In the process of automobile manufacturing, forging is widely used. With the advancement of science and technology and the continuous improvement of workpiece precision requirements, precision forging technology with high efficiency, low cost, low energy consumption, high quality, and other advantages 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 auto parts produced include: automobile clutch engagement ring gear, automobile transmission input shaft parts, bearing rings, automotive constant velocity universal joint sliding sleeve series products, automotive differential gears, automotive front axles, etc.

Common automotive forgings

1. Definition and classification of forging

1. Definition of forging

Forging is a processing method that uses a forging machine to apply pressure to a metal blank to plastically deform it to obtain a forging with certain mechanical properties, certain shape and size, and is one of the two major components of forging (forging and stamping).

as-cast looseness produced by the metal during the smelting process can be eliminated, and the microstructure can be optimized. At the same time, due to the preservation of the complete metal streamline, 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 related machinery, forgings are mostly used in addition to rolling plates, profiles, or welded parts with simple shapes.

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 that uses simple universal tools or directly applies external force to the blank between the upper and lower anvils of the forging equipment to deform the blank to obtain the required geometric shape and internal quality.

Die forging: Refers to the forgings obtained by pressing and deforming metal blanks in a forging die bore with a certain shape. Die forging can be divided into hot forging, warm forging, and cold forging. Warm forging and cold forging are the future development direction of die forging, and also represent the level of forging technology.

Ring rolling: refers to the production of ring-shaped parts of different diameters through special equipment ring-grinding machines, and is also used to produce wheel-shaped parts such as automobile wheels and train wheels.

Special forging: includes roll forging, cross wedge rolling, radial forging, liquid die forging, and other forging methods, which are more suitable for the production of parts with special shapes. For example, roll forging can be used as an effective performing process to greatly reduce the subsequent forming pressure; cross wedge rolling can produce parts such as steel balls and drive shafts; radial forging can produce large forgings such as barrels and stepped shafts.

According to the forging temperature, forging technology can be divided into hot forging, warm forging, and cold forging.

The initial recrystallization temperature of steel is about 727 °C, but 800 °C is generally used as the dividing line, and hot forging is higher than 800 °C; between 300 and 800 °C, it is called warm forging or semi-hot forging. called cold forging. Forgings used in most industries are hot forging. Warm and cold forging is mainly used for forging parts such as automobiles and general machinery. Warm and cold forging can effectively save materials.

According to the movement mode of the forging die, forging can be divided into pendulum rolling, pendulum rotary forging, roll forging, cross wedge rolling, ring rolling, and skew rolling.

3. Forging materials

Forging materials are mainly carbon steel and alloy steel of various compositions, followed by aluminum, magnesium, copper, titanium, etc., and their alloys, are iron-based superalloys, nickel-based superalloys, and cobalt-based superalloys. Deformed alloys are also forged. Or the rolling method is completed, but these alloys are relatively difficult to forge due to their relatively narrow plastic zone, and the heating temperature, opening forging temperature and final forging temperature of different materials have strict requirements.

The raw state of the material 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 selection of forging ratio, reasonable heating temperature and holding time, reasonable initial forging temperature and final forging temperature, reasonable deformation amount, and deformation speed has a lot to do with improving product quality and reducing costs.

2. Commonly used forging methods and their advantages and disadvantages

1. Free forging

Free forging refers to the processing method of forgings that uses simple universal tools or directly applies external force to the blank between the upper and lower anvils of the forging equipment to deform the blank to obtain the required geometric shape and internal quality. Forgings produced by the free forging method are called free forgings.

Free forging

Free forging is mainly based on the production of small batches of forgings. Forging equipment such as forging hammers and hydraulic presses are used to form and process the blanks to obtain qualified forgings. The basic processes of free forging include upsetting, drawing, punching, cutting, bending, torsion, offset, and forging. Free forging adopts the hot forging method.

The free forging process includes the basic process, auxiliary process, and finishing process.

The basic processes of free forging: are upsetting, drawing, punching, bending, cutting, torsion, offset and forging, etc., and the three most commonly used processes in actual production are upsetting, drawing, and punching.

Auxiliary process: pre-deformation process, such as pressing jaws, pressing steel ingot edges, shoulder cutting, etc.

Finishing process: The process of reducing surface defects of forgings, such as removing unevenness and shaping of forgings surface.

Upsetting

Pull out

Punching

advantage:

(1) Great forging flexibility, can produce small parts less than 100kg, and can also produce heavy parts up to 300t;

(2) The tools used are simple general tools;

(3) Forging forming is to gradually deform the billet in sub-regions, so the tonnage of forging equipment required for forging the same forging is much smaller than that of model forging;

(4) The requirements for the accuracy of the equipment are low;

(5) The production cycle is short.

Disadvantages and limitations:

(1) The production efficiency is much lower than model forging;

(2) The forgings have a simple shape, low dimensional accuracy, and rough surface; the labor intensity of workers is high, and the technical level is also high;

(3) It is not easy to realize mechanization and automation.

2. Die forging

Die forging refers to a forging method that uses a die to form a blank on a special die forging equipment to obtain a forging. The forgings produced by this method are accurate in size, small in machining allowance, complex in structure, and high in productivity.

Die forging

According to the different types of equipment used: die forging on the hammer, die forging on the crank press, die forging on flat forging machine and die forging on friction press, etc.

　　The most commonly used equipment for hammer die forging are steam-air die forging hammers, anvils hammers, and high-speed hammers.

Forging die cavity: According to its different functions, it can be divided into two categories: die forging die cavity and billet die cavity.

Forging dies for hammer forging

(1—hammerhead; 2—upper die; 3—flash groove; 4—lower die; 5 —die pad; 6, 7, 10—fastening wedge iron; 8 —parting surface; 9 —die cavity )

1) Die forging die bore

(1) Pre-forging die cavity: The function of the pre-forging die cavity is to deform the blank to the shape and size close to the forging so that when the final forging is performed, the metal can easily fill the die cavity to obtain the required size of the forging. For forgings with simple shapes or small batches, pre-forging die bores may not be provided. The fillet and slope of the pre-forging die cavity are much larger than that of the final forging die cavity, and there is no flash groove.

(2) Final forging die chamber: The function of the final forging die chamber is to deform the blank to the required shape and size of the forging, so its shape should be the same as that of the forging; The size of the forging die bore should be larger than the size of the forging by a shrinkage amount. The shrinkage of steel forgings is 1.5%. In addition, there are flash grooves around the die cavity to increase the resistance of the metal to flow out of the die cavity, promote the metal to fill the die cavity, and accommodate excess metal at the same time.

2) Blank-making die cavity

For forgings with complex shapes, 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 the die cavity can be well filled, it must be pre-made in the billet die cavity.

Bending connecting rod forging process

(1) Lengthening the die cavity: it is used to reduce the cross-sectional area of a certain part of the blank to increase the length of the part. There are two types of drawing die chambers: open type and closed type.

Draw long die cavity: (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 another part so that the metal is distributed according to the shape of the forging. The rolling die cavity is divided into two types: open type and closed type.

Rolling die cavity: (a) open type ; (b) closed type

(3) Bending die cavity: For bent rod die forgings, a bending die cavity is required to bend the blank.

Bending Die

(4) Cutting the die cavity: it forms a pair of knife edges on the corners of the upper die and the lower die to cut off the metal.

cut-off die

advantage:

• High production efficiency. During die forging,      the deformation of the metal is carried out in the die cavity, so the      desired shape can be obtained quickly;
• It can forge forgings with complex shapes, and      can make the distribution of metal streamlines more reasonable and improve      the service life of parts;
• The size of die forgings is more accurate, the      surface quality is better, and the machining allowance is small;
• Save metal materials and reduce cutting workload.
• Under the condition that the batch is sufficient, 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 the forging die is long and the cost is high;
• The investment cost of die forging equipment is larger than that of free forging.

3. Roll forging

　　Roll forging refers to a forging process in which a pair of oppositely rotating fan-shaped dies is used to plastically deform the blank to obtain the desired forging or forging blank.

Schematic diagram of roll forging

The principle of roll forging deformation is shown above. Roll forging deformation is a complex three-dimensional deformation. Most of the deformed material flows along the length direction to increase the length of the blank, and a small part of the material flows laterally to increase the width of the blank. During the roll forging process, the cross-sectional area of the billet root decreases continuously. Roll forging is suitable for 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, picks, turbine blades, etc. The roll forging process uses the principle of roll forming to gradually deform the blank.

Compared with ordinary die forging, roll forging has the advantages of simpler equipment structure, stable production, low vibration and noise, easy automation, and high production efficiency.

4. Tire die forging

　　Tire die forging is a forging method in which the free forging method is used to make billets and then finally form in the tire mold. It is a forging method between free forging and dies forging. There are few die forging equipment, and most of them are free forging hammers that are widely used in small and medium-sized enterprises.

　　There are many types of tire molds used in tire mold forging. Commonly used in production are mold drop, buckle mold, sleeve mold, cushion mold, mold clamping, etc.

Buckle

Open cylinder mold: (a) Integral cylinder mold; (b) Insert cylinder mold; (c) Cylinder mold with cushion

Closed cylinder mold

Closed cylinder dies are mostly used for the forging of rotary body forgings. For example, gears with bosses on both ends are sometimes used for the forging of non-rotating body forgings. Closed barrel dies forging is a flashless forging.

For tire forgings with complex shapes, it is necessary to add two half-dies (that is, add a parting surface ) in the barrel mold to make a combined barrel mold, and the blank is formed in the mold cavity composed of the two half-molds.

Combined cylinder die (1- cylinder die; 2-right half-die; 3-punch; 4-left half-die; 5-forging)

Clamping

The lamination film usually consists of two parts, the upper and lower molds. In order to make the upper and lower dies fit together and prevent the forgings from shifting, guide posts and guide pins are often used for positioning. Clamping is mostly used to produce non-rotary forgings with complex shapes, such as connecting rods, fork-shaped forgings, etc.

Compared with free forging, die forging has the following advantages:

(1) Since the blank is formed in the die cavity, the size of the forging is relatively accurate, the surface is relatively smooth, and the distribution of the streamline structure is relatively reasonable, so the quality is high;

(2) Die forging can forge forgings with complex shapes; since the shape of the forgings is controlled by the die cavity, the blank is formed faster, and the productivity is 1 to 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 the machining time.

Disadvantages and limitations:

(1) Forging hammers with larger tonnage are required;

(2) Only small forgings can be produced;

(3) The service life of the tire mold is relatively low;

(4) The tire mold is generally moved by manpower during work, so the labor intensity is relatively large;

(5) Die forging is used to produce medium and small batches of forgings.

3. Forging defects and analysis

The raw materials for forging are ingots, rolled materials, extruded materials, and forging billets. Rolled material, extruded material, and forging billet are semi-finished products processed by rolling, extrusion, and forging of ingot respectively. In general, the appearance of internal defects or surface defects of ingots is sometimes unavoidable. Coupled with the improper forging process during the forging process, it eventually leads to defects in the forgings. The following briefly introduces some common defects in forgings.

1. Forging defects caused by defects in raw materials usually include:

Surface cracks: Surface cracks mostly occur in 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 the ingot are elongated along the deformation direction during rolling and are exposed to the surface, and develop deep inside. For another example, during rolling, if the surface of the billet is scratched, it will cause stress concentration during cooling, which may crack along the scratch, and so on. If such cracks are not removed before forging, they may expand and cause forging cracks during forging.

Folding: The reason for the formation of folding is that when the metal blank is rolled, due to the incorrect sizing of the groove on the roll, or the burrs generated by the wear surface of the groove are involved during rolling, forming a certain shape with the surface of the material. Angled crease. For steel, there are iron oxide inclusions in the crease and decarburization around it. If the fold is not removed before forging, it may cause the forging to fold or crack.

Scaling: Scaling is a peelable film that is localized on the surface of the rolled stock.

The formation of scarring is due to the splash of molten steel during casting and condensation on the surface of the ingot, which is pressed into a film during rolling and attached to the surface of the rolled material, that is, scarring. After the forging is cleaned by pickling, the film will peel off and become a surface defect of the forging.

Layered fracture: Layered fracture is characterized by a fracture or section that is very similar to a broken slate or bark.

Layered fractures mostly occur in alloy steels (chromium-nickel steel, chromium-nickel-tungsten steel, etc.), and are also found in carbon steel. This defect is caused by the existence of non-metallic inclusions, dendrite segregation, and porosity in the steel, which are elongated along the rolling direction during forging and rolling, making the steel lamellar. If there are too many impurities, there is a risk of delamination cracking in forging. The more serious the layered fracture, the poorer the plasticity and toughness of the steel, especially the low transverse mechanical properties, so the steel with obvious layered defects is unqualified

Bright line (bright area): Bright line is a thin line with reflective ability that is crystallized and shiny on the longitudinal fracture, most of which run through the entire fracture, and most of them occur in the axial part.

Bright lines are mainly due to alloy segregation. The slight bright line has little effect on the mechanical properties, and the serious bright line will obviously reduce the plasticity and toughness of the material.

Non-metallic inclusions: Non-metallic inclusions are mainly formed due to chemical reactions between components or between metals and furnace gas and containers during the cooling process of molten steel for smelting or casting. In addition, during metal smelting and casting, because the refractory material falls into the molten steel, inclusions can also be formed, which are collectively referred to as slag inclusions. On the cross-section of the forging, non-metallic inclusions can be distributed in the form of dots, sheets, chains, or lumps. Serious inclusions can easily cause forgings to crack or reduce the performance of the material.

Carbide segregation: Carbide segregation often occurs in alloy steels with high carbon content. It is characterized by the aggregation of more carbides in local areas. It is mainly caused by ledeburite eutectic carbides and secondary network carbides in the steel, which is not broken and uniformly distributed during billeting and rolling. Carbide segregation will reduce the forging deformation properties of steel and easily cause cracking of forgings. Forgings are prone to local overheating, overburning, and quenching cracks during heat treatment and quenching.

Aluminum alloy oxide film: The aluminum alloy oxide film is generally located on the web of the die forging and near the parting surface. There are fine cracks on the low magnification tissue and swirl patterns on the high magnification tissue. The characteristics of the fracture can be divided into two categories: one is flat flakes, the color ranges from silver gray, light yellow to brown, dark brown; Second, it is a small, dense, and sparkling point-like object.

The aluminum alloy oxide film is formed when the exposed melt surface interacts with water vapor or other metal oxides in the atmosphere during the casting process and is rolled into the interior of the liquid metal during the casting process.

The oxide film in forgings and die forgings has no obvious effect on the longitudinal mechanical properties but has a greater impact on the mechanical properties in the height direction, which reduces the strength properties in the height direction, especially the elongation in the height direction, impact toughness, and height direction resistance. Corrosive properties.

White spot: The main feature of white spots is round or oval silver-white spots on the longitudinal fracture of the billet and small cracks on the transverse fracture. The size of the white spots varies, and the length ranges from 1 to 20 mm or longer. White spots are common in alloy steels such as nickel-chromium steel and nickel-chromium-molybdenum steel and are also found in ordinary carbon steel, which are hidden internal defects. White spots are generated under the combined action of hydrogen and the structural stress during phase transformation and thermal stress. It is more likely to occur when the hydrogen content in the steel is large and the cooling ( or post-forging heat treatment ) after hot pressing is too fast.

Forgings forged from steel with white spots are prone to cracking during heat treatment (quenching), and sometimes even fall off in blocks. The white spot reduces the plasticity of the steel and the strength of the part. It is a stress concentration point. It is like a sharp cutter. Under the action of alternating loads, it is easy to become fatigue cracks and cause fatigue damage. Therefore, white spots are absolutely not allowed in forging raw materials.

Coarse-grained rings: Coarse-grained rings are often defected present on aluminum or magnesium alloy extruded rods.

The extruded rods of aluminum and magnesium alloys supplied after heat treatment often have coarse-grained rings on the outer layer of their circular sections. The thickness of the coarse-grained ring increases gradually from the beginning to the end during extrusion. If the lubrication conditions during extrusion are good, coarse grain rings can be reduced or avoided after heat treatment. Conversely, the thickness of the ring will increase.

　　The reasons for the formation of coarse crystal rings are related to many factors. But the main factor is due to the friction created between the metal and the extrusion barrel during extrusion. This friction causes the outer grains of the extruded bar cross-section to be much more fragmented than the grains in the center of the bar. However, due to the influence of the cylinder wall, the temperature in this area is low, and the recrystallization is not complete during extrusion. During quenching and heating, the unrecrystallized grains recrystallize and grow up and swallow the recrystallized grains, so coarse grains are formed on the surface layer. ring.

with the coarse grain, the ring is easy to crack during forging. If the coarse grain ring remains on the surface of the forging, the performance of the part will be reduced.

Shrinkage tube residue: Shrinkage tube residue is generally caused by the fact that the concentrated shrinkage cavities generated in the riser part of the ingot are not removed cleanly, and remain inside the steel during billeting and rolling.

Dense inclusions, porosity, or segregation generally appear in the area near the remnants of the shrink tube. Irregularly wrinkled crevices in lateral low magnification. It is easy to cause cracking of forgings during forging or heat treatment.

2. Defects caused by improper material preparation and their influence on forgings

Defects caused by improper material preparation are as follows:

Bevel cut: Bevel cut means that the inclination of the end face of the blank relative to the longitudinal axis exceeds the specified allowable value because the bar is not compressed when the sawing machine or punch is unloaded. Severely cut, folds may have formed during the forging process.

end of the blank is bent and has burrs: when the shearing machine or punch is unloading, due to the excessive gap between the blades of the scissors or the cutting die or because the cutting edge is not sharp, the blank has been bent before being cut off. Metal is squeezed into the gap between the blade or die, forming a sagging burr at the end.

Blanks with burrs are prone to local overheating and overburning when heated, and are prone to folding and cracking during forging.

End face of the blank is concave: When unloading on the shearing machine, because the gap between the scissors is too small, the cracks on the metal section and the bottom do not overlap, resulting in secondary shearing. As a result, part of the end metal is pulled off, and the end face is concave. Such billets are prone to folds and cracks during forging.

End cracks: When large-section alloy steel and high carbon steel bars are sheared in a cold state, cracks are often found at the ends 3 to 4 hours after shearing. The main reason is that the unit pressure of the blade is too large, so the blank with the circular section is flattened into an oval shape, and large internal stress is generated in the material at this time. 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. When the hardness of the material is too high, the hardness is uneven, and the material segregation is serious, it is easy to produce shear cracks.

For billets with end cracks, the cracks will further expand during forging.

Gas cutting cracks: Gas cutting cracks are generally located at the end of the billet, which is caused by the fact that the raw materials are not preheated before gas cutting, and the tissue stress and thermal stress are generated during gas cutting.

For billets with gas-cutting cracks, the cracks will further expand during forging. Therefore, it should be cleared in advance before forging.

Cracked convex core: When the lathe is blanking, there is often a convex core in the center of the end face of the bar. During the forging process, since the section of the convex core is small and the cooling is fast, its plasticity is low, but the section of the base part of the billet is large, the cooling is slow, and the plasticity is high. Therefore, the abrupt junction of the cross-section becomes the place where the stress is concentrated, and the plasticity difference between the two parts is large, so under the action of the hammering force, the surrounding of the convex core is prone to cracking.

3. Defects are often caused by the improper heating process

Defects caused by improper heating can be divided into:

(1) Defects caused by the change of the chemical state of the outer layer of the billet due to the influence of the medium, such as oxidation, decarburization, carburization and sulfurization, copper infiltration, etc .;

(2) Defects caused by abnormal changes in the internal structure, such as overheating, overburning and underheating, etc.;

(3) Due to the uneven distribution of temperature inside the blank, the internal stress (such as temperature stress, and tissue stress) is too large and the blank is cracked.

Here are a few of the common pitfalls:

Decarburization: Decarburization refers to the phenomenon that the carbon in the lower layer of the metal is oxidized at high temperatures so that the carbon content of the surface layer is significantly lower than that of the inner layer.

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. Heating in an oxidizing atmosphere is prone to decarburization, high carbon steel is easy to decarburize, and steel with a large silicon content is also easy to decarburize.

Decarburization reduces the strength and fatigue properties of parts and reduces wear resistance.

: Forgings heated by oil furnaces often have carbonization on the surface or part of the surface. Sometimes the thickness of the carburizing layer is 1.5-1.6mm, the carbon content of the carburizing layer is about 1% (mass fraction), and the carbon content of the local point even exceeds 2% (mass fraction), and a ledeburite structure appears.

This is mainly in the case of oil furnace heating, when the position of the billet is close to the nozzle of the oil furnace or in the area where the two nozzles cross-spray the fuel oil, the combustion is not complete because the oil and air are not mixed well, and the result is The surface of the billet forms a reducing carburizing atmosphere, thereby producing the effect of surface carburization.

The increase in carbon makes the machining performance of the forgings worse, and it is easy to hit the knife when cutting.

Overheating: Overheating means that the heating temperature of the metal billet is too high, the residence time is too long within the specified forging and heat treatment temperature range, or the grain size is coarse due to the excessive temperature rise due to thermal effects.

Carbon steels (hypo-eutectoid or hyper-eutectoid steels) tend to appear Widmanderin after overheating. After the martensitic steel is overheated, the intragranular texture often appears, and the tool and die steel is often characterized by a carbide angularity to determine the overheated structure. After the titanium alloy is overheated, obvious β-phase grain boundaries and straight and slender Widmandarin structures appear. The fracture of the alloy steel after overheating will appear as a stone fracture or strip fracture. Overheated structure, due to coarse grains, will cause a decrease in mechanical properties, especially impact toughness.

After normal heat treatment (normalizing and quenching) of general overheated structural steel, the structure can be improved and the performance can be recovered. After (including high temperature normalizing), annealing or quenching, the overheated structure cannot be completely eliminated, and this kind of overheating is often called stable overheating.

Overburning: Overburning means that the heating temperature of the metal billet is too high or the residence time in the high-temperature heating zone is too long. In equal oxidation, a eutectic of fusible oxides is formed, which destroys the connection between grains and drastically reduces the plasticity of the material. Severely over-burned metal will crack with a single blow when it is thickened, and transverse cracks will appear at the over-burned place when it is stretched.

There is no strict temperature boundary between overheating and overheating. Generally, overburning is judged by the oxidation and melting of the grains. For carbon steel, when the grain boundary is melted during over-burning, and the severely oxidized tool and die steel (high-speed steel, Cr12 steel, etc.) is over-burned, the grain boundary melts and fishbone-like ledeburite appears. When the aluminum alloy is overfired, the grain boundary melting triangle and the remelting ball appear. After the forging is over-burned, it is often impossible to save it, and has to be scrapped.

Heating cracks: When heating large ingots with large cross-sectional dimensions and high-alloy steel and superalloy billets with poor thermal conductivity, if the heating rate is too fast in the low-temperature stage, the billet will generate large thermal stress due to the large temperature difference between the inside and outside. In addition, at this time, the billet has poor plasticity due to low temperature. If the value of thermal stress exceeds the strength limit of the billet, heating cracks radiating from the center to the surrounding areas will occur, causing the entire section to crack.

Copper brittleness: Copper brittleness is cracked on the surface of the forging. When observed at high magnification, light yellow copper (or copper solid solution) is distributed along the grain boundaries.

When the billet is heated, if there are copper oxide scraps remaining in the furnace, the oxidized steel is reduced to free copper at high temperature, and the molten steel atoms expand along the austenite grain boundaries, weakening the connection between the grains. In addition, when the copper content in the steel is high [>2% (mass fraction)], such as heating in an oxidizing atmosphere, a copper-rich layer is formed under the iron oxide scale, which also causes steel embrittlement.

4. Defects often caused by improper forging process

Defects caused by improper forging process usually include the following:

Large grains: Large grains are usually caused by too high initial forging temperature and insufficient deformation, excessive final forging temperature, or the degree of deformation falling into the critical deformation zone. The deformation degree of aluminum alloy is too large to form texture; the deformation temperature of superalloy is too low, which may also cause coarse grains when forming a mixed deformation structures. Coarse grains will reduce the plasticity and toughness of the forgings, and the fatigue properties will decrease significantly.

Uneven grains: Uneven grains means that the grains in some parts of the forging are particularly coarse, but some parts are small. The main reason for the uneven grains is that the deformation of the billet is not uniform, so the degree of grain breakage is different, or the degree of deformation of the local area falls into the critical deformation zone, or the local work hardening of the superalloy, or the local grain during quenching and heating. thick. Heat-resistant steels and superalloys are particularly sensitive to grain inhomogeneity. Uneven grains will significantly reduce the durability and fatigue properties of forgings.

Chilling phenomenon: During forging deformation, due to the low temperature or too fast deformation speed, and too fast cooling after forging, the softening caused by recrystallization may not keep up with the strengthening (hardening) caused by deformation, so that the interior of the forging after hot forging The cold deformation structure is still partially retained. The existence of this structure increases the strength and hardness of the forging but reduces the ductility and toughness. The severe chilling phenomenon may cause forging cracks.

Cracks: Forging cracks are usually caused by large tensile stress, shear stress, or additional tensile stress during forging. The location where the crack occurs is usually the location where the stress of the billet is the greatest and the thickness is the thinnest. If there are micro-cracks on the surface and inside of the billet, there are structural defects in the billet, the improper hot working temperature reduces the plasticity of the material, or the deformation speed is too fast, the deformation degree is too large, and exceeds the allowable plasticity index of the material, etc. Cracks may occur in processes such as drawing, punching, reaming, bending, and extrusion.

Cracking: Forging cracks are shallow tortoise-like cracks on the surface of forgings. Surfaces that are tensile stressed in forging forming (eg, underfilled bulges or bent sections) are most prone to this defect.

The internal causes of cracking may be multifaceted:

(1) The material contains too many fusible elements such as Cu and Sn;

(2) When heated at a high temperature for a long time, there is copper precipitation on the surface of the steel, coarse grains on the surface, decarburization, or a surface that has been heated many times;

(3) The sulfur content of the fuel is too high, and there is sulfur infiltration into the surface of the steel material.

Flash cracks: Forging flash cracks are cracks generated at the parting surface during die forging and trimming. The reasons for the flash cracks may be: ① In the die forging operation, the strong flow of the metal caused the piercing phenomenon due to the heavy blow. ②The trimming temperature of magnesium alloy die forgings is too low; the trimming temperature of copper alloy die forgings is too high.

Parting surface cracks: Forging parting surface cracks refer to cracks along the parting surface of forgings. There are many non-metallic inclusions in the raw materials. During die forging, the flow to the parting surface and the concentrated or shrinking tube residues often form parting surface cracks after being squeezed out of the flash during die forging.

Folding: Forging folds are formed by the fusion of oxidized surface metals during metal deformation. It can be formed by the convection of two (or more) metal convections; it can also be formed by the rapid and massive flow of a metal that brings the adjacent part of the surface metal to flow, and the two converge; it can also be due to deformation. The metal is formed by bending and reflowing; it can also be formed by partial deformation of a part of the metal and being pressed into another part of the metal. Folding is related to the shape of raw materials and blanks, the design of molds, the arrangement of forming processes, lubrication conditions, and the actual operation of forging.

Forging folding not only reduces the bearing area of the part but also tends to become a source of fatigue due to the stress concentration here during operation.

Throughflow: Forging throughflow is a form of improper distribution of streamlines. In the flow-through area, the streamlines originally distributed at a certain angle join together to form a flow-through, which may make the grain size difference between the inside and outside of the flow-through area quite different. The reason for the flow-through is similar to that of folding. It is formed by the confluence of two metals or metals with another metal, but the metal in the flow part is still a whole.

The forging flow through the forging reduces the mechanical properties of the forgings, especially when the grains on both sides of the through-flow zone are quite different, and the performance degradation is more obvious.

streamlines of forgings: Unsmooth distribution of streamlines of forgings refers to the occurrence of streamline disturbances such as streamline cutting, backflow, and eddy currents at low magnifications of forgings. If the mold design is improper or the forging method is unreasonably selected, the flow line of the prefabricated blank is disordered; the uneven flow of the metal caused by the improper operation of the worker and the wear of the mold can make the flow line distribution of the forgings not smooth. Streamline irregularities will reduce various mechanical properties, so for important forgings, there are requirements for streamlining distribution.

Casting structure residues: Forging and casting structure residues mainly appear in forgings that use ingots as billets. The as-cast structure mainly remains in the difficult deformation area of the forging. Insufficient forging ratio and improper forging methods are the main reasons for the residual cast structure.

The residual structure of forging and casting will reduce the performance of forgings, especially the impact of toughness and fatigue properties.

Carbide segregation grade does not meet the requirements: Forging carbide segregation grade does not meet the requirements mainly in ledeburite tool steel. The main reason is that the carbides in the forgings are not uniformly distributed, and are concentrated in large blocks or in a network. The main reason for this defect is that the raw material carbide segregation level is poor, the forging ratio is insufficient or the forging method is improper during the forging process. The resulting cutting tools and molds are easy to break when used.

Banded structure: The forged banded structure is a kind of structure in which ferrite and pearlite, ferrite and austenite, ferrite and bainite, and ferrite and martensite are distributed in a band in forgings, they mostly appear in hypo-folded steel, austenitic steel, and semi-martensitic steel. This kind of structure is the band-shaped structure produced during forging deformation under the coexistence of two phases, which can reduce the transverse plasticity index of the material, especially the impact toughness. It is often easy to crack along the ferrite band or the junction of the two phases during forging or part work.

Insufficient local filling: The local filling shortage of forging mainly occurs in ribs, convex corners, corners, and rounded corners, and the size does not meet the requirements of the drawing. The reasons may be ① low forging temperature and poor metal fluidity; ② insufficient equipment tonnage or insufficient hammering force; ③ unreasonable design of billet die, unqualified billet volume or cross-sectional size; ④ accumulation of oxide scale or welding in the die cavity Deformed metal.

Undervoltage: Forging Undervoltage refers to the general increase in the size perpendicular to the parting surface, which may be caused by: ①Low forging temperature. ②The tonnage of the equipment is insufficient, the hammering force is insufficient or the number of hammering times is insufficient.

Misalignment: Forging misalignment is the displacement of the forging along the upper half of the parting surface relative to the lower half. The reasons may be: ①The gap between the slider (hammerhead) and the guide rail is too large; ②The design of the forging die is unreasonable, and there is a lack of locks or guide posts to eliminate the misalignment force; ③The mold is poorly installed.

Axis bending: The axis of the forging is bent, and there is an error with the geometric position of the plane. The reasons may be ① Careless when the forging is ejected from the mold; ② Uneven force during edge trimming; ③ Different cooling rates of each part when the forging is cooled; ④ Improper cleaning and heat treatment.

5. Defects are often caused by the improper cooling process after forging

Defects caused by improper cooling after forging usually include the following:

Cooling cracks: During the cooling process after forging, large thermal stress will be generated inside the forging due to the excessive cooling rate, and large organizational stress may also be caused by the structural transformation. If these stresses exceed the strength limit of the forging, smooth and elongated cooling cracks are produced in the forging.

Reticulated carbide: When forging steel with high carbon content, if the forging temperature is high and the cooling rate is too slow, carbides will be precipitated in a network along the grain boundary. For example, if the bearing steel is cooled slowly at 870-770°C, carbides will precipitate along the grain boundaries.

Forged mesh carbides are prone to quench cracks during heat treatment. In addition, it also deteriorates the performance of the parts.

6. Defects often caused by improper heat treatment after forging

Defects caused by improper post-forging heat treatment process usually include:

Too high hardness or insufficient hardness: The reasons for the insufficient hardness of forgings caused by improper post-forging heat treatment process are: ① quenching temperature is too low; ② quenching heating time is too short; ③ tempering temperature is too high; ④ repeated heating causes forgings The surface is seriously decarburized; ⑤ The chemical composition of the steel is unqualified, etc.

forgings caused by improper post-forging heat treatment process are: ① the cooling after normalizing is too fast; ② the heating time of normalizing or tempering is too short; ③ the chemical composition of the steel is unqualified, etc.

Uneven hardness: The main reason for uneven hardness caused by forging is improper heat treatment process regulations, such as too much furnace loading or too short holding time; partial decarburization of forgings caused by heating, etc.

7. Defects often caused by improper cleaning process of forgings

Defects generated during the cleaning of forgings usually include the following:

Excessive pickling: Excessive forging pickling will make the surface of the forging 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 the acid residue on the surface of the forging due to the unclean surface of the forging.

Corrosion cracks: If there is large residual stress after forging martensitic stainless steel forgings, it is easy to produce fine network corrosion cracks on the surface of the forgings during pickling. If the structure is coarse, the formation of cracks will be accelerated.

Fourth, the application of precision forging in the automotive industry

In recent years, due to the rapid development of precision forging technology, the progress of automobile manufacturing has been promoted. Cold forgings and warm forgings are increasingly used in the automotive industry, and the shape of the product is getting closer and closer to the final shape. Precision forging will develop accordingly with the advancement of future processes and related technologies. In addition, based on the purpose of reducing production cost, reducing product weight, simplifying part design and manufacturing, and enhancing product added value, the field of metal plastic forming is actively developing towards high-precision net-shape forming technology.

Net shape is defined as follows:

(1) Compared with the traditional plastic forming (Plastic Forming), the subsequent machining can be smaller, and the forming process can meet the size and tolerance requirements of the parts.

(2) Forming process that meets the size and tolerance requirements of the parts without the need for subsequent machining at some important parts of the formed parts.

(3) Within the range of the dimensions and tolerances of the parts, the forgings may not require the subsequent machining process.

Metal plastic working is now moving towards three major goals:

(1) Product precision ( net shape parts development)

(2) Process rationalization (minimum investment cost and production cost are the principles of process integration and application)

(3) Automation and labor-saving

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