How are bolts made? Seek an answer

The processing technology of high-strength bolts is as follows: hot-rolled wire rod-(cold-drawn)-spheroidizing (softening) annealing-mechanical descaling-pickling-cold-drawn-cold forging forming-thread processing-heat treatment-inspection 1. Steel design.

In the manufacture of fasteners, the correct selection of fastener materials is an important link, because the performance of fasteners is closely related to their materials. Improper or incorrect material selection may lead to unsatisfactory performance, shortened service life, even accidents or processing difficulties, and high manufacturing costs, so the selection of fastener materials is a very important link. Cold heading steel is a fastener steel with high interchangeability produced by cold heading forming process. Because the metal is plastically formed at room temperature, the deformation of each part is large and the deformation speed is fast, so the performance requirements of cold heading steel raw materials are very strict. On the basis of long-term production practice and user survey, combined with GB/T6478-200 1 Technical Conditions for Cold Heading and Cold Extrusion GB/T699- 1999, High-quality carbon structural steel and JISG 3507-199/kloc, the target steel for cold heading. Too low can not meet the requirements of mechanical properties of parts, so it is set at 0.25%-0.55%. Mn can improve the permeability of steel, but too much Mn will strengthen the matrix structure and affect the cold formability. When the parts are quenched and tempered, it tends to promote the growth of austenite grains, so it is appropriately increased internationally, and it is set at 0.45%-0.80%. Si can strengthen ferrite and reduce cold formability, and the reduction of material elongation is determined to be less than or equal to 0.30%. As an impurity element, S.P. will segregate along grain boundaries, leading to grain boundary embrittlement and damaging the mechanical properties of steel, which should be reduced as much as possible, with P≤0.030% and S≤0.035%. B the maximum boron content is 0.005%, because although boron can significantly improve the permeability of steel, it will also lead to the increase of brittleness of steel. Excessive boron content is very unfavorable to bolts, screws and studs that need good comprehensive mechanical properties. ^5 QSV\X

Secondly, spheroidizing (softening) annealing.

When producing countersunk head screws and hexagon socket bolts by cold heading process, the original structure of steel will directly affect the formability during cold heading. In the process of cold heading, the plastic deformation of local area can reach 60%-80%, so the steel must have good plasticity. When the chemical composition of steel is certain, metallographic structure is the key factor to determine plasticity. It is generally believed that coarse flaky pearlite is not conducive to cold heading, while fine spherical pearlite can significantly improve the plastic deformation ability of steel. For medium carbon steel and medium carbon alloy steel with large amount of high strength fasteners, spheroidizing (softening) annealing is carried out before cold heading to obtain uniform and detailed spheroidized pearlite, which can better meet the actual production needs. For the softening annealing of medium carbon steel wire rod, the heating temperature is usually kept around the critical point of steel, and the heating temperature should not be too high, otherwise the third carburization will precipitate along the grain boundary, leading to cold heading cracking. The medium carbon alloy steel wire is annealed by isothermal spheroidization, heated by AC 1+(20-30%), and then cooled with the furnace to a temperature slightly lower than that of Ar 1, and the temperature is about 7000. The metallographic structure of steel changes from coarse to fine, from flaky to spherical, and the cracking rate of cold heading will be greatly reduced. The softening and annealing temperature of 35\45\ML35\SWRCH35K steel is generally 7 15-735 degrees Celsius; In SCM435\40Cr\SCR435, the heating temperature of steel ball annealing is generally 740-770 degrees Celsius, and the isothermal temperature is 680-700 degrees Celsius.

III. Peeling and descaling 6TlkPM$~2

The process of removing iron scale from cold heading steel wire rod includes peeling, descaling, mechanical descaling and chemical pickling. Using mechanical descaling instead of chemical pickling process of wire rod not only improves productivity, but also reduces environmental pollution. This descaling process includes bending method (round wheel with triangular groove is often used to bend wire repeatedly) and spraying method. The scale removal effect is good, but the residual scale can not be removed (the scale removal rate is 97%), especially when the scale has strong adhesion. Therefore, mechanical descaling is affected by the thickness, structure and stress state of iron oxide scale, and is used for carbon steel wire rod (less than or equal to 6.8 grade) for low-strength fasteners. After mechanical derusting, the wire rod for high-strength fasteners (8.8 or above) should undergo a chemical pickling process to remove all oxide scales. For low carbon steel wire rod, the residual iron scale after mechanical descaling is easy to cause uneven wear of pulling grains. When the die hole of the wire rubs against the external temperature, the iron sheet adheres to it, resulting in longitudinal grain marks on the surface of the wire. When the wire rod is a cold heading flange bolt or a cylindrical head screw, more than 95% of the reasons for the micro-cracks at the head of the wire rod are caused by the scratches on the surface of the wire rod during drawing. Therefore, mechanical derusting method is not suitable for high-speed drawing.

Four, wire drawing process has two purposes: one is to change the size of raw materials; Secondly, fasteners can obtain basic mechanical properties through deformation strengthening. For medium carbon steel, medium carbon alloy steel has another purpose, that is, the flaky cementite obtained by controlled cooling of wire rod can crack as much as possible during drawing, so as to prepare for the subsequent spheroidizing (softening) annealing to obtain granular cementite. However, some manufacturers reduce the number of drawing passes at will in order to reduce the cost. Excessive surface reduction rate increases the work hardening tendency of wire rod and steel wire, which directly affects the cold heading of wire rod and steel wire. If the cross-sectional shrinkage of each pass is not properly distributed, it will also cause torsional cracks in the wire rod steel wire during drawing, which are distributed along the longitudinal direction of the steel wire and have a certain period, and are exposed during the cold heading of the steel wire. In addition, if the lubrication is not good in the drawing process, regular transverse cracks will also occur in the cold-drawn wire rod steel wire. The tangent direction of wire rod and wire drawing die are different at the same time, which will aggravate the wear of one-side pass of wire drawing die, make the inner hole out of round, cause uneven circumferential drawing deformation of wire rod, make the roundness of wire rod out of tolerance, and make the cross section of wire rod bear uneven force during cold heading, which will affect the qualified rate of cold heading. In the drawing process of wire rod steel wire, the surface quality of wire rod steel wire is deteriorated by excessive local reduction rate, while the flaky cementite is not broken by too low reduction rate, so 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 wire rod and wire rod steel wire. The local reduction rate of wire rod and wire rod produced by drawing method is directly controlled at 10%- 15.

Verb (abbreviation of verb) is formed by cold forging.

Usually, the bolt head is formed by cold upsetting plastic processing. Compared with cutting processing, the metal fiber (metal wire) is continuous along the shape of the product without cutting in the middle, thus improving the strength of the product, especially the excellent mechanical properties. The cold heading forming process includes cutting forming, single-click and double-click cold heading and multi-station automatic cold heading. An automatic cold heading machine carries out multi-station processes such as stamping, upsetting, extrusion and reducing diameter in several forming dies respectively. The processing characteristics of the original blank used by single-station or multi-station automatic cold heading machine are determined by the size of the bar with a material length of 5-6 meters or the wire with a weight of 1900-2000 kg, that is, the processing technology is characterized in that the blank is cut and upset by the automatic cold heading machine itself (if necessary) instead of a pre-cut single blank. Before extruding the cavity, the blank must be shaped. The blank meeting the technological requirements can be obtained by shaping. Before upsetting, reducing and forward extrusion, the blank does not need to be formed. After the blank is cut, it is sent to the upsetting and forming station. This station can improve the blank quality, reduce the forming force of the next station 15- 17%, and prolong the service life of the die. Bolts can be reduced several times. 1. The simplest way to cut a blank with a semi-closed cutter is to use a sleeve cutter; The angle of the incision should not be greater than 3 degrees; When using an open cutter, the bevel angle of the incision can reach 5-7 degrees. 2. Short-sized blanks should be able to turn over 180 degrees in the process of transferring from the previous station to the next forming station, which can give full play to the potential of automatic cold heading machine, process fasteners with complex structures and improve the accuracy of parts. 3. Each molding station should be equipped with a punch material returning device, and the female die should be equipped with a sleeve ejection device. 4. The number of forming stations (excluding cutting stations) should generally reach 3-4 stations (more than 5 in special cases). 5. Within the effective service life, the structure of the main slider guide rail and process parts can ensure the positioning accuracy of the punch and the die. 6. The terminal limit switch must be installed on the baffle to control the material selection, and the upsetting force should be controlled. The out-of-roundness of cold-drawn wire rod used for manufacturing high-strength fasteners on automatic cold heading machine should be within the diameter tolerance range, while the out-of-roundness of steel wire used for more precise fasteners should be within the diameter tolerance range of 1/2. If the diameter of the steel wire does not reach the specified size, cracks or burrs will appear in the upsetting part or head of the part. If the diameter is less than the size required by the process, the head will be incomplete, and the edges and corners or enlarged parts will be unclear. The precision that can be achieved by cold heading is also related to the choice of forming method and the working procedure adopted. In addition, it also depends on the structural characteristics, technological characteristics and state of the equipment used, the accuracy, life and wear degree of the tools and dies. For high alloy steel used for cold heading and extrusion, the working surface roughness of cemented carbide die should not be greater than Ra=0.2um. When the working surface roughness of this die reaches Ra=0.025-0.050um, its service life is the highest.

Six, thread processing

Generally, the bolt thread is cold-worked, so that the thread blank within a certain diameter range passes through the thread rolling plate (die), and the thread is formed by the pressure of the thread rolling plate (die). The plastic streamline of the threaded part will not be cut off, and products with increased strength, high precision and uniform quality can be obtained, so it is widely used. In order to manufacture the thread outer diameter of the final product, the required thread blank diameter is different, because it is limited by factors such as thread precision and whether the material is coated. Rolling (rubbing) thread refers to the processing method of forming thread teeth through plastic deformation. It adopts a rolling die with the same pitch and tooth profile as the thread to be processed, which squeezes the cylindrical screw blank while rotating it, and finally transfers the tooth profile on the rolling die to the screw blank to form a thread. The common point of thread rolling (rubbing) is not to have too many rolling revolutions. If there are too many, the efficiency is low, and the thread surface is easy to separate or twist. On the other hand, if the number of revolutions is too small, the thread diameter is easy to be out of round, and the initial pressure of rolling will increase abnormally, thus shortening the life of the die. Common defects of thread rolling: cracks or scratches on the thread surface; Arbitrary deduction; The threaded part is not round. If these defects appear in large quantities, they will be found in the processing stage. If the quantity is small, these defects will spread to users if they are not noticed in the production process, causing trouble. Therefore, the key problems of process conditions should be summarized and these key factors should be controlled in the production process.

After heat treatment, the high-strength fasteners shall be quenched and tempered according to the technical requirements. The purpose of heat treatment and tempering is to improve the comprehensive mechanical properties of fasteners to meet the specified tensile strength and yield ratio of products. Heat treatment process has a vital influence on high strength fasteners, especially their internal quality. Therefore, in order to produce high-quality and high-strength fasteners, advanced heat treatment technology and equipment are necessary. Due to the high production capacity and low price of high-strength bolts, and the relatively fine and precise thread structure, the heat treatment equipment is required to have high production capacity, high degree of automation and good heat treatment quality. Since 1990s, the continuous heat treatment production line with protective atmosphere has dominated, and the vibrating bottom mesh belt furnace is especially suitable for heat treatment and tempering of small and medium-sized fasteners. The quenching and tempering line not only has good furnace sealing performance, but also has advanced computer control of atmosphere, temperature and process parameters, equipment fault alarm and display functions. High-strength fasteners are automatically controlled from feeding, cleaning, heating, quenching, cleaning, tempering and coloring to off-line, effectively ensuring the quality of heat treatment. Decarburization of thread will lead to the fastener tripping when it fails to meet the mechanical performance requirements, which will make the threaded fastener invalid and shorten its service life. Due to the decarbonization of raw materials, if the annealing is improper, the decarbonization layer of raw materials will deepen. In the process of quenching and tempering heat treatment, some oxidizing gases usually enter from outside the furnace. The rust of bar steel wire or the residue on the surface of wire rod steel wire after cold drawing will also decompose after heating in the furnace and react to generate some oxidizing gases. For example, the surface rust of steel wire is composed of iron carbonate and iron hydroxide, which will be decomposed into CO2 and H2O after heating, thus aggravating decarbonization. The results show that the decarbonization degree of medium carbon alloy steel is more serious than that of carbon steel, and the fastest decarbonization temperature is between 700-800 degrees Celsius. Because the attachments on the surface of steel wire decompose rapidly to synthesize CO2 and H2O under certain conditions, if the furnace gas of continuous mesh belt furnace is not properly controlled, the decarbonization of screw will be out of tolerance. When high strength fasteners are cold heading, raw materials and annealed decarburization layer not only still exist, but also are squeezed to the top of threads. For the fastener surface that needs quenching, the required hardness cannot be obtained, and the mechanical properties (especially strength and wear resistance) are reduced. In addition, the surface of steel wire is decarburized, and the expansion coefficients of surface and internal tissues are different, so surface cracks may appear during quenching. Therefore, in the process of quenching and heating, it is necessary not only to protect the top of the thread from decarburization, but also to properly carbon coat the fasteners whose raw materials have been decarburized, and adjust the advantage of protective atmosphere in the mesh belt furnace to make it basically equal to the original carbon content of the carbon-coated parts, so that the decarburized fasteners can slowly return to the original carbon content, and the carbon potential should be set at 0.42%-0.48%, and the carbon coating temperature should be the same as the quenching heating temperature, so as to avoid coarse grains and affect the mechanical properties. The possible quality problems of fasteners during quenching and tempering mainly include: insufficient hardness in quenching state; Uneven hardness in quenching state; Quenching deformation is out of tolerance; Quenching cracking. Such problems on site 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.

Finally, to sum up, the technological factors affecting the quality of high-strength fasteners include steel design, spheroidizing annealing, peeling and rust removal, drawing, cold heading, thread processing, heat treatment, etc., and sometimes they are the superposition of many factors.