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Second, spheroidizing (softening) annealed countersunk screws, hexagonal cylinder head bolts produced by the cold heading process, the original structure of the steel will directly affect the forming ability of cold heading processing. The plastic deformation of the local area during the cold rolling process can reach 60%-80%, and the steel must have good plasticity for this purpose. When the chemical composition of steel is constant, metallographic structure is the key factor determining the pros and cons of plasticity. It is generally considered that coarse flaky pearlite is not conducive to cold heading, and fine spherical pearlite can significantly improve the plastic deformation of steel. Medium carbon steel and medium carbon alloy steel with high strength fasteners are spheroidized (softened) before cold heading to obtain uniform and fine spheroidized pearlite to better meet the actual production needs. For the softening annealing of medium carbon steel wire rod, the heating temperature is selected to be kept above and below the critical point of the steel, and the heating temperature is generally not too high, otherwise three cementites will be precipitated along the grain boundary, causing cold cracking, but The wire rod of medium carbon alloy steel is isothermal spheroidizing annealing. After heating by AC1+ (20-30%), the furnace is cooled to slightly lower than Ar1, and the temperature is about 700 degrees Celsius for a certain period of time, and then the furnace is cooled to about 500 degrees Celsius to be air-cooled. The metallographic structure of the steel is coarse and thin, from sheet to ball, and the cold cracking rate is greatly reduced. The softening annealing temperature of 3545ML35SWRCH35K steel is generally 715-735 degrees Celsius.
Third, the shelling and descaling cold steel wire rod to remove the iron oxide plate process for stripping, descaling, mechanical descaling and chemical pickling two methods. The chemical pickling process that replaces the wire rod by mechanical descaling not only improves productivity but also reduces environmental pollution. The descaling process includes a bending method (commonly using a circular wheel with a triangular groove to repeatedly bend the wire rod), a spray method, etc., and the descaling effect is good, but the residual iron scale cannot be removed (the scale removal rate is 97%). ), especially when the scale of iron oxide is very strong, therefore, mechanical descaling is affected by the thickness, structure and stress state of the iron, and is used for carbon steel wire rods for low-strength fasteners (less than or equal to 6.8). High-strength fasteners (greater than or equal to 8.8) are mechanically descaled with wire rods to remove all iron oxide scales and then de-scaled by chemical pickling. For low carbon steel wire rods, the iron scale left by mechanical descaling is likely to cause uneven wear of the grain draft. When the die draft hole is adhered to the iron sheet due to the friction of the wire rod, the surface of the wire rod is longitudinally grained, and the wire rod is cold-twisted with a flange bolt or a cylinder head screw, and the head is slightly cracked. More than 95% is caused by scratches on the surface of the wire during the drawing process. Therefore, mechanical descaling should not be used for high speed drawing.
Fourth, the drawing and drawing process has two purposes, one is to resize the raw material; the other is to obtain the basic mechanical properties of the fastener through deformation strengthening. For medium carbon steel, medium carbon alloy steel has another purpose, namely It is to make the sheet-like cementite obtained after the wire rod is controlled to be cold as much as possible during the drawing process, and prepare for the subsequent spheroidization (softening) annealing to obtain the granular cementite. However, some manufacturers reduce the cost. If the drawing pass is reduced arbitrarily, the excessive reduction rate increases the work hardening tendency of the wire rod wire, which directly affects the cold heading performance of the wire rod wire. If the distribution of the reduction rate of each pass is not suitable, the wire rod will also produce torsional cracks during the drawing process. This kind of crack along the longitudinal direction of the steel wire and having a certain period of time is exposed during the cold rolling process of the wire. In addition, if the lubrication is not good during the drawing process, the cold-drawn wire rod wire may have a transverse crack regularly. When the wire rod exits the wire die, the tangential direction is different from the drawing die, which will cause the wear of the single-edge hole type of the wire drawing die to be intensified, and the inner hole is out of roundness, resulting in uneven drawing deformation of the wire in the circumferential direction. The roundness of the steel wire is extremely poor, and the cross-sectional stress of the steel wire is uneven during the cold rolling process, which affects the pass rate of cold heading. During the drawing process of the wire rod, the excessive partial reduction rate causes the surface quality of the steel wire to deteriorate, while the excessively low surface reduction rate is not conducive to the fracture of the sheet cementite, and it is difficult to obtain as many granular cementite as possible. That is, the spheroidization rate of cementite is low, which is extremely unfavorable to the cold heading performance of the steel wire. The bar material and the wire rod steel wire produced by drawing are partially controlled in the range of 10%-15%.
5. Cold forging forming Generally, the head of the bolt is formed by cold heading plastic processing. Compared with the cutting process, the metal fiber (metal strand) is continuous along the shape of the product, and there is no cutting in the middle, thus improving the product strength, especially Excellent mechanical properties. The cold heading forming process includes cutting and forming, single-station clicking, double-clicking cold heading and multi-station automatic cooling. An automatic cold heading machine performs stamping, upsetting, extrusion and reduction of the multi-station process in several forming concave molds. The processing characteristics of the original blank used in the single-station or multi-station automatic cold heading machine are determined by the size of the bar with a material length of 5-6 m or the wire rod weight of 1900-2000 KG, that is, the characteristics of the processing technology. In the case of cold heading, instead of using a pre-cut single piece blank, the automatic cold heading machine itself is used to cut and upset (if necessary) the blank from the bar and the wire rod wire. The blank must be shaped before the cavity is squeezed. By shaping, a blank that meets the process requirements can be obtained. The blank does not need to be shaped before upsetting, reducing the diameter and pressing. After the blank is cut, it is sent to the upsetting station. The station can improve the quality of the blank, reduce the forming force of the next station by 15-17%, and extend the life of the mold. The bolt can be made multiple times. 1. The semi-closed cutting tool is used to cut the blank. The simplest method is to use a sleeve-type cutting tool; the angle of the slit should not be greater than 3 degrees; and when the open-cut tool is used, the angle of the slit can be up to 5 -7 degrees. 2. The short-size blank should be able to be flipped 180 degrees during the transfer from the previous station to the next forming station. This will enable the potential of the automatic cold heading machine to process complex fasteners and improve part accuracy. 3. The punch returning device should be installed at each forming station, and the die should be equipped with a sleeve type topping device. 4. The number of forming stations (excluding the cutting station) should generally reach 3-4 stations (more than 5 in special cases). 5. During the effective use period, the structure of the main slider guide and the process components can ensure the positioning accuracy of the punch and the die. 6. The terminal limit switch must be installed on the baffle that controls the material selection. Attention must be paid to the control of the forging force. The out-of-roundness of the cold-dash wire used in the manufacture of high-strength fasteners on automatic cold heading machines should be within the diameter tolerance. For more precise fasteners, the out-of-roundness of the steel wire should be limited to 1/ 2 Within the diameter tolerance range, if the wire diameter does not reach the specified size, the upset or head of the part will crack or form a burr. If the diameter is smaller than the required size of the process, the head will be incomplete. The edges or corners are not clear. The precision that can be achieved by cold heading is also related to the choice of the forming method and the process used. In addition, it depends on the structural characteristics of the equipment used, the process characteristics and its state, the accuracy of the tooling, the life and the degree of wear. For high-alloy steels used for cold heading and extrusion, the working surface roughness of cemented carbide molds should not be large Ra=0.2um. When the roughness of such working surfaces reaches Ra=0.025-0.050um, it has the highest life.
Sixth, the thread processing bolt thread is generally cold-worked, so that the thread blank in a certain diameter range passes through the 滚 (rolling) wire plate (die), and the thread is formed by the pressure of the wire plate (rolling die). A product in which the plastic flow line of the threaded portion is not cut, the strength is increased, the precision is high, and the quality is uniform, and thus it is widely used. In order to produce the outer diameter of the thread of the final product, the required blank diameter is different because it is limited by factors such as the accuracy of the thread and the presence or absence of plating of the material. Rolling (æ“) pressing thread refers to a processing method of forming a thread by plastic deformation. It uses a rolling (twisting plate) mold with the same pitch and shape as the thread to be machined, and squeezes the cylindrical blank while rotating the screw blank, finally transferring the tooth shape on the rolling mold to On the screw blank, the thread is formed. The common point of the rolling (æ“) pressing thread processing is that the number of rolling revolutions does not have to be too much. If too much, the efficiency is low, and the surface of the thread is prone to separation or disorder. On the other hand, if the number of revolutions is too small, the diameter of the thread is easily rounded, and the initial pressure during the rolling is abnormally increased, resulting in a shortened life of the mold. Common defects in rolling threads: surface cracks or scratches on the threaded part; chaotic buckle; the threaded part is out of round. If these defects occur in large quantities, they will be discovered during the processing stage. If the number of occurrences is small, the production process will not notice that these defects will flow to the user, causing trouble. Therefore, the key issues of processing conditions should be summarized and these key factors controlled in the production process.
7. Heat-treated high-strength fasteners shall be quenched and tempered according to technical requirements. Heat treatment and quenching and tempering is to improve the comprehensive mechanical properties of the fastener to meet the tensile strength and yield ratio specified by the product. The heat treatment process has a crucial impact on high-strength fasteners, especially its intrinsic quality. Therefore, in order to produce high-quality high-strength fasteners, advanced heat treatment technology must be available. Due to the large production volume and low price of the high-strength bolts, the threaded parts are relatively fine and relatively precise structures. Therefore, the heat treatment equipment must have the capability of large production capacity, high degree of automation and good heat treatment quality. Since the 1990s, the continuous heat treatment production line with protective atmosphere has dominated. The shock-bottom type, mesh belt furnace is especially suitable for heat treatment and quenching of small and medium-sized fasteners. In addition to the good sealing performance of the furnace, the quenching and tempering line also has advanced atmosphere, computer control of temperature and process parameters, equipment fault alarm and display function. High-strength fasteners from the feeding-cleaning-heating-quenching-cleaning-tempering-coloring to the lower line, all automatic control operation, effectively guarantee the quality of heat treatment. The decarburization of the thread causes the fastener to trip first when the resistance required by the mechanical properties is not reached, causing the threaded fastener to fail and shorten the service life. Due to the decarburization of the raw materials, if the annealing is improper, the decarburization layer of the raw materials will be deepened. During the quenching and tempering heat treatment, some oxidizing gas is usually brought in from the outside of the furnace. The rust of the bar steel wire or the residue on the surface of the wire rod after cold drawing will also decompose after being heated in the furnace, and the reaction will generate some oxidizing gas. For example, the surface rust of steel wire, which is composed of iron carbonate and hydroxide, decomposes into CO2 and H2O after heating, thereby aggravating decarburization. Studies have shown that medium carbon alloy steel is more decarburized than carbon steel, and the fastest decarburization temperature is between 700-800 degrees Celsius. Since the attachment of the steel wire surface is decomposed to synthesize CO2 and H2O under certain conditions, if the continuous mesh belt furnace gas is improperly controlled, the screw decarburization will be excessively poor. When high-strength fasteners are formed by cold heading, the raw material and the annealed decarburized layer are not only still present, but are also extruded to the top of the thread, and the required hardness is not obtained for the surface of the fastener to be quenched. Mechanical properties (especially strength and wear resistance) are reduced. In addition, the surface of the steel wire is decarburized, the surface layer has different expansion coefficients from the internal structure, and surface cracks may occur during quenching. For this reason, in the quenching heating, the top of the thread should be protected from decarburization, and the fasteners with decarburized raw materials should be moderately covered with carbon, and the advantages of the protective atmosphere in the mesh belt furnace should be adjusted to the original parts covered with carbon. The carbon content is basically equal, so that the decarburized fastener is slowly restored to the original carbon content, and the carbon potential is set at 0.42%-0.48%. The carbon coating temperature is the same as the quenching heating, and cannot be performed at high temperature. In order to avoid coarse grains, affecting mechanical properties. The quality problems that may occur in the quenching and quenching process of fasteners are: insufficient hardness in quenched state; uneven hardness in quenched state; excessive quenching deformation; quenching cracking. Such problems appearing on the site are often related to raw materials, quenching heating and quenching cooling. Correctly formulating heat treatment processes and standardizing production operations often avoid such quality accidents.
8. In summary, the process factors affecting the quality of high-strength fasteners include steel design, spheroidizing annealing, shell descaling, drawing, cold heading, thread processing, heat treatment, etc., sometimes The superposition of factors. We know that fastener defects are caused by the volatility of product quality characteristics. Only by accurately understanding the process factors in the product manufacturing process, and thus generating a great driving force for continuous improvement of quality, can we obtain more through the continuous improvement of quality. Profit and stronger competitiveness!
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Bolt heat treatment method
Bolt processing technology is: hot rolled wire rod - (cold dial) - spheroidizing (softening) annealing - mechanical descaling - pickling - cold dialing - cold forging forming - thread processing - heat treatment - inspection one, steel design in fasteners In manufacturing, the correct selection of fastener materials is an important part because the properties of fasteners are closely related to their materials. If the materials are improperly selected or incorrect, the performance may not meet the requirements, the service life is shortened, or accidents or processing difficulties are encountered, and the manufacturing cost is high. Therefore, the selection of fastener materials is a very important link. Cold heading steel is a highly interchangeable fastener steel produced by a cold heading process. Since it is formed by metal plastic working at normal temperature, the deformation amount of each part is large and the deformation speed is high. Therefore, the performance requirements of the cold heading steel raw material are very strict. On the basis of long-term production practice and user use research, combined with GB/T6478-2001 "Technical conditions for cold heading and cold extrusion steel" GB/T699-1999 "Quality carbon structural steel" and target JISG3507-1991 "cold 镦The characteristics of steel carbon steel wire rods are determined by the material requirements of 8.8 grade, 9.8 bolt bolts, and various chemical elements. If the C content is too high, the cold forming performance will be lowered; if it is too low, the mechanical properties of the parts will not be met, so it is set to 0.25%-0.55%. Mn can improve the permeability of steel. However, if it is added too much, it will strengthen the matrix structure and affect the cold forming property. When the parts are quenched and tempered, there is a tendency to promote the growth of austenite grains. Therefore, it is appropriately improved on an international basis. It is 0.45% - 0.80%. Si can strengthen the ferrite, and the cold forming performance is lowered, and the material elongation is lowered to be less than or equal to 0.30%. SP is an impurity element, and their existence will cause segregation along the grain boundary, resulting in grain boundary embrittlement, which impairs the mechanical properties of the steel, and should be reduced as much as possible, and P is less than or equal to 0.030%, and S is less than or equal to 0.035%. B. The maximum boron content is 0.005%, because the boron element has a significant effect of improving the permeability of the steel, but at the same time, the brittleness of the steel is increased. Excessive boron content is highly detrimental to workpieces that require good overall mechanical properties such as bolts, screws and studs.