during forging.
The end of billet is bent and burred
When cutting on the cutting machine or punch, the blank has been bent before cutting because of the excessive gap between the blades of scissors or cutting dies or because the edges are not sharp. As a result, part of the metal is squeezed into the gap between the blades or dies to form the end drooping burr.
Burred billet is easy to cause local overheating and overheating when heated, and easy to fold and crack when forged.
End face depression of billet
When cutting on the shear machine, because the gap between the scissors is too small, the cracks on and below the metal section do not coincide, resulting in secondary shearing. As a result, part of the end metal is pulled off and the end surface is concave. Such billets are prone to folding and cracking during forging.
End crack
In cold shearing of large section alloy steel and high carbon steel bars, cracks are often found at the end of the bars 3 to 4 hours after shearing. The main reason is that the unit pressure of the blade is too large to flatten the round section billet into an ellipse, which results in great internal stress in the material. The flattened end surface strives to restore its original shape. Under the action of internal stress, cracks often occur within a few hours after cutting. Shear cracks are also prone to occur when the hardness is too high, the hardness is not uniform and the material segregation is serious.
For blanks with end cracks, the cracks will further expand during forging.
Gas Cutting Crack
Gas cutting cracks are usually located at the end of the blank, which is caused by the tissue stress and thermal stress of the raw material before gas cutting without preheating.
For blanks with gas cutting cracks, the cracks will further expand during forging. Therefore, it should be cleared beforehand before forging.
Cracking of convex core
When cutting material on lathe, convex core is often left in the central part of bar end face. In the forging process, the plasticity of the convex core is low because of its small section and fast cooling, but the section of the blank matrix is large, the cooling rate is slow and the plasticity is high. Therefore, at the sudden change of the cross-section, it becomes the place where stress concentration occurs, and the plasticity of the two parts is quite different. Therefore, under the action of hammering force, the cracking around the convex core is easy to occur.
3. Defects often caused by improper heating process
Defects caused by improper heating can be divided into:

  • (1) Defects such as oxidation, decarbonization, carburization, sulphurization and copper infiltration caused by the change of the histochemical state of the outer layer of billet due to the influence of medium;
  • (2) Defects caused by abnormal changes in internal structure, such as overheating, overheating and non-thermal penetration;
  • (3) Because of the uneven distribution of temperature in the billet, the internal stress (such as temperature stress and tissue stress) is too large and the billet cracks occur.

Here are some of the common shortcomings.
Decarbonization refers to the phenomenon that the surface carbon of metals is oxidized at high temperatures, which makes the carbon content of the surface layer significantly lower than that of 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. It is easy to decarbonize when heated in oxidizing atmosphere, high carbon steel and steel with high silicon content.
Decarbonization decreases the strength and fatigue properties of parts and the wear resistance.
Carbonization often occurs on the surface or part of the surface of forgings heated by oil furnace. Sometimes the thickness of the carburized layer is 1.5-1.6 mm, the carbon content of the carburized layer is about 1% (mass fraction), and the carbon content of the local point is even more than 2% (mass fraction). Ledeburite structure appears.
This is mainly due to the incomplete combustion of oil and air when the billet is near the nozzle of the oil furnace or in the area where the two nozzles cross-inject fuel, resulting in the formation of a reductive carburizing atmosphere on the surface of the billet, resulting in the effect of surface carburization.
Carbon addition deteriorates the machinability of forgings and makes cutting easier.
Overheating refers to the phenomenon that the heating temperature of metal billet is too high, or the residence time is too long in the prescribed temperature range of forging and heat treatment, or the grain coarsening is caused by the excessive temperature rise due to the thermal effect.
Widmanstatten structure often occurs in carbon steels (hypoeutectoid or hypereutectoid steels) after overheating. After the martensitic steel is overheated, the intragranular texture often appears. The superheated structure of tool and die steels is usually determined by the angularization of primary carbide. After superheating, obvious beta phase grain boundary and straight and slender Widmanstatten structure appeared in the titanium alloy. After superheating, the fracture of alloy steel will appear stony or strip fracture. The mechanical properties, especially impact toughness, of superheated structure will be reduced due to the coarse grain size.
After normal heat treatment (normalizing and quenching), the structure of overheated structural steels can be improved and their properties can be restored. This kind of overheating is often called unstable overheating. However, after normal normalizing (including high temperature normalizing), annealing or quenching, the overheated structure of alloy structural steels can not be completely eliminated. In addition, this type of overheating is often referred to as stable overheating.
Over burning
Overburning refers to the excessive heating temperature of metal billet or the long residence time in the high-temperature heating zone. Oxygen and other oxidizing gases in the furnace penetrate into the voids between metal grains and oxidize with iron, sulphur and carbon, forming eutectic of fusible oxides, destroying the relationship between grains and dramatically reducing the plasticity of materials. The metal with serious overburning will crack at the slightest blow when it is rough removed, and the transverse crack will appear at the overburning place when it is drawn out for a long time.
There is no strict temperature boundary between overheating and overheating. Overburning is generally judged by the appearance of oxidation and melting of grains. For carbon steel, fishbone-like ledeburite occurs at grain boundaries due to melting during overburning and over-burning of severely oxidized tool and die steels (high-speed steel, Cr12 steel, etc.). The grain boundary melting triangle zone and remelting sphere occur when the aluminium alloy is overfired. After the forgings are burned, they are often irreparable and have to be scrapped.
Heating Crack
When large ingot with large section size and high alloy steel and superalloy billet with poor thermal conductivity are heated at low temperature, if the heating speed is too fast, the billet will have great thermal stress due to large temperature difference between inside and outside. In addition, the plasticity of the billet is poor because of the low temperature. If the value of thermal stress exceeds the strength limit of the billet, a radial heating crack will occur from the center to all sides, which will cause the whole section to crack.
Copper brittleness
Copper brittle cracks on the surface of forgings. At high magnification, pale yellow copper (or copper solid solution) is distributed along grain boundaries.
When billet is heated, such as residual copper oxide chips in furnace, steel oxide is reduced to free copper at high temperature, and molten steel atoms expand along austenite grain boundary, which weakens the relationship between grains. In addition, when the content of copper in steel is higher [> 2% (mass fraction)], if heated in oxidizing atmosphere, copper-rich layer is formed under the iron oxide skin, which also causes brittleness of steel.
4. Defects often caused by improper forging process
Defects caused by improper forging process are usually the following
Large grain
Large grains are usually caused by excessive initial forging temperature and insufficient deformation degree, or excessive final forging temperature, or the degree of deformation falling into the critical deformation zone. The deformation degree of aluminium alloy is too large to form texture, and the deformation temperature of superalloy is too low to form mixed deformation structure, which may also cause coarse grains.
Coarse grain size will reduce the plasticity and toughness of forgings, and the fatigue properties will be significantly reduced.
Grain inhomogeneity
Grain inhomogeneity refers to the fact that the grains in some parts of forgings are especially coarse, but smaller in some parts. The main reason for the uneven grain size is that the uneven deformation around the billet causes the different degree of grain breakage, or the degree of local deformation falls into the critical deformation zone, or the local work hardening of superalloy, or the local grain size is large when quenched and heated. Heat-resistant steels and superalloys are particularly sensitive to grain inhomogeneity. Non-uniform grain size will significantly reduce the durability and fatigue properties of forgings.
Cold hardening phenomenon
The softening caused by recrystallization may not keep up with the strengthening (hardening) caused by deformation due to low temperature or too fast deformation speed and too fast cooling after forging, so that the cold deformation structure remains partly 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 cracks.
Forging cracks are usually caused by large tensile stress, shear stress or additional tensile stress during forging. Cracks usually occur in the parts with the greatest stress and the thinnest thickness. If there are micro-cracks on the surface and inside of the billet, or there are structural defects in the billet, or if the plasticity of the material is reduced due to improper hot working temperature, or if 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 the processes of coarsening, drawing, punching, expanding, bending and extrusion.
Forging tortoise crack is a shallow tortoise crack on the surface of forgings. This defect is most likely to occur on the surface of the tension stress in forging forming (e.g., the unfilled protruding part or the bending part).
The internal causes of cracking may be multifaceted:

  • (1) There are too many fusible elements such as copper and Sn.
  • (2) When the steel is heated for a long time at high temperature, copper precipitation, coarse grain size, decarbonization or multiple heating surfaces occur on the steel surface.
  • (3) The sulphur content of fuel is too high and there is sulphurizing on the surface of steel.

Flying edge 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. During die forging operation, the phenomenon of steel bar piercing is caused by heavy blow. (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
Forging parting surface cracks refer to the cracks along the parting surface of forgings. There are many non-metallic inclusions in the raw materials, and the cracks on the parting surface are often formed after the flow and concentration to the parting surface during die forging or the remnants of shrinkage pipe are squeezed to the flying edge during die forging.
Forging folding is formed by the combination of oxidized surface metals during metal deformation. It can be formed by the convection of two (or more) strands of metal; it can also be formed by the rapid mass flow of one metal bringing the surface metal of the adjacent part with it to flow; it can also be formed by the bending and reflux of the deformed metal; it can also be the partial deformation of some metals and the pressed person. Another part of the metal is formed inside. Folding is related to the shape of raw material and blank, the design of die, the arrangement of forming process, lubrication and the actual operation of forging.
Forging folding not only reduces the bearing area of parts, but also often becomes the source of fatigue due to the stress concentration here.
Through flow
Forging penetration is a form of improper streamline distribution. In the cross-flow zone, streamlines originally distributed at a certain angle converge to form cross-flow, and the grain sizes inside and outside the cross-flow zone may be quite different. The reason of penetration is similar to folding. It is formed by the confluence of two metals or one metal with another metal, but the metal in the penetrating part is still a whole.
The mechanical properties of forgings are reduced by forging through-flow, especially when the grains on both sides of the through-flow band are quite different.
Streamline distribution of forgings is not smooth
The irregular distribution of streamlines in forgings refers to the disorder of streamlines, such as streamline cut-off, backflow and eddy current, occurring at low forgings. If the die design is improper or the forging method is unreasonable, the flow line of prefabricated blank is disordered; if the worker operates improperly and the die wears and tears, the metal will flow unevenly, the flow line of forgings will not be distributed smoothly. Streamline irregularity will reduce the mechanical properties of various kinds of forgings, so there is a requirement of streamline distribution for important forgings.
Casting Tissue Residues
The residual structure of forging casting mainly occurs in Forgings Using ingots as blanks. As-cast structure mainly resides in the difficult deformation zone of forgings. Inadequate forging ratio and improper forging method are the main reasons for residual casting structure.
The residual structure of forging casting will degrade the properties of forgings, especially the impact toughness and fatigue properties.
Carbide segregation level does not meet the requirements
Forging carbide segregation grade does not meet the requirements mainly occurs in ledeburite tool and die steels. The main reason is that the distribution of carbides in forgings is not uniform, and the carbides are concentrated in large blocks or distributed in a network. The main reason for this defect is that the carbide segregation grade of raw material is poor, and the forgings with this defect are easy to be overheated and cracked locally when heat treatment and quenching is carried out because of insufficient forging ratio or improper forging method. The blades and dies made are easy to crack when used.
Zonal tissue
Forging banded structure is a kind of structure with banded distribution of ferrite and pearlite, ferrite and austenite, ferrite and bainite as well as ferrite and martensite in forgings. They mostly occur in sub-eutectic folded steel, austenitic steel and semi-martensitic steel. This kind of structure is the banded structure produced during forging deformation under the condition of two-phase coexistence, which can reduce the transverse plastic index of materials, especially impact toughness. It is easy to crack along the ferrite band or the intersection of two phases when forging or parts are working.
Insufficient local filling
Deficiency of local filling in forging mainly occurs in ribs, convex corners, turning corners and rounded corners, and the size does not meet the requirements of the pattern. The causes may be as follows: 1) low forging temperature, poor metal fluidity; 2) insufficient equipment tonnage or hammering force; 3) unreasonable design of blank die, unqualified blank volume or section size; 4) accumulation of oxide scale or welded deformed metal in the chamber.
Forging underpressure refers to the general increase of dimension perpendicular to the parting surface, which may be caused by:

  • (1) low forging temperature.
  • (2) Insufficient tonnage of equipment, insufficient hammering force or insufficient hammering times.

Forging offset is the displacement of forgings along the upper part of the parting surface relative to the lower part. The reasons may be as follows:

  • 1) the gap between slider (hammer head) and guide rail is too large;
  • 2) the design of forging die is unreasonable, lacking of locks or guide pillars to eliminate the misalignment force;
  • 3) the installation of die is not good.

Axis Bending
Forging forging axes are bent, which is in error with the geometric position of the plane. The causes may be as follows:

  • 1) inadvertent drawing of forgings;
  • 2) uneven force during trimming;
  • 3) different cooling rates of various parts of forgings during cooling;
  • 4) improper cleaning and heat treatment.

5. 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 occur in the inner part of the forging due to the excessive cooling rate, and it may also be caused by the transformation of the structure. If these stresses exceed the strength limit of the forging, smooth and slender cooling cracks will occur in the forging.
Reticulated carbide
When forging steels with high carbon content, if the forging temperature is high and the cooling rate is too slow, the carbides will precipitate in a network along the grain boundary. For example, carbides precipitate along grain boundaries when bearing steel is slowly cooled at 870-770 (?)
The forged carbides are easy to cause quenching cracks during heat treatment. In addition, it also deteriorates the performance of parts.
6. Defects often caused by improper heat treatment after forging
Defects caused by improper heat treatment after forging usually include:
Excessive or insufficient hardness
The reasons for inadequate hardness of forgings caused by improper heat treatment after forging are:

  • (1) too low quenching temperature;
  • (2) too short quenching heating time;
  • (3) too high tempering temperature;
  • (4) serious decarburization of forgings surface caused by repeated heating;
  • (5) unqualified chemical composition of steel, etc.

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

  • (1) too fast cooling after normalizing;
  • (2) too short heating time after normalizing or tempering; 
  • (3) unqualified chemical composition of steel.

Uneven hardness
The main reason of uneven hardness caused by forging is improper heat treatment process, such as too much primary charging or too short holding time, or partial decarbonization of forgings caused by heating.
7. Defects often occurring in improper cleaning process of forgings
There are usually several kinds of defects in cleaning forgings.
Excessive pickling
Excessive pickling in forging will make the surface of forgings loose and porous. This defect is mainly caused by the excessive depth of acid and the long residence time of forgings in the pickling tank, or by the unclean surface cleaning of forgings and acid residue on the surface of forgings.
Corrosion Crack
If there is a large residual stress after forging martensitic stainless steel forgings, it is easy to produce small network corrosion cracks on the surface of forgings during acid pickling. If the structure is coarse, the formation of cracks will be faster.

Forging flange is one of the best mechanical properties of flange products. Its raw material is usually tube billet. After cutting, it is hammered continuously to eliminate segregation and looseness in ingot. The price and mechanical properties are one grade higher than those of common cast flanges. Flange is a part that connects pipes to pipes and valves and connects them to the end of pipes. It is also used in the import and export of equipment. Flange is used to connect pipes and valves to the end of pipes. It is an accessory product of pipeline. The main materials of forging flange are carbon steel, alloy steel and stainless steel. The main standards are national standard, electric standard, American standard, German standard, Japanese standard and so on. The main anticorrosion treatments are oiling and galvanizing. Forging flange has good pressure and temperature resistance, which is generally suitable for high pressure and high temperature working environment.

Source: China Flange Manufacturer – Yaang Pipe Industry (www.steeljrv.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.)

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