Forging has a long history in China, which has been continued by the production method of hand workshops. It was probably in the early 20th century. It only gradually emerged as a mechanical industrialized production method in the railroad, military, and shipbuilding industries. The main sign of this transformation was using machines with powerful forging capabilities.
The machining method of forging was widely used in the automobile manufacturing process. With the progress of science and technology and the increasing requirements for workpiece precision, precision forging technology, which has the advantages of high efficiency, low cost, low energy consumption, and high quality, is being used more and more widely. According to the different deformation temperatures of metal plastic forming, precision cold forging forming can be divided into cold forging forming, temperature forming, sub-hot forging forming, hot fine forging forming, etc. The production of automotive parts includes automotive clutch engagement gear rings, automotive transmission input shaft parts, bearing rings, automotive isometric universal joint slide series products, automotive differential gear, automotive front axle, etc.
The definition and classification of forging
Definition of forging
What is forging? Forging is forging machinery applying pressure to the metal billet to produce plastic deformation to obtain a certain mechanical property, a certain shape, and size forgings processing methods; forging (forging and stamping) is one of the two major components.
Through forging can eliminate defects such as cast looseness produced by the metal in the smelting process and optimize the microstructure. In contrast, the mechanical properties of forgings are generally better than castings of the same material due to the preservation of the complete metal flow line. Related machinery in the high load, working conditions of the important parts, and the simpler shape of the available rolled plate, profile, or welded parts, more forgings.
The classification of forging
According to the different production tools, forging technology can be divided into free forging, modular forging, lapping rings, and special forging.
Free forging: refers to the processing method of forging parts with simple general-purpose tools or between the upper and lower anvils of forging equipment to apply external force directly to the billet to deform the billet and obtain the desired geometric shape and internal quality.
Die forging: refers to the metal billet in a certain shape of the forging die chamber pressure deformation and obtaining forgings. Die forging can be divided into hot die forging, warm forging, and cold forging. Warm forging and cold forging is the future development direction of die forging but also represent the level of forging technology.
Ring lapping: refers to the production of ring-shaped parts of different diameters by a special equipment ring lapping machine, which is also used to produce car wheels, train wheels, and other wheel-shaped parts.
Special forging: including roll forging, wedge cross-rolling, radial forging, liquid die forging, and other forging methods, which are more suitable for producing certain special-shaped parts. For example, roll forging can be used as an effective pre-forming process to reduce the subsequent forming pressure significantly; wedge cross-rolling can produce parts such as steel balls and drive shafts; radial forging can produce large barrels, step shafts, and other forgings.
According to the forging temperature, forging technology can be divided into hot forging, warm forging, and cold forging.
The beginning recrystallization temperature of steel is about 727℃, but 800℃ is commonly used as the division line, higher than 800℃ is hot forging; between 300 and 800℃ is called warm forging or semi-hot forging, forging at room temperature is called cold forging. Forgings used in most industries are hot forging, warm forging, and cold forging are mainly used in the forging of parts for automobiles, general machinery, etc. Warm forging and cold forging can be effective in saving materials.
According to the movement of the forging die, forging can be divided into pendulum rolling, pendulum rotary forging, roll forging, wedge cross-rolling, ring rolling, and oblique rolling.
Forging materials are mainly carbon steel and alloy steel of various compositions, followed by aluminum, magnesium, copper, titanium, and other alloys, iron-based high-temperature alloys, nickel-based high-temperature alloys, cobalt-based high-temperature alloys deformation of the alloy is also completed by forging or rolling. Still, due to their plastic zone, these alloys are relatively narrow so the forging will be relatively difficult; the heating temperature of different materials, open forging temperature, and final forging temperature have strict requirements.
The material’s original state is a bar, ingot, metal powder, and liquid metal. The ratio of the cross-sectional area of the metal before deformation to the cross-sectional area after deformation is called the forging ratio.
The correct choice of forging ratio, reasonable heating temperature and holding time, reasonable initial and final forging temperature, and reasonable deformation volume and deformation speed is relevant to improving product quality and reducing costs.
The commonly used forging methods and their advantages and disadvantages
Free forging refers to the processing method of forging parts with simple general-purpose tools or between the upper and lower anvils of forging equipment to apply external force directly to the billet so that the billet is deformed to obtain the desired geometric shape and internal quality of forgings. The forgings produced by the free forging method are called free forgings.
Free forging mainly produces a small batch of forgings, using forging hammers, hydraulic presses, and other forging equipment to form and process the billet to obtain qualified forgings. The basic free-forging process includes upsetting, drawing, punching, cutting, bending, twisting, shifting, and forging. Free forging is done by hot forging.
Free forging process
Including basic process, auxiliary process, and finishing process.
The basic processes of free forging: are upsetting, drawing, punching, bending, cutting, twisting, misalignment and forging, etc., and the three most common processes in actual production are upsetting, drawing, and punching.
Auxiliary process: pre-deformation process, such as pressure jaws, pressure ingot edges, shoulder cutting, etc.
Finishing process: reducing the surface defects of forgings, such as removing the unevenness of the forging surface and shaping, etc.
- Flexibility in forging, allowing the production of small parts of less than 100kg and heavy parts of up to 300t or more;
- The tools used are simple general-purpose tools;
- Forging forming is to deform the billet gradually in areas. Thus the tonnage of forging equipment required to forge the same forgings is much smaller than that of model forging;
- Low precision requirements for the equipment;
- Short production cycle.
Disadvantages and limitations:
- Much lower production efficiency than model forging;
- Simple shape, low dimensional accuracy, and rough surface of forgings; high labor intensity of workers and high technical level required;
- Need to realize mechanization and automation.
Die forging is a method to obtain forgings by using a die to shape the blank on special die forging equipment. This method produces forgings with precise dimensions, small machining allowances, and complex structures with high productivity.
The different classifications of the equipment used: are hammer forging, crank press forging, flat forging machine, and friction press forging.
The most common equipment used for die forging on the hammer is a steam-air die forging hammer, no anvil seat hammer, and a high-speed hammer.
Forging die chamber:
Its functions can be divided into two categories forging die chamber and billet-making die chamber.
1) Die forging chamber
(1) Pre-forging die chamber:
The role of the pre-forging die chamber is to deform the blank to a 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. Forgings with simple shapes or small batches may not have a pre-forging chamber. The rounding and bevel of the pre-forging chamber are much larger than that of the final forging chamber, and there is no flying edge groove.
(2) Final forging chamber:
The role of the final forging die chamber is to make the final deformation of the blank to the shape and size required by the forging; therefore, its shape should be the same as the shape of the forging; but because the forging is shrinking when cooling, the size of the final forging die chamber should be a shrinkage amount larger than the size of the forging. The shrinkage of steel forgings is taken as 1.5%. In addition, flying edge grooves around the die chamber increase the resistance of metal flowing out from the chamber to promote the metal filled with the die chamber while accommodating the excess metal.
2）Die chamber for billet making
For complex-shaped forgings, to make the shape of the blank conform to the shape of the forging so that the metal can be reasonably distributed and well-filled with the die chamber, it is necessary to make the blank in the billet-making die chamber in advance.
(1) Drawing chamber:
It is used to reduce the cross-sectional area of a part of the blank to increase the length of that part. There are two types of chambers: open and closed.
(2) Rolling chamber:
It is used to reduce the cross-sectional area of one part of the blank to increase the cross-sectional area of another so that the metal is distributed according to the shape of the forging. The rolling die chamber is divided into open and closed types.
(3) Bending die chamber:
A bending die chamber is required to bend the blank for bent rod forgings.
(4) Cut off die chamber:
It comprises a pair of cut-offs on the corners of the upper and lower dies to cut off the metal.
- Higher production efficiency. When die forging, the deformation of metal in the die chamber, so it can be faster to obtain the desired shape;
- The ability to forge complex shaped forgings, make the metal flow distribution more reasonable and improve the service life of the parts;
- More accurate dimensions, better surface quality, and smaller machining allowances for die forgings
- Save metal materials and reduce the workload of cutting and machining;
- Under a sufficient batch, it can reduce the cost of parts.
Disadvantages and limitations:
- The weight of the drop-forged parts is limited by the capacity of general drop-forging equipment, mostly below 7OKg;
- Long manufacturing cycle and high cost of forging dies;
- The investment cost of die-forging equipment is higher than that of free forging.
Roll forging is a process that uses a pair of fan-shaped dies rotating in opposite directions to produce plastic deformation of the billet to obtain the required forgings or forging billets.
The roll forging deformation principle is shown above. Roll forging deformation is a complex three-dimensional deformation. Most of the deformed material flows in the length direction to increase the length of the billet, while a small portion of the material flows laterally to increase the width of the billet. The root cross-sectional area of the billet decreases continuously during the roll forging process. Roll forging is suitable for the deformation processes such as shaft parts drawing, slab rolling, and material distribution along the length direction.
Roll forging can produce connecting rods, twist drill bits, wrenches, wrenches, hoes, picks, and turbine blades. Roll forging uses the roll forming principle to deform the blank gradually.
Compared with ordinary die forging, roll forging has a simpler equipment structure, smooth production, vibration, small noise, easy-to-achieve automation, and high production efficiency.
Tire die forging is a forging method using the free forging method to make billets, and then in the final shape of the tire die, it is a forging method between free forging and die forging. In the die forging equipment is less, most of the free forging hammers in small and medium-sized enterprises are commonly used.
Tire die forging uses many types of tire die; the production commonly used are type fall, buckle dies, set die, pad die, combined die, etc.
Closed barrel die is mostly used for forging rotary forgings. Such as the two ends of the gears with tabs, etc., sometimes also used to forge non-rotary forgings. Closed barrel forging is forging without flying edges.
For complex-shaped die forgings, it is necessary to add two half-dies (i.e., increase a parting surface) in the cylinder die to make a combined cylinder die, and the blank is formed in the die chamber composed of two half-dies.
The film usually comprises two parts: the upper and lower die. Guide pillars and pins are often used for positioning to make the upper and lower dies fit together and not make the forgings mix shift. The closed die mostly produces non-rotary forgings with complex shapes, such as connecting rods, fork forgings, etc.
Compared with free forging, die forging has the following advantages:
- Higher quality because the billet is formed in the die chamber, so the forging size is more accurate, the surface is more polished, and the distribution of the streamlined organization is more reasonable;
- The die chamber controls the shape of the forging, so the billet is formed faster, and the productivity is 1 to 5 times higher than that of free forging;
- Fewer remaining blocks, thus smaller machining allowance, which can save metal materials and reduce machining person-hours.
Disadvantages and limitations:
- Larger tonnage forging hammers are required;
- Only small forgings can be produced;
- The lower service life of the die;
- The work generally relies on the workforce to move the tire die; thus, the labor intensity is higher;
- Tire die forging is used to produce medium and small batches of forgings.
Forging defects and analysis
The raw materials used for forging are ingots, rolled, extruded, and forging billets. The rolled material, extruded material, and forging billet are the semi-finished products processed by rolling, extrusion, and the forging of ingots. In general, the appearance of internal or surface defects of ingots is sometimes unavoidable. Coupled with the improper forging process, it eventually leads to defects in the forgings. The following is a brief introduction to some common defects in forgings.
Defects in forgings caused by defects in the raw material are usually
Surface cracks mostly occur in rolled and forged bars, which are generally linear in shape and in the same direction as the main deformation of rolling or forging. There are many reasons for this defect, such as subcutaneous air bubbles within the ingot elongating in the direction of deformation during rolling on one side and exposure to the surface and deeper into the interior. Another example is in the rolling, the billet’s surface, such as scratched, cooling will cause stress concentration, which may be cracked along the scratch, etc. If this crack is not removed before forging, forging may expand to cause forging cracks.
Fold formation is caused when the metal billet in the rolling process, due to incorrect sizing of the type groove on the roll, or due to the type groove wear surface generated by the burr in the rolling was involved in the formation and the material surface into a certain angle of inclination crease. For steel, iron oxide inclusions are in the fold, surrounded by decarburization. Fold, if not removed before forging, may cause forging folding or cracking.
Knotting is a peelable film in a localized area on the surface of the rolling stock.
The formation of scars is due to the casting of steel splash and condensation on the surface of the ingot; rolling is pressed into a film and attached to the surface of the rolled material, that is, scars. After forging forgings by pickling cleaning, the film will be peeled off and become a forging surface defect.
Laminar fracture is characterized by its fracture or section, and the broken slate bark is very similar.
Laminar fracture mostly occurs in alloy steel (chromium-nickel steel, chromium-nickel tungsten steel, etc.), and carbon steel is also found. This defect is due to non-metallic inclusions in the steel, dendritic segregation, porosity laxity, and other defects in the forging and rolling process along the rolling direction is elongated so that the steel is lamellar. If there are too many impurities, the forging is at risk of laminar rupture. The more serious the laminar fracture, the worse the plasticity and toughness of steel, especially since the transverse mechanical properties are very low, so steel with obvious laminar defects is not qualified.
Bright line (bright area)
The bright line is presented in the thin line’s longitudinal fracture crystalline shiny reflective ability, mostly throughout the fracture, which is produced in the axial part.
Bright lines are mainly caused by alloy segregation. Thin bright lines have little effect on mechanical properties; serious bright lines will significantly reduce the plasticity and toughness of the material.
Non-metallic inclusions are mainly formed during the molten or cast steel cooling due to chemical reactions between the components or between the metal and the furnace gas and container. In addition, refractory materials falling into the steel in metal melting and casting can also form inclusions; such inclusions are collectively referred to as slag. In the cross-section of the forging, non-metallic inclusions can be dotted, flakes, chains, or clumps of distribution. Severe inclusions will likely cause forging cracking or reduce the material’s performance.
Carbide segregation often occurs in alloy steels with high carbon content. A large number of carbide aggregates in the local area characterizes it. It is mainly caused by eutectic carbides and secondary reticulated carbides in steel, which are not broken up and are evenly distributed during opening and rolling. Carbide segregation will reduce the steel forging deformation performance, easy to cause forging cracking. Forging heat treatment quenching is easy to local overheating, overburning, and quenching cracking.
Aluminum oxide film
The aluminum oxide film is generally located on the forging web near the parting surface. In the low-time organization is micro-fracture; in the high-time organization is swirled; in the fracture, characteristics can be divided into two categories: one is a flat sheet, color from silver gray, light yellow to brown, dark brown; the second is a small dense and with the flash of dots.
The aluminum oxide film is the melt casting process of the open melt liquid surface and the atmosphere of water vapor or other metal oxides formed when the interaction of the oxide film in the process of transfer casting is rolled into the internal formation of liquid metal.
The oxide film in forgings and die-forged parts has no significant effect on the longitudinal mechanical properties. Still, it has a greater effect on the height direction mechanical properties, which reduces the height direction strength properties, especially the height direction elongation, impact toughness, and height direction corrosion resistance.
Round or oval silver-white spots on the longitudinal fracture of the billet and small cracks on the transverse fracture mainly characterize white spots. The size of the white spot varies, and the length is from 1 to 20 mm or longer. White spots are common in alloy steels such as Ni-Cr and Ni-Cr-Mo, are also found in ordinary carbon steels, and are hidden internal defects. White spots are produced by the combined effect of hydrogen and tissue stress during phase transformation and thermal stress. They are more likely to occur when the steel contains more hydrogen and cools too quickly after hot pressure processing (or post-forging heat treatment).
Forgings forged from steel with white spots are prone to cracking during heat treatment (quenching) and sometimes even fall off in pieces. The white point reduces the plasticity of the steel, and the strength of the part, is the stress concentration point; it is like a sharp cutter, and under the action of alternating load, it is easy to become fatigue cracks and lead to fatigue damage. Therefore, white spots are not allowed in forging raw materials.
Coarse crystal ring
A coarse crystal ring is often a defect on the aluminum or magnesium alloy extrusion bar.
After heat treatment of aluminum, magnesium alloy extrusion bars supplied often have coarse crystal rings in the round section of the outer layer. The thickness of the rough crystal ring, from the beginning to the end of the extrusion, is gradually increased. If the extrusion of good lubrication conditions, then after heat treatment, can reduce or avoid coarse crystal ring. On the contrary, the thickness of the ring will increase.
The cause of coarse crystal rings is related to many factors. But the main factor is the friction between the metal and the extrusion barrel during the extrusion process. This friction causes the outer surface grains of the extruded bar cross-section to be much more broken than the grains at the center of the bar. However, due to the influence of the bar wall, the temperature in this area is low, and the extrusion is not fully recrystallized, quenching and heating of the recrystallized grains recrystallized and grew engulfed the recrystallized grains, so the formation of coarse crystal ring in the surface layer.
Coarse crystal ring billet forging is easy to crack, such as coarse crystal environmental protection in the forging surface layer, which will reduce the performance of the parts.
Shrinkage residue is generally due to the ingot riser part of the shrinkage concentration not being removed clean, open billet, and rolling residual in the steel inside and produced.
The area near the shrinkage residue is generally dense with inclusions, loosening, or segregation. Gaps that are irregularly wrinkled in the low transverse times. Susceptible to cracking of forgings during forging or heat treatment.
Defects caused by improper material preparation and their effects on forgings
There are several defects caused by improper material preparation.
Slanting is caused by not pressing the bar tightly enough when feeding on a saw or punching machine, resulting in the slanting of the billet end face relative to the longitudinal axis exceeding the specified allowable value. Severe skew may result in folding during the forging process.
Bending of the billet end with burrs
When the material is fed from the shearing machine or punching machine, the blank is bent before it is cut due to excessive clearance between the blades of the scissors or cutting die or due to unsharp edges, as a result of which part of the metal is squeezed into the clearance of the blades or die, forming a sagging burr at the end.
Blanks with burrs are prone to local overheating and overburning when heated and folding and cracking when forged.
Blank end depression
In the shear bed up and down the material, due to the gap between the scissors being too small, the metal section on the upper and lower cracks do not overlap, resulting in secondary shear; the result is that part of the end metal is pulled off, the end face into a depression. Such blanks are prone to folding and cracking when forging.
In the cold shear large sections of alloy steel and high carbon steel bar material are often found in the shear 3 to 4h after the end of the crack. Mainly due to the unit pressure of the blade being too large, so the round section of the billet flattened into an oval when the material produced a large internal stress. And the flattened end surface to strive to restore the original shape, the role of internal stress is often cracked within a few hours after cutting the material. Material hardness is too high, uneven hardness and material segregation are more serious when also prone to shear cracking.
The crack will be further extended during the forging of blanks with end cracks.
Gas-cutting cracks are usually located at the end of the billet. They are caused by the tissue and thermal stresses generated during gas-cutting when the raw material is not preheated before gas-cutting.
The crack will be further expanded when forging the billet with gas cut crack. Therefore, it should be pre-cleared before forging.
Convex core cracking
When the lathe is dismantled, a convex core is often left in the center of the end face of the bar. During forging, the plasticity is low because the section of the convex core is small and cools quickly, but the section of the base part of the billet is large and cools slowly with high plasticity. Therefore, at the intersection of the abrupt change in the section becomes a stress concentration, coupled with the large difference in plasticity between the two parts, it is easy to cause cracking around the convex core under the action of hammering force.
Improper heating process often produces defects
Defects arising from improper heating can be divided into:
- (1) The influence of the medium so that the outer layer of the billet tissue chemical state changes caused by defects, such as oxidation, decarburization, carburization and sulfurization, copper penetration, etc.;
- (2) Defects caused by abnormal changes in the internal organization, such as overheating, overburning, and failure to heat through, etc.;
- (3) Due to the uneven distribution of temperature inside the billet, causing excessive internal stress (such as temperature stress, tissue stress) and the billet cracking, etc.
Several of these common defects are described below:
Decarburization refers to the oxidation of carbon in the surface layer of the metal at high temperatures, making the carbon content of the surface layer significantly lower than that of the interior.
The depth of the decarburized layer is related to the composition of the steel, the composition of the furnace gas, the temperature, and the holding time at this temperature. Oxidizing atmosphere heating is prone to decarburization, high carbon steel is easy to decarburize, and steel containing much silicon is also easy to decarburize.
Decarburization reduces the strength and fatigue properties of the parts and weakens the wear resistance.
Forgings heated by the oil furnace, often in the surface or part of the surface carbon increase phenomenon. Sometimes the thickness of the carbon layer is up to 1.5 ~ 1.6mm, the carbon layer of carbon content up to 1% (mass fraction) or so, and the focal point of the carbon content of even more than 2% (mass fraction), the appearance of site organization.
This is mainly in the case of oil furnace heating when the position of the billet is close to the oil furnace nozzle or in the area of the two nozzles cross injection of fuel because the oil and air are not well mixed. Therefore incomplete combustion results in the formation of a reducing carburizing atmosphere on the billet’s surface, resulting in the effect of surface carbonation.
The result is a reduced carburizing atmosphere on the billet’s surface, creating a surface carburizing effect. The carburizing deteriorates the machinability of the forging and makes it easier to hit the tool when cutting.
Overheating refers to the heating temperature of the metal billet being too high, or in the specified forging and heat treatment temperature range for too long, or due to the thermal effect of the temperature rise being too high and caused by the phenomenon of grain coarseness.
After overheating, carbon steel (sub-eutectoid or over-eutectoid steel) often appears in Weiss’s organization. Martensitic steel, after overheating, often appears as an intercrystallite weave; a carbide angularity often characterizes tool steel to determine the overheating organization. Titanium alloys have obvious β-phase grain boundaries and flat elongated Weiss organization after superheating. The fracture of alloy steel after superheating will appear as a stone or strip fracture. Superheated organization, due to coarse grain size, will cause a reduction in mechanical properties, especially impact toughness.
General overheating of structural steel after normal heat treatment (normalizing, quenching), the organization can be improved, and the performance is restored; this overheating is often called unstable overheating; and the severe overheating of structural alloy steel by the general normalizing (including high-temperature normalizing), annealing or quenching treatment, the overheating organization cannot be eliminated, this overheating is often called stable overheating.
Overburning refers to the metal billet heating temperature is too high or in the high-temperature heating zone for too long, the furnace oxygen and other oxidizing gases penetrate the gaps between the metal grains, and oxidation with iron, sulfur, carbon, etc., the formation of a fusible oxide of the eutectic, destroying the connection between the grains so that the plasticity of the material is sharply reduced. Overburning heavy metal, withdrawal of course when a light blow on the crack, pulling long will appear in the overburning transverse cracks.
Overburning and overheating do not have a strict temperature boundary—generally, grain oxidation and melting features to determine overburning, and for carbon steel, overburning grain boundary melting, severe oxygen chemical mold steel (high-speed steel, Cr12 steel, etc.) overburning, the grain boundary due to melting, and fishbone Leydenite. Aluminum alloy overburning appears as grain boundary melting triangle, re-melting ball, etc. After overburning, forgings are often unsalvageable and have to be scrapped.
In heating large ingots with large cross-sectional dimensions and poor thermal conductivity of high-alloy steel and high-temperature alloy billets, if the low-temperature stage heating speed is too fast, the billet is due to the large temperature difference between inside and outside and generates large thermal stress. In addition, at this time, the billet, due to low temperature and poor plasticity, if the value of the thermal stress exceeds the strength limit of the billet, will produce a radiating heating crack from the center to the surrounding area, so the entire section is cracking.
Copper brittle on the surface of the forging is cracked. When observed at high magnification, a yellowish copper (or copper solid solution) is distributed along the grain boundaries.
When the billet is heated, such as residual copper oxide chips in the furnace, at high temperatures, oxidized steel reduces to free copper, molten steel atoms along the austenite grain boundary expansion, weakening the connection between the grains. In addition, the high copper content in steel [> 2% (mass fraction)], such as heating in an oxidizing atmosphere and the formation of a copper-rich layer under the iron oxide skin, also causes steel to be too brittle.
Defects often produced by improper forging process
Defects arising from improper forging processes are usually the following:
Large grains are usually caused by too high a forging temperature and low deformation, or too high a final forging temperature, or the degree of deformation falls into the critical deformation zone. The aluminum alloy deformation degree is too large, causing weave formation; high-temperature alloy deformation temperature is too low, and the formation of mixed deformation organization may also cause coarse grains.
Coarse grains will reduce the plasticity and toughness of the forgings, and the fatigue performance will be significantly reduced.
Uneven grain size
Grain inhomogeneity refers to the forging of some parts of the grain being coarse and some parts being smaller. Grain unevenness is mainly due to the uneven deformation of the billet in various places so that the degree of grain fragmentation is not the same, or the degree of deformation of the local area falls into the critical deformation zone, or high-temperature alloy local processing hardening, or quenching and heating of the local grain course. Heat-resistant steel and high-temperature alloys are particularly sensitive to grain inhomogeneity. Grain inhomogeneity will make the forging lasting performance, and fatigue performance will be significantly reduced.
Cold hard phenomenon
Forging deformation due to low temperature or deformation speed being too fast, as well as forging after cooling too fast, may make the recrystallization caused by the softening cannot keep up with the strengthening (hardening) caused by deformation so that the hot forging internal part still retains the cold deformation organization. The existence of this organization improves the strength and hardness of the forging but reduces the plasticity and toughness. Severe cold hardening may cause forging cracking.
Forging cracks are usually caused by large tensile, tangential, or additional tensile stresses. Cracking usually occurs in the billet area where the greatest stress and the thickness are thinnest. If there are micro cracks on the surface and inside of the billet, if there are organizational defects in the billet, if the heat processing temperature is not appropriate to reduce the plasticity of the material, or if the deformation speed is too fast and the degree of deformation is too large, exceeding the allowable plastic pointer of the material, etc., cracks may occur in the process of withdrawal, drawing, punching, reaming, bending, and extrusion.
Forging cracking is a shallow turtle-like crack on the surface of the forging. This defect will likely occur on surfaces subjected to tensile stress in forging (e.g., unfilled projections or parts subjected to bending).
The internal causes of tortoise cracking may be multiple:
- (1) Material combined with Cu, Sn, and other fusible elements too much;
- (2) High-temperature heating for a long time, the surface of the steel material with copper precipitation, coarse surface grain, decarburization, or after multiple heating of the surface;
- (3) The fuel contains too much sulfur; sulfur penetrates the steel surface.
Flying edge cracking
Forging edge cracks occur at the parting surface during die forging and edge cutting. Flying edge crack may be caused by:
- In the die forging operation, due to the heavy blow to make the metal flows strongly through the phenomenon of ribbing.
- The cutting edge of magnesium alloy die forging is too low; the copper alloy dies forging is too high.
Cracking on the die surface
Forging parting surface cracks are cracks along the parting surface of the forging. Raw material non-metallic inclusions, die forging to the die surface flow, and concentration or shrinkage residue in the die forging after squeezing the flying edge often formed after the die surface crack.
Forging folding is the metal deformation process that oxidizes the metal convergence surface layer to form. It can be formed by the convergence of two (or more) metal convection; it can also be formed by a rapid and massive flow of metal will the neighboring part of the surface metal with the flow of the two convergences; it can also be due to deformation of metal bending, reflux, and formation; can also be part of the local metal deformation, was pressed into another part of the metal and formed. Folding is related to the shape of the raw material and billet, the design of the die, the arrangement of the forming process, the lubrication, and the actual operation of the forging.
Forging folding reduces the part’s load-bearing area and tends to become a source of fatigue due to the stress concentration here during work.
Forging through-flow is a form of improper distribution of flow lines. In the flow-through area, the original angular distribution of flow lines converges to form a flow-through and may make a relatively large difference in the size of the grains inside and outside the flow-through area. Through-flow is similar to folding and is formed by two or a stream of metal with another stream, but the metal in the through-flow section is still whole.
Forging through the flow of forgings to reduce the mechanical properties of the forgings, especially when the flow on both sides of the grain difference is relatively large, the performance reduction is more obvious.
Forging flow line distribution is not smooth.
Forging streamline distribution is not smooth refers to the low forging times on the occurrence of streamline cut-off, reflux, vortex, and other streamline disorder phenomena. If the mold design or forging method selection is not reasonable, prefabricated blank streamline disorder; workers’ improper operation, mold wear and tear, and uneven flow of metal can make the forging streamline distribution not smooth. Poor flow lines will make a variety of mechanical properties to reduce, so for important forgings, there are requirements for the distribution of flow lines.
Casting tissue residue
Forging casting tissue residue is mainly found in forgings with ingots as billets. We are casting organization residues mainly in the difficult deformation area of forgings. The forging ratio needs to be, and improper forging method is the main reason for the residual casting organization.
Forging casting tissue residue will decline the forging performance, especially impact toughness and fatigue performance.
The carbide segregation level does not meet the requirements.
The forging carbide segregation level does not meet the requirements, mainly in the Leyland tool steel. Mainly the carbide distribution in forgings is not uniform in large pieces of concentrated distribution or a network distribution. The main reason for this defect is that the raw material carbide segregation level is poor, coupled with the forging ratio is not enough or the forging method is not appropriate with these defective forgings, heat treatment quenching is easy to local overheating and quenching crack. Made of sharpening tools and dies are easy to collapse when used.
Forging band organization is ferrite and pearlite, ferrite and austenite, ferrite and bainite, and ferrite and martensite in forgings in a band distribution of an organization; they are mostly found in sub-folding steel, austenitic steel, and semi-martensitic steel. This organization, in the case of two-phase coexistence of forging deformation band organization, can reduce the transverse plasticity of the material pointer, especially impact toughness in forging or parts work often easily to crack along the ferrite band or the junction of the two phases.
Forging local underfill mainly occurs in the rib, convex corner, corner, and rounded parts; the size needs to meet the requirements of the drawing. The reasons may be: ① low forging temperature, poor metal fluidity; ② insufficient equipment tonnage or hammering force; ③ unreasonable design of the billet die, billet volume or cross-section size is not qualified; ④ die chamber in the accumulation of oxide or welding deformation of the metal.
The pressure of forging refers to the general increase in size perpendicular to the direction of the die surface, which may be caused by the following:
- Low forging temperature.
- Insufficient equipment tonnage, insufficient hammering force, or an insufficient number of hammerings.
Forging shift is the displacement of the upper part of the forging along the parting face relative to the lower part. The reasons may be ① the gap between the slide (hammerhead) and the guide rail is too large; ② the forging die design is unreasonable; the lack of lock or guide pillar to eliminate the mix shift force; ③ poor die installation.
They were forging forgings axis bending, with the plane of the geometric position of the error. The causes may be ① inattention when the forging is out of the die; ② uneven force when cutting the edge; ③ different cooling speed of each part when the forging is cooled; ④ improper cleaning and heat treatment.
3.5 Defects often produced by the improper cooling process after forging
Defects arising from improper cooling after forging are usually the following.
Forging the cooling process internally will be too fast due to the cooling rate and produce large thermal stress, which may also be due to tissue transformation caused by large tissue stress. If these stresses exceed the strength limit of the forging, the forging will produce smooth and slender cooling cracks.
Forging in the forging of steel’s high carbon content, if the stopping temperature is high and the cooling rate is too slow, it will cause carbides along the grain boundaries in a precipitation network. For example, bearing steel in 870 – 770 ℃ slow cooling, the carbide precipitation along the grain boundary.
Forging mesh carbide in the heat treatment is prone to cause quenching cracks. In addition, it also makes the parts of the use of performance deterioration.
Forging heat treatment process is often improperly produced defects
Defects arising from improper post-forging heat treatment process are usually.
Hardness is too high or not hard enough
Forging due to improper post-forging heat treatment process and the forging hardness is not enough because ① quenching temperature is too low; ② quenching heating time is too short; ③ tempering temperature is too high; ④ multiple heating caused by severe decarburization of the forging surface; ⑤ the chemical composition of steel is not qualified.
Forging due to improper post-forging heat treatment process and the forging hardness is too high because ① cooling too fast after normalizing; ② normalizing or tempering heating time is too short; ③ the chemical composition of the steel is not qualified.
Forging caused by uneven hardness is the main reason for improper heat treatment process regulations, such as a furnace charge too much or holding time being too short; heating caused by local decarburization of forgings, etc.
3.7 Forging cleaning process is often improperly produced defects
Forging forgings cleaning defects usually have the following:
Excessive forging pickling will make the surface of the forging lose and porous. This defect is mainly due to the acid depth being too high and the forging in the pickling tank for too long, or due to the forging surface cleaning is not clean, the acid residue on the forging surface caused by.
Forging martensitic stainless-steel forging after forging, if there is large residual stress, pickling can easily produce small mesh corrosion cracks on the forging surface. If the organization is coarse will accelerate the formation of cracks.
The application of precision forging in the automotive industry
In recent years, the rapid development of precision forging technology has promoted the progress of the automotive manufacturing industry. Cold forgings and warm forgings are increasingly used in the automotive industry, the product’s shape is getting closer to the final shape, and precision forging will be developed accordingly with the future progress of the process and related technology. In addition, the field of metal forming is actively moving toward high-precision net-shape technology to reduce production costs, reduce product weight, simplify part design and manufacturing, and increase the added value of products.
The definition of net forming is as follows:
- (1) Compared to conventional plastic forming (Plastic Forming), a forming process can meet a part’s dimensional and tolerance requirements with less subsequent mechanical processing.
- (2) Forming process that can meet a part’s dimensional and tolerance requirements without subsequent machining at important local locations of the formed part.
- (3) Within the dimensional and tolerance range of the part, the forging can be formed without subsequent mechanical processing.
Metal plastic processing is now moving toward three major goals:
- (1) Product precision (net shape part development);
- (2) Process rationalization (the principle of process integration and application with minimum investment cost and production cost);
- (3) Automation and labor-saving.