The high-strength bolt manufacturing process
The processing technology of high-strength bolts is: hot-rolled wire rod - (cold drawing) - spheroidizing (softening) annealing - mechanical descaling - pickling - cold drawing - cold forging - thread processing - heat treatment - inspection
1. Steel design
In the manufacture of fasteners, the correct selection of fastener materials is an important part, because the performance of fasteners is closely related to their materials. If the material is improperly or incorrectly selected, the performance may not meet the requirements, the service life may be shortened, or even accidents or processing difficulties may occur, and the manufacturing cost will be high. Therefore, the selection of fastener materials is a very important link. Cold heading steel is steel for fasteners with high interchangeability produced by the cold heading forming process. Because it is formed by metal plastic processing at room temperature, each part has a large amount of deformation and a high deformation speed. Therefore, the performance requirements of cold heading steel raw materials are very strict.
On the basis of long-term production practice and user research, combined with GB/T6478-2001 "Technical Conditions for Cold Heading and Cold Extrusion Steel" GB/T699-1999 "High-Quality Carbon Structural Steel" and target JISG3507-1991 "Cold Heading" The characteristics of "Carbon Steel Wire Rod for Steel", taking the material requirements of grade 8.8 and grade 9.8 bolts and screws as an example, the determination of various chemical elements. If the C content is too high, the cold forming performance will be reduced; if it is too low, the mechanical properties of the parts cannot be met, so it is set at 0.25%-0.55%. Mn can improve the permeability of steel, but adding too much will strengthen the matrix structure and affect the cold forming performance; when the parts are quenched and tempered, there is a tendency to promote the growth of austenite grains, so it should be appropriately improved on an international basis. It is 0.45%-0.80%. Si can strengthen the ferrite and reduce cold formability. S.P. is an impurity element, and its existence will cause segregation along the grain boundary, resulting in the embrittlement of the grain boundary and damage to the mechanical properties of the steel. It should be reduced as much as possible. B. The maximum value of boron content is 0.005%, because although boron has the effect of significantly improving the permeability of steel, it will also increase the brittleness of steel. Excessive boron content is very unfavorable for workpieces such as bolts, screws, and studs that require good comprehensive mechanical properties.
2. Spheroidizing (softening) annealing
When the countersunk head screws and hexagon socket head bolts are produced by the cold heading process, the original structure of the steel will directly affect the forming ability of the cold heading process. In the process of cold heading, the plastic deformation of the local area can reach 60%-80%, so the steel must have good plasticity. When the chemical composition of the steel is constant, the metallographic structure is the key factor to determine the plasticity. It is generally believed that the coarse flaky pearlite is not conducive to cold heading forming, while the fine spherical pearlite can significantly improve the plastic deformation ability of the steel. For medium carbon steel and medium carbon alloy steel with a large amount of high-strength fasteners, spheroidizing (softening) annealing is performed before cold heading, so as to obtain uniform and fine spheroidized pearlite to better meet the actual production needs.
For the softening annealing of medium carbon steel wire rods, the heating temperature should be kept above and below the critical point of the steel, and the heating temperature should not be too high, otherwise tertiary cementite will precipitate along the grain boundary, resulting in cold heading cracking. The wire rod of medium carbon alloy steel is annealed by isothermal spheroidization. After heating at AC1+ (20-30%), the furnace is cooled to slightly lower than Ar1, the temperature is about 700 degrees Celsius for an isothermal period, and then the furnace is cool to about 500 degrees Celsius and air-cooled. The metallographic structure of the steel changes from coarse to fine, from flake to spherical, and the cracking rate of cold heading will be greatly reduced. The general area of softening annealing temperature for 35\45\ML35\SWRCH35K steel is 715-735 degrees Celsius; while the general area of spheroidizing annealing heating temperature for SCM435\40Cr\SCR435 steel is 740-770 degrees Celsius, and the isothermal temperature is 680-700 degrees Celsius.
Three, peeling and descaling
The process of removing iron oxide plate from cold heading steel wire rod is stripping and descaling. There are two methods: mechanical descaling and chemical pickling. Replacing the chemical pickling process of wire rods with mechanical descaling not only improves productivity but also reduces environmental pollution. This descaling process includes the bending method (the round wheel with triangular grooves is commonly used to repeatedly bend the wire rod), the spray nine methods, etc., the descaling effect is good, but the residual iron scale cannot be removed (the removal rate of iron oxide scale is 97%) ), especially when the scale of the iron oxide scale is very strong, therefore, mechanical descaling is affected by the thickness, structure and stress state of the iron scale. Carbon steel wire rods used for low-strength fasteners (less than or equal to 6.8)
High-strength fasteners (greater than or equal to grade 8.8) use wire rods after mechanical descaling to remove all iron oxide scales, and then go through a chemical pickling process to compound descaling. For low carbon steel wire rods, the iron sheet left by mechanical descaling is likely to cause uneven wear of grain drafting. When the grain draft hole adheres to the iron sheet when the wire rod rubs against the external temperature, the surface of the wire rod produces longitudinal grain marks. More than 95% are caused by scratches on the surface of the steel wire during the drawing process. Therefore, the mechanical descaling method is not suitable for high-speed drawing.
4. Pulling
The drawing process has two purposes, one is to modify the size of the raw materials; the other is to obtain basic mechanical properties of the fasteners through deformation and strengthening. For medium carbon steel, medium carbon alloy steel also has another purpose, that is, to make the wire rod. The flaky cementite obtained after controlled cooling is cracked as much as possible during the drawing process to prepare for the subsequent spheroidization (softening) annealing to obtain granular cementite. However, some manufacturers arbitrarily reduce the drawing in order to reduce costs. The excessive reduction rate increases the work hardening tendency of the wire rod, which directly affects the cold heading performance of the wire rod. If the distribution of the reduction ratio of each pass is not appropriate, it will also cause torsional cracks in the wire rod during the drawing process.
In addition, if the lubrication is not good during the drawing process, it can also cause regular transverse cracks in the cold drawn wire rod. The tangential direction of the wire rod and the wire drawing die is not concentric at the same time when the wire rod is rolled out of the die, which will cause the wear of the unilateral hole pattern of the wire drawing die to aggravate, making the inner hole out of round, and cause uneven drawing deformation in the circumferential direction of the wire. The roundness of the steel wire is out of tolerance, and the cross-sectional stress of the steel wire is not uniform during the cold heading process, which affects the cold heading pass rate. During the drawing process of the wire rod, the excessive surface reduction ratio will deteriorate the surface quality of the steel wire, while the too low surface reduction ratio is not conducive to the crushing of the flaky cementite, and it is difficult to obtain as much 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. For the bar and wire rod produced by the drawing method, the partial surface reduction rate is directly controlled within the range of 10%-15%.
5. Cold forging
Usually, the forming of the bolt head adopts cold heading plastic processing. Compared with the cutting process, the metal fiber (metal wire) is continuous along with the shape of the product, and there is no cut in the middle, thus improving the strength of the product, especially the mechanical properties. The cold heading forming process includes cutting and forming, single-position single-click, double-click cold heading, and automatic cold heading at multi-task positions.
An automatic cold heading machine performs multi-tasking processes such as stamping, heading forging, extrusion, and diameter reduction in several forming dies. The processing characteristics of the original blank used by the single-station or multi-tasking automatic cold heading machine are determined by the size of the material size of the bar with a length of 5-6 meters or the size of the wire rod with a weight of 1900-2000KG, that is, the characteristics of the processing technology. The point is that cold heading does not use pre-cut single blanks, but uses the automatic cold heading machine itself to cut and upset (if necessary) blanks from bars and wire rods. Before extruding the cavity, the blank must be shaped. A blank that meets the technical requirements can be obtained by shaping. Before upsetting, reducing, and positive extrusion, the blank does not need to be shaped. After the blank is cut, it is sent to the upsetting and shaping station. This station can improve the quality of the blank, reduce the forming force of the next station by 15-17%, and prolong the life of the mold, and the bolts can be made with multiple diameter reductions.
1. The easiest way to cut the blank with a semi-closed cutting tool is to use a sleeve cutting tool; the angle of the incision should not be greater than 3 degrees; and when an open cutting tool is used, the bevel angle of the incision can reach 5 -7 degrees.
2. The short-sized blank should be able to turn 180 degrees during the transfer from the previous station to the next forming station so that the potential of the automatic cold heading machine can be exerted, the fasteners with complex structures can be processed, and the precision of the parts can be improved.
3. Each forming station should be equipped with a punch return device, and the die should be equipped with a sleeve-type ejector device.
4. The number of forming stations (excluding cutting stations) should generally reach 3-4 stations (more than 5 in special cases).
5. During the effective service period, the structure of the main slider guide rail and 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, and attention must be paid to the control of the upsetting force. The out-of-roundness of the cold dial wire used in the manufacture of high-strength fasteners on an automatic cold heading machine should be within the diameter tolerance range, while the out-of-roundness of the steel wires for more precise fasteners should be limited to 1/1/2. 2 Within the diameter tolerance range, if the diameter of the steel wire does not reach the specified size, cracks will appear in the upsetting part or head of the part, or burrs will be formed. If the diameter is smaller than the size required by the process, the head will be incomplete. , the edges and corners or the thickened parts are not clear. The precision that can be achieved by cold heading is also related to the selection of the forming method and the procedure used.
In addition, it also depends on the structural characteristics of the equipment used, the process characteristics and its state, the precision of the tool and dies, the life, and the degree of wear. For high alloy steel used in cold heading forming and extrusion, the roughness of the working surface of the cemented carbide mold should not be greater than Ra=0.2um. When the roughness of the working surface of this type of mold reaches Ra=0.025-0.050um, it has the highest life.
6. Thread processing
The bolt thread is generally cold worked so that the thread blank within a certain diameter range passes through the rubbing (rolling) wire plate (die), and the thread is formed by the pressure of the wire plate (rolling die). The plastic streamline of the threaded part is not cut off, the strength is increased, the precision is high, and the quality is uniform, so it is widely used.
In order to make the outer diameter of the thread of the final product, the required thread blank diameter is different, because it is limited by factors such as thread accuracy and whether the material is coated or not. Rolling (rubbing) thread is a processing method that uses plastic deformation to form thread teeth. It uses a rolling (screwing plate) die with the same pitch and tooth shape as the thread to be processed, while extruding the cylindrical screw blank, rotating the screw blank, and finally transferring the tooth shape on the rolling die to the On the screw blank, the thread is formed. The common point of rolling (rubbing) thread processing is that the number of rolling revolutions does not need to be too much. If it is too much, the efficiency will be low, and the surface of the thread teeth will easily cause separation or a random buckle. On the contrary, if the number of revolutions is too small, the diameter of the thread is easy to be out of round, and the pressure at the initial stage of rolling increases abnormally, resulting in a shortening of the life of the die.
Common defects of rolled threads: cracks or scratches on the surface of the threaded part; random buckles; out of roundness of the threaded part. If these defects occur in large numbers, they will be discovered during the processing stage. If the number of occurrences is small, the production process does not notice these defects and then circulates to users, causing trouble. Therefore, the key issues of processing conditions should be summarized, and these key factors should be controlled in the production process.
7. Heat treatment
High-strength fasteners must be quenched and tempered according to technical requirements. The purpose of heat treatment and tempering is to improve the comprehensive mechanical properties of fasteners to meet the specified tensile strength value and yield ratio of the product. The heat treatment process has a crucial impact on high-strength fasteners, especially their intrinsic quality. Therefore, in order to produce high-quality high-strength fasteners, advanced heat treatment technology and equipment must be available. Due to the large production volume and low price of high-strength bolts, and the threaded part is a relatively fine and relatively precise structure, the heat treatment equipment is required to have a large production capacity, high degree of automation, and good heat treatment quality.
Since the 1990s, the continuous heat treatment production line with a protective atmosphere has dominated, and the shock bottom type and mesh belt furnace are especially suitable for heat treatment and tempering of small and medium-sized fasteners. In addition to the good sealing performance of the furnace, the quenching and tempering line also has advanced computer control of atmosphere, temperature and process parameters, equipment failure alarm, and display functions. High-strength fasteners are automatically controlled and operated from feeding-cleaning-heating-quenching-cleaning-tempering-coloring to offline, which effectively ensures the quality of heat treatment. The decarburization of the thread will cause the fastener to trip before the resistance required by the mechanical properties is reached, which will cause the failure of the threaded fastener and shorten the service life. Due to the decarburization of the raw material, if the annealing is improper, the decarburized layer of the raw material will be deepened. In the process of quenching and tempering heat treatment, some oxidizing gas is generally brought in from outside the furnace. The rust of the bar wire or the residue on the surface of the wire rod after the cold drawing will also decompose after being heated in the furnace, and some oxidizing gases will be generated by the reaction. For example, the surface rust of steel wire, which is composed of iron carbonate and hydroxide, will be decomposed into CO2 and H2O after heating, thus aggravating decarburization. Studies have shown that the degree of decarburization of medium carbon alloy steel is more serious than that of carbon steel, and the fastest decarburization temperature is between 700-800 degrees Celsius. Because the attachments on the surface of the steel wire decompose and synthesize CO2 and H2O very quickly under certain conditions, if the furnace gas of the continuous mesh belt furnace is not properly controlled, the decarburization of the screw will be too poor. When the high-strength fasteners are formed by cold heading, the raw materials and the annealed decarburized layer not only still exist, but also are extruded to the top of the thread. For the surface of the fastener that needs to be quenched, the required hardness cannot be obtained. Mechanical properties (especially strength and wear resistance) are reduced.
In addition, the surface of the steel wire is decarburized, the surface layer and the internal structure have different expansion coefficients, and surface cracks may occur during quenching. For this reason, during quenching and heating, the top of the thread should be protected from decarburization, the fasteners whose raw materials have been decarburized should be properly carbonized, and the advantages of the protective atmosphere in the mesh belt furnace should be adjusted to the original carbon-coated parts. The carbon content is basically the same so that the decarburized fasteners can be slowly restored to the original carbon content. The carbon potential is preferably set at 0.42%-0.48%. , so as to avoid coarse grains and affect mechanical properties. The quality problems that may occur in the process of quenching and tempering fasteners mainly include insufficient hardness in the quenched state; uneven hardness in the quenched state; excessive quenching deformation; quenching cracking. Such problems in the field are often related to raw materials, quenching heating, and quenching cooling. Correctly formulating the heat treatment process and standardizing the production operation process can often avoid such quality accidents.
