In forging production, in addition to ensuring the required shape and size of the forging, it must also meet the performance requirements put forward during the use of the parts, which mainly include: strength index, plastic index, impact toughness, fatigue strength, fracture initial and Stress corrosion resistance, etc. For parts working at high temperature, there are also high temperature instantaneous tensile properties, durable properties, anti-deformation properties and thermal fatigue properties.

Influence of Forging on Metal Microstructure and Properties

The raw materials for forging are ingots, rolled products, extruded products and forging billets. Rolled material, extruded material and forging billet are semi-finished products formed by rolling, extruding and forging of ingots respectively. In forging production, using reasonable process and process parameters, the structure and properties of raw materials can be improved through the following aspects:

1. Break the columnar crystals, improve the macrosegregation, change the as-cast structure to the forged structure, and weld the internal pores under suitable temperature and stress conditions to improve the density of the material;

2. The ingot is forged to form a fibrous structure, and further through rolling, extrusion and die forging, the forgings can obtain a reasonable distribution of fiber directions;

3. Control the size and uniformity of grains;

4. Improve the distribution of the second phase (for example: alloy carbides in ledeburite steel);

5. To make the organization get deformation strengthening or deformation strengthening, etc.

Due to the improvement of the above structure, the plasticity, impact toughness, fatigue strength and durability of the forgings are also improved, and then the required hardness, strength and plasticity of the parts can be obtained through the final heat treatment of the parts. performance.

However, if the quality of the raw materials is poor or the forging process used is unreasonable, forging defects may occur, including surface defects, internal defects or unqualified performance.

Influence of raw materials on the quality of forgings

The good quality of raw materials is a prerequisite to ensure the quality of forgings. If the raw materials are defective, it will affect the forming process of the forgings and the final quality of the forgings.

If the chemical elements of the raw materials exceed the specified range or the content of impurity elements is too high, it will have a greater impact on the forming and quality of the forgings. Appears hot and crisp. In order to obtain intrinsically fine-grained steel, the residual aluminum content in the steel needs to be controlled within a certain range, such as 0.02% to 0.04% (mass fraction) of A1 acid. If the content is too small, it will not be able to control the grain size, and it is easy to make the essential grain size of the forgings unqualified; if the aluminum content is too much, it is easy to form wood grain fractures under the condition of forming fibrous structure during pressure processing. Tear-shaped fractures, etc. For another example, in austenitic stainless steel, the more n, Si, Al, and Mo are contained, the more anionite phase, the easier it is to form band cracks during forging, and make the parts magnetic.

For example, there are defects such as shrinkage tube residue, subcutaneous foaming, severe carbide segregation, and coarse non-metallic inclusions (slag inclusions) in the raw materials, which are easy to cause cracks in the forgings during forging. Defects such as dendrites, severe porosity, non-metallic inclusions, white spots, oxide films, segregation bands and mixed metals in the raw materials can easily cause the performance of forgings to decline. Surface cracks, folds, scars, and coarse-grained rings of raw materials are likely to cause surface cracks in forgings.

Influence of forging process on the quality of forgings

The forging process generally consists of the following steps, namely blanking, heating, forming, cooling after forging, pickling and heat treatment after forging. If the forging process is improper, a series of forging defects may occur.

The heating process includes furnace loading temperature, heating temperature, heating speed, holding time, furnace gas composition, etc. Improper heating, such as too high heating temperature and too long heating time, will cause defects such as decarburization, overheating, and overburning.

For the bad material with large cross section, poor thermal conductivity and low plasticity, if the heating speed is too fast and the holding time is too short, the temperature distribution will be uneven, thermal stress will be caused, and the forging blank will be cracked.

The forging forming process includes deformation method, deformation degree, deformation temperature, deformation speed, stress state, tool and die condition and lubrication conditions. Overlap, flow through, eddy current, as-cast structure residue, etc.

During the cooling process after forging, if the process is improper, it may cause cooling cracks, white spots, network carbides, etc.

Influence of Forging Microstructure on Microstructure and Properties after Final Heat Treatment

Austenitic and ferritic heat-resistant stainless steels, superalloys, aluminum alloys, magnesium alloys, etc. have no allotropic transformation during the heating and cooling process, as well as some copper alloys and titanium alloys, which are produced during the forging process. The structural defects cannot be improved by heat treatment.

Materials with allotropic transformation during heating and cooling, such as structural steel and martensitic stainless steel, etc., due to some structural defects caused by improper forging process or some defects left over from the original material, the forgings after heat treatment Quality has a big impact. An example is as follows:

1. The microstructure defects of some forgings can be improved during post-forging heat treatment, and satisfactory microstructure and properties can still be obtained after final heat treatment of forgings. For example, coarse grains and Widmandering structures in generally overheated structural steel forgings, hypereutectoid steels and bearing steels with slight network carbides caused by improper cooling, etc.

2. The structural defects of some forgings are difficult to eliminate with normal heat treatment, and can be improved by measures such as high-temperature normalizing, repeated normalizing, low-temperature decomposition, and high-temperature diffusion annealing.

3. The structural defects of some forgings cannot be eliminated by the general heat treatment process, resulting in the performance of the forgings after the final heat treatment being reduced or even unqualified. For example, severe stone fractures and facet fractures, overburning, ferrite strips in stainless steel, carbide meshes and strips in ledeburite high alloy tool steels, etc.

4. The structural defects of some forgings will further develop and even cause cracking during the final heat treatment. For example, if the coarse grain structure in alloy structural steel forgings is not improved during heat treatment after forging, it will often cause coarse martensite and unqualified properties after carbon, nitriding and quenching; material, quenching often causes cracking.

Different forming methods have different stress and strain characteristics due to different stress conditions, so the main defects that may occur are also different. For example, the main defect when the billet is upsetting is the cracks in the longitudinal or 45° direction on the side surface, and the as-cast structure is often left on the upper and lower ends of the upsetting ingot; the main defect when the rectangular section billet is elongated is the transverse crack on the surface and corner cracks, internal diagonal cracks and transverse cracks; the main defects in open die forging are filling, folding and misalignment.

Different types of materials, due to their different compositions and structures, have different microstructure changes and mechanical behaviors during heating, forging and cooling processes. Therefore, when the forging process is not appropriate, the defects that may occur also have their particularities. For example, the defects of ledeburite high alloy tool steel forgings are mainly coarse carbide particles, uneven distribution and cracks, the defects of superalloy forgings are mainly coarse grains and cracks; the defects of austenitic stainless steel forgings are intergranular chromium depletion, The resistance to intergranular corrosion is reduced, ferrite band structure and cracks, etc.; the defects of aluminum alloy forgings are mainly coarse grains, folding, eddy currents, and flow through.

Special alloy forging is much more difficult than ordinary carbon steel and low-alloy structural steel. To produce special alloy forgings with geometric dimensions and metallurgical quality that meet the requirements, corresponding technical measures must be taken according to different characteristics. These measures are mainly Including process measures, forging and die design measures, quality control measures, testing of forging performance (based on selection of thermodynamic parameters), simulation of forging process, first piece trial forging, etc. Among the above measures, the process measures that need to be taken are described as follows:

1. Reasonably select the temperature, heating speed and holding time of the blank into the furnace

For special alloys with low thermal conductivity (especially large-sized billets), they should be heated at a low speed with the furnace or preheated in the furnace at 800 ° C ~ 850 ° C and held for a period of time, rather than directly heated in the furnace at the initial forging temperature, and then heated It is heated to the initial forging temperature or transferred to a high temperature furnace at the initial forging temperature; the holding time at the initial forging temperature depends on the alloy, which is generally several times longer than that of alloy structural steel.

Other blanks of aluminum, magnesium, copper and their alloys with higher thermal conductivity than steel can be directly put into the high-temperature furnace, the heating speed can be accelerated and the holding time can be appropriately shortened.

2. Properly select the degree of deformation

In view of the characteristics of high alloying degree of special alloys, serious macrosegregation of ingots and forgings, low plasticity and wide critical deformation range, in the forging process, it is necessary to strictly control the degree of deformation of each fire and even the degree of deformation of each stroke of the equipment (deformation degree If it is too large, it may be forged and cracked, and if the degree of deformation is too small, it may fall into the deformation zone, resulting in local grain growth and uneven structure).

3. Strictly control the final forging temperature

In view of the characteristics of high recrystallization temperature and obvious cold work hardening tendency of special alloys, it is necessary to increase or greatly increase the final forging temperature. The forging temperature is about 300°C higher.

4. Reasonably choose the working speed of the forging equipment

For some special alloys that are sensitive to strain rate and slow recrystallization speed, hydraulic presses, mechanical presses and screw presses with lower loading speed should be selected for forging; for high-speed steels with serious mesh fractures, in order to completely crush them For two-shaped broken objects, a forging hammer with high working speed and strong impact force should be selected for forging.

5. Reasonable selection of forging equipment

For some special alloys with high deformation resistance, slow recrystallization speed and high final forging temperature, compared with the same size of broken indium and low-alloy structural steel parts, it is necessary to choose the ability to see or load 3 to 5 times or more. Large forging equipment.

In the same case, molybdenum, magnesium and steel and their alloys with low deformation resistance should choose smaller equipment.

6. Protect the blank

In view of the characteristics that alloy element depleted layer and bile layer or absorbing harmful gas are easily formed on the surface of special alloy during heating process, under possible conditions, it should be heated in a protective atmosphere, or a protective coating should be used to protect the blank, for example, to give The blanks are coated with protective glass lubricant.

7. Double lubricate the blank and mold

In view of the characteristics of high deformation resistance of special alloys, large equipment energy (load) and narrow forging temperature range, in addition to lubricating molds in accordance with the routine, the blank should also be protected and lubricated to minimize friction and blank temperature drop, so as to reduce the required tonnage of equipment . It is recommended to use glass shield lubricants to protect and lubricate the blanks when forging superalloys, titanium alloys and stainless steels.

8. Select the process of forging under compressive stress as much as possible

In view of the characteristics of high alloying degree and low plasticity of special alloys, it is recommended to choose the forging process under compressive stress such as extrusion, closed die satin, and closed upsetting and extrusion on the flat forging machine as much as possible.

9. “Strike while the iron is hot”

In view of the characteristics of high final forging temperature and narrow forging temperature range of special alloys, forging workers are required to be skilled and responsive, and every link from the release of the blank to the transfer to forging must be fast and accurate, and try to shorten the forging pairs as much as possible. The purpose of “strike while the iron is hot”.

10. Strictly implement the preheating system of tools and molds

In view of the characteristics of high final forging temperature, narrow forging temperature range and low thermal conductivity of special alloys, which are sensitive to mold chilling, all clamping dies in contact with forgings must be preheated in strict accordance with regulations.

The above is about the forging process of special alloy forgings. Through the processing of the above process, the plasticity, impact toughness, fatigue strength and durability of the forgings have also been improved, and then the final heat treatment of the parts can be obtained. Good comprehensive properties such as required hardness, strength and plasticity.

To put it simply, forging is to smash metal into the shape you want, and casting is to melt the metal and then re-solidify it into the shape you want. Forging is to mechanically shape metal into the shape you want. (There are two methods, cold and hot.) Casting is to melt the metal and pour it into a prepared sand mold or mold. After cooling, it solidifies into the shape you want. .

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). Through forging, defects such as 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.

A processing method of using 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. One of the two major components of forging. Through forging, the as-cast looseness and welding holes of the metal can be eliminated, and the mechanical properties of forgings are generally better than those of castings of the same material. For important parts in machinery with high load and severe working conditions, forgings are mostly used in addition to rolling plates, profiles or welded parts with simple shapes.

Forging can be divided into

①Open forging (free forging)

Using impact force or pressure to deform the metal between the upper and lower abutting irons (anvil blocks) to obtain the required forgings, there are mainly two types of manual forging and mechanical forging.

②Closed mode forging

The metal blank is compressed and deformed in a forging die cavity with a certain shape to obtain a forging, which can be divided into die forging, cold heading, rotary forging, extrusion, etc. According to the deformation temperature, forging can be divided into hot forging (processing temperature is higher than the recrystallization temperature of the billet metal), warm forging (lower than the recrystallization temperature) and cold forging (normal temperature). Forging materials are mainly carbon steel and alloy steel of various compositions, followed by aluminum, magnesium, titanium, copper, etc. and their alloys. The original state of the material is bar, ingot, metal powder and liquid metal. The ratio of the cross-sectional area of ​​the metal before deformation to the die cross-sectional area after deformation is called the forging ratio. Correct selection of forging ratio has a lot to do with improving product quality and reducing costs.

The difference between casting and forging

1. The production process is different

Casting is a forming. After the metal is melted into liquid, it is cast into a casting cavity suitable for the shape of the part, and after it is cooled, solidified and cleaned, the processing method of the part or blank is obtained. The foundry major focuses on the metal smelting process and the control of the process during the casting process.
Forging is slow forming. The method of applying pressure, extrusion, hammering and other methods to the metal blank with a forging machine makes the metal material in the plastic state a processing method of a workpiece with a certain shape and size. Forging is plastic forming in the solid state, which can be divided into hot working and cold working, such as extrusion, drawing, pier thickening, punching, etc.

2. Different uses

Forging is generally used in the processing of forgings of a certain shape and size. Casting is a relatively economical blank forming method, and is generally used on parts with complex shapes.

3. Different advantages and disadvantages

Forging advantages:

Through forging, defects such as as-cast looseness produced in 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.

Casting advantages:

1. It can produce parts with complex shapes, especially blanks with complex inner cavities.

2. Wide adaptability, metal materials commonly used in industry can be cast, ranging from a few grams to several hundred tons.

3. Wide source of raw materials and low prices, such as scrap steel, scrap parts, chips, etc.

4. The shape and size of the casting are very close to the parts, which reduces the amount of cutting and belongs to no cutting processing.

5. Widely used, 40% ~ 70% of agricultural machinery and 70% ~ 80% of the weight of machine tools are castings.

Casting Disadvantages:

1. The mechanical properties are not as good as forgings, such as coarse structure and many defects.

2. In sand casting, single-piece and small-batch production is labor-intensive.

3. The quality of castings is unstable, there are many processes, and the influencing factors are complex, which are prone to many defects.

Due to the different production processes of casting and forging, the uses of the processed products are different, and the advantages and disadvantages of the products are different. Forging uses hammering and other methods to make a metal material in a plastic state into a workpiece with a certain shape and size, and to change its physical properties. Casting is a process in which metal is melted into a liquid and then poured into a mold. After cooling, solidification, and cleaning, a casting of the desired shape can be obtained. It can be made into various objects with complex shapes.

Cold forging is a process in which metals are formed by forging at room temperature. Cold forging includes deformation forms such as upsetting, die forging and extrusion, and is a cold bulk forming. The cold forging process was developed from cold extrusion and gradually extended to cold die forging and cold precision forging.

Compared with hot die forging, the main advantage of cold forging is that the blank is not heated, so there are no forging dimensions and inherent quality problems caused by heating, and heating equipment is omitted. Cold forgings have high dimensional accuracy, small roughness values, reduce cutting, save materials and reduce costs, and are easy to realize mechanized and automated production. Therefore, it is widely promoted and applied in industries such as automobiles, motorcycles, bicycles, tractors, home appliances, textile machinery, military industry and aviation industry, and is expanding day by day.

Types of cold forgings

1. Extrusion: cold extrusion, which can be divided into positive extrusion, reverse extrusion and compound extrusion.

2. Upsetting category: cold upsetting can be divided into upsetting and partial upsetting
.
3. Die forging: It is finally formed by cold forging, which can be divided into small burr die forging and closed burr-free die forging.

4. Precision forging: cold precision bevel gear tooth profile, spline and straight tooth profile.

Cold forging process characteristics

Cold forging products are of good quality, processed at room temperature, with high dimensional accuracy, smooth surface and good mechanical properties. It can also process forgings with complex shapes, reduce cutting, save material consumption and reduce costs.

The deformation resistance is large, and measures to reduce the deformation resistance must be taken: adjusting the chemical composition of raw materials, pre-treatment of the blank, improving the deformation process, improving the hardness and finish of the mold, and controlling the pressurization speed of the equipment.

Cold deformation strengthening and thermal effect, in the process of cold forging, as the degree of deformation increases, the cold deformation strengthening phenomenon in which the strength and hardness of the metal also increase. Cold deformation strengthening can be used to strengthen the strength and hardness of the product and improve the mechanical properties.

Due to the conversion of mechanical energy into thermal energy, the temperature of the billet will increase, known as thermal effect. To control the billet temperature does not exceed 400 ℃.

The friction force is large, the cold forging deformation force is large, the unit pressure is also large, and the friction force between the metal and the die surface is also high, so the lubrication conditions must be improved to reduce the friction force.

Mechanization and automation, processing at room temperature, good labor conditions, easy to realize mechanization and automation, suitable for mass production.

The cold forging drawing is designed according to the part drawing, which is the main basis for formulating the process, designing the mold and checking the forging. According to the design characteristics, it is divided into cold extrusion and cold die forging design.

Design points of cold die forgings

1. Cold-forged or cold-extruded parts should be as axially symmetrical as possible to ensure that the metal flows evenly during the forming process.

2. Determine the shape, size, location and technical requirements of cold forging without cutting after direct forming according to the part drawing.

3. Parting surface: cold forgings can be divided into two forms: small burr open die forging and closed die forging. The position of the parting surface of the small burr is usually set on the largest diameter or the end face of the largest diameter.

4. Machining allowances and tolerances: After cold forging, no machining allowances will be made for the parts that are not machined, which are the same as the dimensions and tolerances of the parts drawing. There is no need to exact product accuracy for the remaining parts to be machined.

5. Ejection slope and fillet radius: During cold forging, an ejector is generally provided, and the ejection slope is about 0°~3°. The fillet radius is based on R2, and it can be increased or decreased according to the requirements of the part drawing. However, during cold forging, if the fillet is too large, it will affect the forming.

The main technical problem of using the cold forging process is that the metal has high strength in the cold state, and it has a strengthening effect during the deformation process. Focusing on this central issue, only effective measures can be taken from the aspects of raw materials, blank pretreatment, cold forging deformation process, molds and equipment, etc., in order to achieve success.

The forging products of the forging factory are plastically deformed by forging processing. The forging process is a processing method for the blanks or parts that use the external force to plastically deform the forging raw materials and obtain the size, shape and performance required for the forgings. Through forging processing can eliminate defects such as as-cast looseness caused by metal in the smelting process, optimize the microstructure, and at the same time greatly enhance the performance of forgings in use due to the preservation of the complete metal forging flow line.

Forging is one of the main methods for the production of blanks and parts in mechanical manufacturing. It is often divided into free forging and die forging. Compared with other processing methods, forging processing has the following characteristics:

1. Improve the internal structure of forgings and improve mechanical properties. After the forging blank is forged, its structure and properties are improved and improved. Forging processing can eliminate defects such as pores, shrinkage cavities and dendrites in the metal ingot, and can be coarsened due to plastic deformation and recrystallization of the metal. The grain is refined to obtain a dense metal structure, thereby improving the mechanical properties of the forging. In the design of the part, if the direction of the force of the part and the direction of the fiber structure are correctly selected, the impact resistance of the forging can be improved.

2. The utilization rate of materials is high. Metal plastic forming is mainly rearranged by the relative position of the metal body structure without the need to cut the metal.

3. Higher productivity. Forging processing is generally carried out by using a press and a forging hammer.

4. The accuracy of blanks or forgings is high. With advanced technology and equipment, it can achieve less or no cutting.

5. The metal material used for forging should have good plasticity so that it can be plastically deformed without breaking under the action of external force. Among the commonly used metal materials, cast iron is a brittle material with poor plasticity and cannot be used for forging. Copper, aluminum and alloys thereof in steel and non-ferrous metals can be processed under cold or hot conditions.

6. It is not suitable for forming forgings with complex shapes. Forging processing is formed in the solid state. Compared with casting, the flow of metal is limited, and it is generally required to adopt heating and other technological measures. It is difficult to manufacture parts or blanks with complex shapes, especially those with complex internal cavities.

Since forging has the above characteristics, important parts subjected to impact or alternating stress (such as transmission spindle, ring gear, connecting rod, track wheel, etc.) should be processed by forging blanks, so forging processing in machinery manufacturing, mining, light industry Heavy industry and other industries have been widely used.