Complete Knowledge of Forging Technology
1. What is forging?
Table of Contents
Forging technology: a processing method uses forging machinery to apply pressure to metal billets, causing them to undergo plastic deformation to obtain forgings with certain mechanical properties, shapes, and sizes.
According to the forming method, forging can be divided into:
Open forging (i.e., free forging)
There are two main methods of using impact or pressure to deform the metal between the upper and lower anvils to obtain the required components: manual forging and mechanical forging. Free forging is a processing method that places heated metal billets on top of forging equipment and between the lower iron, applying impact or pressure to directly cause plastic deformation of the billets, thereby obtaining the required forgings. Free forging is suitable for producing single, small batch, and heavy forgings due to its simple shape and flexible operation. Free forging is divided into manual free forging and machine free forging. Manual free forging has low production efficiency and high labor intensity and is only used to repair or produce simple, small, and small batches of forgings. In modern industrial production, machine-free forging has become the main method of forging production, and it plays a particularly important role in heavy machinery manufacturing.
Closed mode forging
The metal billet undergoes compression deformation in a forging die chamber with a certain shape to obtain a forging, which can be divided into die forging (i.e., die forging, also known as model forging, where the heated billet is placed in a forging die fixed to the forging equipment to form the forging. The forging die structure of die forging includes a single-die chamber forging die. A multi-die chamber forging die), cold heading (i.e., stamping at room temperature to make the stamped part press according to the shape of the forging die chamber), rotary forging (i.e., the formed metal parts are forged and extruded into shape in a rotating state), extrusion (obtaining the required shape by exerting force on the formed parts).
According to the deformation temperature forging can be divided into:
- Hot forging (forging is carried out at a processing temperature higher than the recrystallization temperature of the billet metal);
- Warm forging (forging occurs when the processing temperature is lower than the recrystallization temperature);
- Cold forging (forging at room temperature during processing).
The main materials used for forging are various components of carbon steel and alloy steel, followed by aluminum, magnesium, titanium, copper, and their alloys. The original state of materials includes bars, ingots, metal powders, and liquid metals. The ratio of the cross-sectional area of a metal before deformation to the cross-sectional area of the mold after deformation is called the forging ratio. The correct selection of forging ratio is closely related to improving product quality and reducing costs.
2. The advantages of the forging process
Improve the structure of the metal, improve the mechanical properties.
After pressure processing, the microstructure and properties of metal materials are improved. Plastic processing can eliminate defects such as pores, shrinkage cavities, and dendrites in metal ingots. Due to the plastic deformation and recrystallization of metals, coarse grains can be refined to obtain dense metal structures, thereby improving the mechanical properties of metals. In the design of parts, if the force direction and fiber organization direction of the parts are correctly selected, the impact resistance of the parts can be improved.
The utilization rate of materials is high.
The metal plastic forming is mainly based on the redistribution of the volume of the metal, without removing the metal, so the material utilization rate is high.
Presses and molds generally form plastic forming, and the production efficiency is high. For example, the efficiency of using a multi-station cold heading process to process inner hexagonal screws is much higher than that of using the bar cutting process.
The accuracy of blanks or parts is high.
Applying advanced technology and equipment can achieve less cutting or no cutting processing. For example, the tooth profile of the precision forged bevel gear can be used directly without cutting, and the blade with a complex surface shape can achieve the required precision only by grinding after precision forging.
3. Learn and discuss the machining method of open-mode forging (i.e., free forging)
3.1 Free forging
A processing method uses impact force or pressure to cause plastic deformation of metal between the upper and lower anvils to obtain the required shape, size, and internal quality of forgings. During free forging, except for the metal parts in contact with the upper and lower anvils, which are constrained, the metal billet can freely deform and flow in all other directions without external constraints, making it impossible to accurately control the development of deformation.
Classification of free forging: manual forging and machine forging. Hand forging can only produce small forgings, and the productivity is also relatively low. Machine forging is the main method of free forging.
The characteristics of free forging are simple tools, strong universality, and a short production preparation cycle. The mass range of free forging can range from less than one kilogram to two to three hundred tons. For large forgings, free forging is the only processing method, which makes it particularly important in the manufacturing of heavy machinery. For example, parts such as water turbine spindles, multi-crank crankshafts, large connecting rods, and important gears all bear significant loads during operation, requiring high mechanical properties. Free forging is often used to produce blanks.
3.1.1 Process flow of free forging
Different forging methods have different processes, and the general order is:
- Forging blank cutting; Forging billet heating; (Roll forging blank preparation);
- Forging forming, trimming; Intermediate inspection, inspecting the dimensions and surface defects of forgings;
- Heat treatment of forgings to eliminate forging stress and improve metal cutting performance;
- Cleaning, mainly to remove surface oxide scale;
- Inspection: Generally, forgings need to undergo appearance and hardness inspection, while important forgings must also undergo chemical composition analysis, mechanical properties, residual stress testing, and non-destructive testing.
188.8.131.52 Forging blanking
Cut the bar into the required length before forging according to the forging ratio or the weight of the forging required by the customer.
There are two main blanking methods: cutting blanking and forging equipment blanking:
- Cutting blanking: using a saw blade, saw blade, saw belt, thin grinding wheel, and turning tool to cut the forging blank. The cutting-end face is flat, but the notch wears material, and the productivity is low. It is mostly used for forging blanks with many varieties, small batches, or high requirements for notch quality.
- They were forging equipment blanking, shearing, breaking, heating after cutting with a chopping knife, and other methods. The shape of the knife edge is similar to the bar’s cross-section. Small-size bars are mostly used cold shear. For some alloy steels and larger carbon steel bars, to prevent cracks in the fracture surface, they must also be heated to 350-550 °C to shear.
If a multi-station hot forging automaton is used, it can also be hot cut at the forging temperature. The cutting efficiency is high, which is suitable for mass production. There is no material loss in the incision, but the quality of the shear end face is poor. The precision shearing process and equipment can improve the flatness of the shearing end face and reduce the weight error of the blanking. The way to improve the shear accuracy varies with the material. The main methods are shearing the bar in the clamping state and high-speed shearing. The non-perpendicularity between the end face and the axis after shearing can be less than 1°, and the weight error is within 0.5-1%.
Breaking and cutting is to cut a small gap by sawing or gas cutting at the place where the bar needs to be broken, then pad the two ends of the bar to make the gap hang in the air, and apply pressure on the back of the gap to break the bar. This method is suitable for steel with poor fracture plasticity.
The forging equipment used for blanking is mainly a shearing machine, which can also be used for blanking by mechanical press and screw press. The shearing machine now produced in China can cut carbon steel round bars with a diameter of 230 mm.
184.108.40.206 Heating of forging billet
The metal decreases with the increase in temperature. When the metal is heated to a certain temperature, the plasticity increases, and the deformation resistance decreases. The purpose of heating before forging is to eliminate the internal stress of the metal and increase the plasticity of the metal. Within a reasonable range, the higher the
temperature, the better the plasticity.
- Heating temperature: The forging blank is generally heated to the allowable initial forging temperature of the metal. To ensure a uniform temperature inside and outside, the surface of the forging billet should be kept for a certain time after heating to the required temperature. The holding time is related to the thermal conductivity of the metal, the cross-sectional size of the forging billet, and the placement state in the furnace. The heating rate of the cold billet should be manageable to prevent excessive temperature differences between the surface and the core and large thermal stress in the core. Thermal stress in the heart is easy to cause cracks. Commonly used temperature-measuring instruments include thermocouples for measuring furnace temperature and optical pyrometers for measuring metal surface temperature.
- Heating method: In ancient times, open fire directly heated forging. Modern forging billet heating uses a variety of coal, oil, gas, and electric industrial furnaces, including intermittent chamber furnaces, trolley furnaces, resistance furnaces, induction furnaces, and continuous furnaces. The induction furnace has the advantages of fast heating speed, uniform temperature, small footprint, and easy automatic control. It has been widely used in the production line of small and medium-sized die forgings. Forging billet heating consumes much energy, so it is necessary to improve the thermal efficiency of industrial furnaces and the management and operation of heating.
I am forging temperature of common metals (initial forging temperature-final forging temperature), carbon steel: 1200-800C; alloy steel 1150-800C; alloy tool steel (high-speed steel): 1180-900C; stainless steel: 1300-850C; aluminum alloy: 480-380C; copper alloy: 900-700C.
The defects that are prone to occur during heating (at high temperatures, the iron in the steel is oxidized in the furnace gas to form oxides such as FeO, Fe3O4, and Fe2O3, known as oxide skins. The formation of an oxide scale will increase the loss of metal. The oxidation burning loss rate of the general intermittent flame furnace is 2-3 %, and the induction heating is less than 0.5 %. In addition, the oxide scale will also aggravate the wear of the mold, reduce the accuracy of the forging, and cause
the surface to be rough, thereby increasing the machining allowance and material consumption. The oxide scale also hinders the conduction of heat, prolongs the heating time, and affects the furnace bottom’s life and the industrial furnace’s mechanized operation. In addition to the oxide scale, the oxidation
will also reduce the carbon content of the surface layer of the steel, form a decarburization layer, and reduce the hardness and strength of the surface layer of the forging. The generation of oxide skin is more unfavorable to precision forging). The heating process is now being continuously improved.
220.127.116.11 Roll forging blank preparation
Generally, roll forging can be used as the front process of slender materials.
18.104.22.168 Free forging forming
Free forging is a processing method that uses impact force or pressure to make the metal deform freely in all directions between the upper and lower anvil surfaces without any restrictions to obtain the required shape and size and certain mechanical properties of the forgings, referred to as free forging. Free forging is divided into manual free forging and machine free forging. Manual forging mainly forges small parts, and the production efficiency is relatively low. Machine forging is the main way of free forging at present.
The characteristics of free forging: simple tool, strong versatility, short production preparation cycle. Free forgings can range from less than one kilogram to two or three hundred tons. For large forgings, free forging is the only processing method, which makes free forging play a particularly important role in heavy machinery manufacturing. For example, parts such as turbine spindles, multi-turn crankshafts, large connecting rods, and important gears are subjected to large loads during operation, requiring high mechanical properties, and free forging methods are often used to produce blanks. Because the shape and size of free forgings are mainly controlled by manual operation, the precision of forgings is low, the machining allowance is large, the labor intensity is large, and the productivity is low. Free forging is mainly used in single piece, small batch production, repair and production of large forgings, and trial production of new products.
22.214.171.124.1 Free forging process
Free forging process: basic process, auxiliary process, and finishing process.
The basic process involves producing a certain degree of plastic deformation of metal billets to obtain the desired shape size or improve material properties. It is a necessary deformation process in the forging forming process, such as upsetting, elongation, bending, punching, cutting, twisting, and displacement. The most commonly used processes in actual production are upsetting, elongation, and punching.
Upsetting: The process of forging along the axial direction of the workpiece to reduce its length and increase its cross-sectional area. Commonly used for forging gear blanks, flanges, discs, and other parts, it can also be used as a preparatory process for punching hollow forgings such as forging rings and sleeves.
Upsetting can be divided into full and partial upsetting: (a) full and (b) partial upsetting.
When upsetting, the blank should not be too long, and the height to diameter ratio should be less than 2.5 to avoid upsetting, bending, thin waist, interlayer, and other phenomena. The upsetting part of the billet must be uniformly heated to prevent uneven deformation.
Elongation: The process of forging along the axis perpendicular to the workpiece to reduce its cross-sectional area and increase its length. They are commonly used for forging parts such as shafts and rods.
Circular billets are usually forged into a square shape before being elongated and finally forged into the desired shape or elongated using a V-shaped anvil.
The following figure shows the elongation diagram:
As shown, during the forging process, the billet needs to be continuously flipped around the axis.
Punching: the operation process of punching a through hole or a blind hole on the workpiece by using a punch. Commonly used for forging hollow forgings such as gears, sleeves, and rings, holes with a diameter less than 25mm are generally not forged but are processed using drilling methods.
When punching through holes on thin billets, a punch can be used to punch them out in one go. If the blank is thick, it can be punched on one side of the blank to a depth of 2/3 of the hole depth; then the punch can be pulled out, the workpiece can be flipped, and punched through from the opposite side to avoid burrs being punched around the hole.
When punching on both sides with a solid punch, cylindrical billets will experience distortion. The degree of distortion is related to the diameter D0, height H0, and aperture d1 of the blank before punching. The smaller the D0/d1, the more severe the distortion. In addition, when the punching height is too large, it is easy to offset the hole. Therefore, the ratio of the diameter D0 of the blank used for punching to the aperture d1 (D0/dl) should be greater than 2.5, and the height of the blank should be smaller than the diameter of the blank.
1- Punch; 2- Blank; 3- Backing ring; 4- Core material
The pre-deformation process is called the auxiliary process (pressing the jaw, cutting the shoulder, etc.) to facilitate the operation of the basic process.
The process reduces surface defects of forgings (such as correction, rounding, leveling, etc.).
126.96.36.199.2 Development of free forging process regulations
Developing process regulations and writing process cards are essential technical preparations for free-forging production and are the basis for organizing production, standardizing operations, and controlling and inspecting product quality. To formulate process regulations, it is necessary to combine production conditions, equipment capacity, and technical level and strive for advanced technology, reasonable economy, and safe operation to guide production correctly. Free forging process regulations: drawing forging drawings based on part drawings, calculating the quality and size of the billet, determining the forging process, selecting forging equipment, determining the heating specifications of the billet, and filling in process cards.
188.8.131.52.3 Draw a free-forging diagram
Based on the part drawing and combined with the characteristics of the free forging process, the graph is the core content of the process specification and the basis for formulating the forging process and forging inspection. The forging drawing must accurately and comprehensively reflect the special content of the forging, such as rounded corners, slope, etc., as well as the technical requirements for the product, such as performance, organization, etc.
When drawing, the following factors are mainly considered:
- Dressing: for the structures that are difficult to forge by free forging method, such as keyway, tooth groove, back knife groove, small hole, blind hole, and step, a part of metal must be added temporarily to simplify the shape of forgings. This part of the metal added to simplify the forging shape for free forging is called dressing.
- Forging allowance: to increase the allowance for cutting on the machined surface of the part, which is called the forging allowance. The size of the forging allowance is related to the material, shape, size, batch size, actual production conditions, and other factors of the part. The larger the part, the more complex the shape, the greater the margin.
- Forging tolerance: forging tolerance is the allowable variation of the nominal size of the forging, and its value is related to the shape and size of the forging and is affected by the specific production conditions.
184.108.40.206.4 Calculate the mass and size of the billet
Determine the quality of the blank: The quality of the blank used for free forging is the sum of the quality of the forging and the quality of the various metals consumed during forging, which can be calculated by the following formula:
Gblank=Gforging + Gburning loss + Gmaterial head
In the formula:
- Gblank is the mass of the billet in kilograms;
- Gforging – mass of the forging, in kg;
- Gburning loss – the mass of the billet burned due to surface oxidation during heating, in kg. The first heating takes 2% -3% of the mass fraction of the heated metal, and the subsequent heating takes 1.5% -2.0%;
- The mass of the metal that has been washed or cut off during the forging process of the G material head, in kilograms; For example, during punching, the blank, the core in the middle, and the material head generated by trimming the end.
For large forgings, when using steel ingots as billets for forging, it is also necessary to consider the quality of the cut steel ingot head and tail.
Determine the size of the billet: based on the principle of constant volume during plastic processing and the forging ratio, height to diameter ratio, etc., of the basic process types used (such as elongation, upsetting, etc.) to calculate the cross-sectional area, diameter, or side length of the billet.
220.127.116.11.5 Select forging process
The selection of the free forging process should be determined based on the characteristics of the process and the shape of the forging. Generally speaking, disk parts often use processes such as upsetting (or stretching upsetting) and punching; Shaft parts often use processes such as elongation, shoulder cutting, and forging steps, and the selection of free forging processes is related to the number of heats (i.e., the number of heating times of the billet) and the degree of deformation throughout the entire forging process. The required number of heats and the process of forming the blank in each heat should be specified and written on the process card.
18.104.22.168.6 Select forging equipment
According to the nature of the force acting on the billet, free-forging equipment is divided into two categories: forging hammers and hydraulic presses.
The hammer produces an impact force that causes the metal billet to deform. The mass of the falling part expresses the tonnage of a forging hammer. The commonly used forging hammers in production are air and steam air hammers. The air hammer uses an electric motor to drive the piston to generate compressed air, causing the hammer head to move back and forth for hammering. Its characteristics are simple structure, convenient operation, and easy maintenance, but its tonnage is small and can only be used to forge small forgings below 100kg. The steam air hammer uses steam and compressed air as power, and its tonnage is slightly larger, which can be used to produce forgings with a mass of less than 1500kg. The hydraulic press generates static pressure to deform the metal billet. At present, large hydraulic presses can reach over 10,000 tons and can forge 300 tons of forgings. Due to the long duration of static pressure, it is easy to achieve a large forging depth. Therefore, hydraulic press forging can obtain forgings with a fine grain structure throughout the cross-section. Hydraulic presses are the only forming equipment for large forgings, and the production of large advanced hydraulic presses often marks the level of industrial technology development in a country. In addition, the hydraulic press operates smoothly, with no vibration during metal deformation, low noise, and good working conditions. But hydraulic press equipment is huge and expensive. The selection of free forging equipment should be based on factors such as forging size, quality, shape, and basic forging process and combined with actual production conditions. For example, using ingots or large cross-section blanks as blanks for large forgings may require multiple upsetting and drawing operations. Operating on the forging hammer isn’t easy, and the center is not easily penetrated. However, on a hydraulic press, due to its large stroke, the lower anvil can move back and forth, and an upsetting platform can be used for upsetting. Therefore, most large forgings are produced on a hydraulic press.
22.214.171.124.7 Determine the forging temperature range
The forging temperature range refers to the temperature range between the initial and final forging temperatures.
The forging temperature range should be selected as wide as possible to reduce fires and improve productivity. The initial forging temperature for heating is generally 100 to 200 °C below the solidus line to ensure the metal does not overheat or burn. The final forging temperature is generally 50-100 °C higher than the metal’s recrystallization temperature to ensure complete recrystallization after forging and to obtain fine grain structure inside the forging. The forging temperature range of carbon steel and low alloy structural steel is generally based on the iron-carbon equilibrium phase diagram, and the final forging temperature is selected above the Ar3 point to avoid cracking caused by phase transformation during forging. Due to the influence of alloying elements, the initial forging temperature of high alloy steel decreases, the final forging temperature increases, and the forging temperature range narrows.
126.96.36.199.8 Structural Processability of Free Forgings
The design principle of the structural process of free forging is to meet the performance requirements; the forging shape should be as simple as possible and easy to forge. In practical operation, the following points should be avoided:
- To avoid forged forgings with conical or inclined structures, special tools need to be manufactured, and the forming of forgings is also relatively difficult, which makes the process complex, inconvenient to operate, and affects the efficiency of equipment use. Therefore, it should be avoided as much as possible.
- Avoiding the formation of spatial curves at the intersection of geometric bodies; otherwise, it isn’t easy to form forgings. Instead, the intersection of planes at the intersection eliminates spatial curves and makes forging forming easier.
- Avoid strengthening ribs, bosses, I-shaped, elliptical, or other irregular sections and shapes. Otherwise, it isn’t easy to obtain them using free forging methods. If special tools or processes are used for production, it will reduce productivity and increase product costs.
- To avoid sharp changes in the cross-sectional area or complex shapes of forgings, they can be designed as a combination of several simple components. After each simple piece is forged and formed, it is welded or mechanically connected to form an integral part.
188.8.131.52.9 Edge cutting
Remove the redundant material after forging to achieve a product that matches the drawing.
184.108.40.206.10 Intermediate inspection, inspection of dimensions and surface defects of forgings
Intermediate inspection, also known as process inspection, is carried out according to the size and weight requirements of the customer. If the size and weight cannot meet the customer’s requirements, it is necessary to heat the forging to meet the customer’s size and weight requirements. Surface defects are mainly inspected for cracks (stainless steel forgings are particularly prone to cracks, and special attention should be paid during inspection). Is the surface uneven and uneven?
220.127.116.11.11 Heat treatment of forgings
Used to eliminate forging stress and improve metal cutting performance: mechanical properties are often required differently depending on the purpose of the forging, and adjustments to the mechanical properties of the forging are achieved through heat treatment.
Generally, there are four basic processes for heat treatment: annealing, normalizing, quenching, and tempering.
Annealing is the process of heating a workpiece to an appropriate temperature, using different holding times based on the material and workpiece size, and then slowly cooling it to achieve or approach an equilibrium state of the internal structure of the metal, achieve good process and service performance, or prepare for further quenching.
Normalizing is heating a workpiece to a suitable temperature and cooling it in air. The effect of normalizing is similar to annealing, except that the resulting microstructure is finer and is commonly used to improve the cutting performance of materials. It is also sometimes used for final heat treatment of parts with low requirements.
Quenching is the rapid cooling of a workpiece in a quenching medium, such as water, oil, other inorganic salts, organic water solutions, etc., after heating and insulation. After quenching, the steel parts become hard, but at the same time, they become brittle.
Tempering reduces the brittleness of steel parts by holding the quenched steel parts for a long time at an appropriate temperature above room temperature but below 650℃ and then cooling them. This process is called tempering.
In the quenching and tempering process, the surface mechanical properties of forged parts vary slightly depending on the cooling method:
- Oil cooling: The cooling time is short, and the cooling surface temperature is relatively uniform, which makes the crystal arrangement on the surface of the forging more uniform and denser, making its surface have stronger hardness and wear resistance. Moreover, during the cooling process, it does not come into contact with air, which does not cause the generation of oxides and saves material for forged parts.
- Wind cooling: The cooling time is relatively long, and the cooling surface temperature is uniform. However, after cooling, the surface crystal structure of the forged part is not tight enough and does not have strong hardness. During the cooling process, if it is exposed to the air for a long time, it is easy to generate corresponding oxides on the metal surface, which is a bit wasteful to the material of the forged parts.
- Water cooling: Its properties are between oil cooling and air cooling, but there may still be material waste during oxidation machining on its surface. Annealing, normalizing, quenching, and tempering are the “four fires” in overall heat treatment, among which quenching and tempering are closely related and often used together, and none of them are indispensable. The “Four Fires” have evolved into different heat treatment processes with different heating temperatures and cooling methods. Combining quenching and high-temperature tempering to obtain strength and toughness is called quenching and tempering. After quenching certain alloys to form supersaturated solid solutions, they are kept at room temperature or a slightly higher appropriate temperature for a longer period to improve the hardness, strength, or electrical magnetism of the alloy. This heat treatment process is called aging treatment. The method of effectively and tightly combining pressure processing deformation with heat treatment to achieve good strength and toughness coordination of the workpiece is called deformation heat treatment. The heat treatment carried out in a negative-pressure atmosphere or vacuum is called vacuum heat treatment. It cannot only prevent oxidation and decarburization of the workpiece, maintain the surface smoothness of the treated workpiece, and improve the performance of the workpiece but also undergo chemical heat treatment by introducing an infiltration agent.
Surface heat treatment is a metal heat treatment process that only heats the surface of a workpiece to change its mechanical properties. To only heat the surface of the workpiece without excessive heat transfer to the interior of the workpiece, the heat source used must have a high energy density, that is, to provide a large amount of heat energy per unit area of the workpiece, so that the surface or local area of the workpiece can reach a high temperature in a short time or instantaneously. The main methods of surface heat treatment include flame quenching and induction heating heat treatment. Commonly used heat sources include flames such as acetylene or propane, induced current, laser, and electron beam.
Chemical heat treatment is a metal heat treatment process that involves changing the chemical composition, structure, and properties of the surface layer of a workpiece. The difference between chemical heat treatment and surface heat treatment is that the latter changes the chemical composition of the surface layer of the workpiece. Chemical heat treatment is the process of heating a workpiece in a medium (gas, liquid, solid) containing carbon, nitrogen, or other alloying elements and holding it for a long time, thus allowing the surface of the workpiece to infiltrate elements such as carbon, nitrogen, boron, and chromium. After infiltration of elements, other heat treatment processes, such as quenching and tempering, are sometimes required. The main methods of chemical heat treatment include carburization, nitriding, and metal infiltration.
Heat treatment is essential for the post-treatment of forged parts, as it can ensure and improve various properties of the workpiece, such as wear resistance, corrosion resistance, etc. It can also improve the microstructure and stress state of the blank to facilitate various cold and hot processing.
18.104.22.168.12 Cleaning, mainly to remove surface oxide skin
During the heating process of the previous process, the workpiece is completely exposed to the air. Due to the high surface temperature of the workpiece, corresponding oxides are easily generated. Before the final inspection, the surface oxides must be cleaned to prevent deviation during the inspection.
22.214.171.124.13 Inspection of forgings
General forgings must undergo appearance and hardness inspection, while important forgings must undergo chemical composition analysis, mechanical properties, residual stress testing, and non-destructive testing:
Appearance inspection: refers to the visual inspection of the surface condition of the parts under natural light or corresponding light irradiation, such as whether there are cracks, scars, folded debris, etc.
Hardness testing: Generally, hardness refers to surface hardness (Vickers hardness), and other hardness descriptions include Rockwell hardness, Brinell hardness, Vickers hardness, and Shore hardness.
- Rockwell hardness: Under the specified conditions, the indenter (diamond cone, steel ball, or carbide ball) is pressed into the sample’s surface in two steps and removed. After the main test force, the indentation residual depth h is measured under the initial test force, and the indentation residual depth h represents the hardness.
- Vickers hardness: A diamond indenter with a regular pyramid body presses into the sample’s surface under experimental force. After maintaining the specified time, the experimental force is removed, and the diagonal length of the indentation on the sample’s surface is measured. The quotient of the experimental force divided by the surface area of the indentation is the Vickers hardness value. The Vickers hardness test has a wide measurement range and can measure almost all metal materials currently used in industry. It can be measured from very soft materials (several Vickers hardness units) to very hard materials (3000 Vickers hardness units). The biggest advantage of the Vickers hardness test is that its hardness value is independent of the size of the test force. As long as the material has a uniform hardness, the test force can be chosen arbitrarily, and its hardness value remains unchanged. This is equivalent to having a unified scale within a wide range of hardness. This is superior to the Rockwell hardness test. However, the testing of Vickers hardness also has its drawbacks. The efficiency of Vickers hardness testing is low, requiring high testing techniques and high requirements for the surface smoothness of the sample. Usually, specialized samples need to be made, which is cumbersome and time-consuming to operate. It is usually only used in the laboratory, and its symbol is HBS.
- Brinell hardness: Use a steel ball or hard alloy ball of a certain diameter to press into the sample’s surface with the specified experimental force (F) and remove the pressure after the specified holding time. Measure the indentation diameter L on the surface of the sample, and the quotient obtained by dividing the experimental force by the indentation area on the indentation surface is the Brinell hardness value. The Brinell measurement method is not applicable for harder steel or thinner plates, and its symbol is HBW.
- Vickers hardness: Use hard steel indenter with a certain shape, which is pressed into the sample’s surface under the action of standard spring test force. The material hardness is determined by the depth of the indenter, and a depth of 0.01mm is defined as a Vickers hardness unit. The Vickers hardness unit is expressed as HW. Vickers hardness is generally only applicable to aluminum alloy products.
- Shore hardness: Shore hardness is generally used to test plastic and silicone products, and its value is expressed as the ability of silicone rubber to resist the insertion of hard objects (HA, HD). Shore hardness (HS) is generally a standard for representing the elastic deformation of metals.
Other types of hardness include Leeb hardness, Mohs hardness, and Rockwell hardness, which are not commonly used and will not be described.
Chemical composition analysis: When receiving a customer inquiry, the chemical composition content of the material is confirmed. Then, the steel is made based on the chemical composition, or the corresponding billet is ordered from the steel factory. After the final forged product is compared with the chemical composition, the chemical composition is inspected. Generally, 2-3 samples are taken occasionally during steelmaking to check whether the required chemical composition ratio is met. After forging the finished product, Firstly, sample from the product (ensuring that sampling does not affect the final use of the product) and inspect it using a spectrometer.
Related mechanical performance test: a test that determines the characteristics of a material under certain environmental conditions under the action of force or energy, also known as material mechanical performance test. The main content of the experiment is to measure the strength of the material (which refers to the ability of the material to resist deformation and damage under external forces. Commonly used strength indicators include yield strength and tensile strength, which are important criteria for material selection in part design), hardness, rigidity (whether the material has elasticity), plasticity (the characteristic of producing significant deformation without damage under external forces), macroscopic and toughness (generally relative brittleness), etc. Measuring material mechanical properties is closely related to the design calculation, material selection, process evaluation, and material inspection of mechanical products. The measured mechanical performance data not only depends on the material itself but also the experimental conditions. The location and direction of sampling, the shape and size of the sample, and the characteristics of the applied force during the test, including loading speed, environmental medium composition, and temperature, all affect the results of the test. To ensure the relative comparability of test results, a unified standard test method is usually developed, and the test conditions are specified one by one for compliance during testing.
Residual stress test: Residual stress refers to the force exerted by an object to maintain internal structure (or internal molecular structure) balance without external force.
Residual stress test: Residual stress refers to the force exerted by an object to maintain internal structure (or internal molecular structure) balance without external force.
In the process of mechanical manufacturing, most manufacturing processes cause residual stress, but the general reasons are mainly: uneven plastic deformation, uneven temperature, and uneven phase transformation; Improper residual stress may reduce fatigue strength, cause stress corrosion (fracture phenomenon caused by the combined action of stress and corrosive medium), lose dimensional accuracy, and even cause product deformation. So it is often required that the final product undergo stress adjustment or stress relief. The commonly used method is tempering treatment, which utilizes the thermal relaxation effect of residual stress to reduce or eliminate residual stress. The usual methods for detecting stress are X-ray diffraction, ultrasonic, and magnetic methods.
Non destructive testing: refers to the quality inspection of the surface or interior of the inspected component without damaging the working condition of the workpiece or raw materials. The
commonly used inspection methods include X-ray inspection, ultrasonic inspection, magnetic particle inspection, penetration inspection, eddy current inspection γ radiographic testing, fluorescent testing, dye penetrant testing, and other methods (currently, the following methods are used for forgings and castings).
- Magnetic particle inspection: When the workpiece is magnetized, if there are defects on the surface of the workpiece, magnetic leakage occurs due to the increase in magnetic resistance at the defect location, forming a local magnetic field. The magnetic particle displays the shape and position of the defect here, thus determining the existence of the defect.
- Ultrasonic testing: When ultrasonic waves propagate in the tested material, the acoustic characteristics and internal organizational changes of the material have a certain impact on the propagation of ultrasonic waves. The technology of understanding the material performance and structural changes through detecting the degree and condition of ultrasonic waves is called ultrasonic testing. Ultrasonic testing methods usually include penetration, pulse reflection, tandem, etc.
- Dye penetrant inspection: The basic principle of dye penetrant inspection is to use a capillary phenomenon to allow the penetrant to penetrate the defect, and after cleaning, the surface penetrant is removed. The residual penetration in the defect is then adsorbed by the capillary action of the imaging agent to achieve the purpose of detecting defects.
- Fluorescent testing: By utilizing the properties of fluorescent substances that emit light under ultraviolet radiation, fluorescent substances are applied to the surface of parts, and surface defects are inspected using fluorescence. Fluorescent testing can be used to inspect magnetic and non-magnetic materials and non-metallic materials.
- X-ray inspection: By penetrating the metal being inspected with X-rays, the internal structure of the metal is visualized to determine whether there are defects, thereby distinguishing whether the internal and surface structure is intact and whether there are defects.
3.2 Forging Tools
In free forging, the tools used are hammer presses and hydraulic presses. Here, we mainly discuss hydraulic presses. The key parameter is the magnitude of the forging force, which is used to represent the tonnage of the machine equipment (1N is approximately equal to 0.1KG force, and an 80000 kN hydraulic press is equivalent to an 8000 ton hydraulic press). The working method of hydraulic presses uses upper and lower forging anvils and simple tools for free forging, mainly used for single piece and small batch production.
Generally speaking, a hydraulic press with a larger tonnage can forge larger parts. The forging level of a country can reflect its machine manufacturing level, and the tonnage of hydraulic presses also indicates the level and level of development of a country in the mechanical industry.
4. Learn and discuss the machining methods of circular and semi-circular forged parts
The production of current ring components is completed through the binding process.
The characteristics of the rolling process: Rolling requires first upsetting and punching the billet on the press and then entering the radial and axial ring rolling mill for rolling. The characteristics are that the billet rotates, the deformation is continuous, the reduction is small, and it has surface deformation characteristics. Rolling is a continuous deformation technology accumulating local deformation to achieve overall formation. It gradually rolls the small diameter thick section circular billet into a large diameter thin section ring, which is labor-saving, energy-saving, material saving, and high productivity. The significant characteristics of low production cost and wide product range, as well as the ability to form a closed circular distribution of product metal flow lines, greatly improve the comprehensive mechanical properties of the parts and have been widely applied in many industrial fields.
The tying ring is mainly achieved by following the following steps:
1) Cutting: Saw the bar into sections; that is, according to the size and weight of the ring, cut the bar into blanks for the ring.
2) Prefabrication: Heat the material section uniformly from room temperature to a high plasticity and low resistance hot deformation temperature, and then upset and punch the hot section to make a rolling ring blank.
The thick pier is shown in the following figure:
Punching is shown in the following figure:
3) Rolling forming by ring rolling machine: Place the blank into a heating furnace for heating, and when the temperature reaches the hot deformation temperature (aluminum alloy ring with a hot deformation temperature of 550 °C-350 °C; steel material ring with a hot deformation temperature of 1200 °C-800 °C), take it out and roll it into the pass of the ring rolling machine for forming. During the rolling process, maintain good lubrication between the ring and the roller. When the ring is deformed through multiple turns and the diameter expands to the predetermined size, the Forging of the outer step section rings with a specified shape and size;
Of course, to ensure that the rolled ring meets the size requirements, the size of the rolled ring should have a machining allowance based on the required size. For this reason, in ring rolling, the size of the ring near the end face of the expanding machine is usually controlled so that the outer diameter of the forged ring is a positive deviation and the inner diameter is a negative deviation. A certain machining allowance is left inside and outside the ring to meet subsequent machining requirements.
To reduce extrusion pressure, prevent mold damage, and improve the surface quality of parts, lubrication measures must generally be taken. To prevent lubricants from being squeezed out and losing their lubrication effect, steel parts are often treated with phosphating to create a porous structure on the billet’s surface to store lubricants and provide lubrication under high pressure. Commonly used lubricants include mineral oil, soybean oil, soap solution, etc.
4) Mechanical cutting of forgings into products: The products are processed into finished products using mechanical processing methods such as lathes and drilling machines.
5) The main equipment used for ring binding is a rolling press, which operates on the principle that the actively rotating rolling roller gradually presses down, causing the diameter of the circular blank placed on the core roller to expand correspondingly and the cross-sectional shape and size of the blank also change until it meets the requirements of the product. The rolling machine has guide and control rollers on both sides of the core roller. The guide roller plays a stabilizing role in supporting the blank during the rolling process. The control roller sends a signal when the workpiece is rolled to a predetermined diameter, playing a role in controlling the size.
Half circle refers to removing partial arcs according to customer requirements after processing rings of the same diameter. During manufacturing, the intermediate inspection, heat treatment, and final inspection items are the same as free forging and will not be repeated.