Heat treatment method of the hottest bolt

Heat treatment method of bolts

bolt processing technology: hot rolled wire rod - (cold drawing) - spheroidizing (softening) annealing - mechanical descaling - acid pickling - cold drawing - cold forging forming - thread processing - heat treatment - Inspection I. steel design in fastener manufacturing, the correct selection of fastener materials is an important part, because the properties of fasteners are closely related to their materials. If the material is selected improperly or incorrectly, it may cause the performance to fail to meet the requirements, shorten the service life, or even cause accidents or processing difficulties, and high manufacturing costs. Therefore, 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 it is formed by metal plastic processing at room temperature, the deformation of each part is large and the deformation speed is high. Therefore, the performance requirements of cold heading steel raw materials are very strict. On the basis of long-term production practice and user use research, combined with the characteristics of gb/t "technical conditions of steel for cold heading and cold extrusion", gb/t "high quality carbon structural steel" and JISG "carbon steel wire rod for cold heading steel", taking the material requirements of grade 8.8 and 9.8 bolts and screws as examples, the determination of various chemical elements. If the content of C is too high, the cold formability will be reduced; If it is too low, it cannot meet the requirements of mechanical properties of parts, so it is set as 0.25% - 0.55%. Mn can improve the permeability of steel, but excessive addition will strengthen the matrix structure and affect the cold formability; During the quenching and tempering of parts, there is a tendency to promote the growth of austenite grains, so it is appropriately increased on the international basis, which is set as 0.45% - 0.80%. Si can strengthen ferrite and reduce cold formability. The reduction of material elongation is determined as si less than or equal to 0.30%. S. P. is an impurity element, whose existence will produce segregation along the grain boundary, leading to grain boundary embrittlement and damaging the mechanical properties of the steel. It should be reduced as much as possible, with p less than or equal to 0.030% and s less than or equal to 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 brittleness of steel, and then the zero clearing of torque value in the software will increase. The boron content is too high, which is very disadvantageous to bolts, screws and studs, which need good comprehensive mechanical properties

second, when spheroidizing (softening) annealing countersunk head screws and hexagon socket head bolts are produced by cold heading process, the original structure of steel will directly affect the forming ability during cold heading. After cold heading, the plastic deformation of local area in one-way can reach 60% - 80%. Therefore, the steel must have good plasticity. When the chemical composition of steel is certain, the metallographic structure is the key factor to determine the plasticity. It is generally believed that coarse large pearlite is not conducive to cold heading. If there is no specified speed for forming, while fine spherical pearlite can significantly improve the plastic deformation ability of steel. For medium carbon steel and medium carbon alloy steel with a large amount of high-strength fasteners, spheroidizing (softening) annealing is carried out 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 is usually selected to keep the temperature above and below the critical point of the steel. The heating temperature is generally not too high, otherwise three times of cementite will precipitate along the grain boundary, resulting in cold heading cracking. For medium carbon alloy steel wire rods, isothermal spheroidization annealing is adopted. After ac1+ (%) heating, the furnace is cooled to slightly lower than AR1, and the temperature is about 700 ℃ isothermal for a period of time, and then the furnace is cooled to about 500 ℃ and out of the furnace for air cooling. The metallographic structure of steel changes from coarse to fine, from flake to spherical, and the cracking rate of cold heading will be greatly reduced. The softening annealing temperature of 3545ml35swrch35k steel is generally Celsius

third, shelling and descaling cold heading steel wire rod to remove iron oxide plate. The process is peeling and descaling. There are two methods: mechanical descaling and chemical pickling. The chemical pickling process of replacing wire rods with mechanical descaling not only improves productivity, but also reduces environmental pollution. This descaling process includes bending method (round wheel with triangular groove is commonly used to repeatedly bend wire rods), spray nine method, etc. the descaling effect is good, but the residual iron scale cannot be removed (the iron oxide scale removal rate is 97%), especially when the iron oxide scale has strong adhesion. Therefore, mechanical descaling is affected by the thickness, structure and stress state of the iron scale, and is used for carbon steel wire rods for low-strength fasteners (less than or equal to grade 6.8). After mechanical descaling with wire rods for high-strength fasteners (grade 8.8 or above), in order to remove all iron oxide scales, the chemical pickling process is followed, that is, composite descaling. For low carbon steel wire rod, the residual iron sheet of mechanical descaling is easy to cause uneven wear of particle draft. When the grain drawing hole is adhered to the iron sheet due to the friction of the wire rod against the external temperature, the longitudinal grain mark is produced on the surface of the wire rod. When the wire rod cold upsets the flange bolt or cylindrical head screw, more than 95% of the reasons for the micro crack on the head are caused by the scratch on the wire surface during the drawing process. Therefore, mechanical descaling is not suitable for high-speed drawing

fourth, the drawing process has two purposes, one is to reform the size of raw materials; The second is to obtain basic mechanical properties of fasteners through deformation strengthening. For medium carbon steel and medium carbon alloy steel, there is another purpose, that is, to make the sheet cementite obtained after controlled cooling of wire rod crack as much as possible in the drawing process, so as to prepare for the subsequent spheroidization (softening) annealing to obtain granular cementite. However, some manufacturers arbitrarily reduce the drawing passes in order to reduce costs, Excessive area reduction increases the work hardening tendency of wire rod and directly affects the cold heading performance of wire rod. If the distribution of the reduction ratio of each pass is not appropriate, the wire rod will also produce torsional cracks during the drawing process. Such cracks distributed longitudinally along the wire and with a certain period will be exposed during the cold heading process of the wire. In addition, if the lubrication is not good during the drawing process, it can also cause the cold drawn wire rod to have transverse cracks regularly. The tangent direction of the wire drawing die at the same time when the wire drawing die is rolled up is not concentric with the drawing die, which will aggravate the wear of the unilateral hole of the drawing die, cause the inner hole to be out of round, cause the drawing deformation of the wire in the circumferential direction to be uneven, and make the roundness of the steel wire out of tolerance. In the process of cold heading, the stress on the cross-section of the steel wire is uneven, which will affect the qualification rate of cold heading. During the drawing process of wire rod, the excessive partial area reduction rate deteriorates the surface quality of the steel wire, while the low area reduction rate is not conducive to the crushing of sheet 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 detrimental to the cold heading performance of the steel wire. For bar stock and wire rod produced by drawing, the partial area reduction rate is controlled within the range of 10% - 15%

v. cold forging forming generally, the forming of bolt head adopts cold heading plastic processing. Compared with cutting processing, metal fibers (metal wires) are continuous along the shape of the product without cutting in the middle, which improves the strength of the product, especially excellent mechanical properties. The cold heading forming process includes cutting and forming, single station click, 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 diameter reduction in several forming dies. The processing characteristics of the original blank used by the single station or multi station automatic cold heading machine are determined by the size of the bar material with a length of meters or the wire rod with a weight of kg, that is, the characteristic of the processing technology is that the cold heading forming is not a pre cut single blank, but a blank (if necessary) cut and upset by the bar material and wire rod by the automatic cold heading machine itself. Before extruding the cavity, the blank must be reshaped. The blanks that meet the process requirements can be obtained by reshaping. The blank does not need to be reshaped before upsetting, reducing and forward extrusion. After cutting, the blank 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%, and prolong the service life of the die. Multiple reduction can be used to manufacture bolts. 1. The simplest way to cut blanks with semi closed cutting tools is to use sleeve cutting tools; The angle of the incision should not be greater than 3 degrees; When the open cutting tool is used, the oblique angle of the incision can reach. 2. During the transfer process from the previous station to the next forming station, the short size blank should be able to turn 180 degrees, which can give full play to the potential of the automatic cold heading machine, process Fasteners with complex structure, and improve the accuracy of parts. 3. Each forming station should be equipped with a punch return device, and the female die should be equipped with a sleeve type jacking device. 4. The number of forming stations (excluding cutting stations) should generally reach to stations (more than 5 in special cases) to lay a foundation for the healthy development of the next step. 5. Within the valid service life, the structure of the main slider guide rail and process parts can ensure the positioning accuracy of the punch and die. 6. Terminal limit switches must be installed on the baffle that controls material selection, and attention must be paid to the control of upsetting force. The out of roundness of the cold drawn wire rod used in the manufacture of high-strength fasteners on the automatic cold heading machine should be within the diameter tolerance, while the out of roundness of the wire of the more precise fasteners should be limited to 1/2 of the diameter tolerance. If the wire diameter does not reach the specified size, the upsetting part or head of the part will have cracks or form burrs. If the diameter is less than the size required by the process, the head will be incomplete, Edges and corners or thickened parts are not clear. The accuracy of cold heading is also related to the choice of forming method and the process used. In addition, it also depends on the structural characteristics, process characteristics and status of the equipment used, the accuracy, service life and wear degree of tools and dies. For high alloy steel used for cold heading and extrusion, the working surface roughness of cemented carbide dies should not be greater than ra=0.2um, and the working surface roughness of such dies should reach ra=0 050um, with the highest service life

VI. thread processing bolt threads are generally cold processed, so that the thread blank within a certain diameter passes through the thread rolling plate (die), and the thread is formed by the pressure of the thread rolling plate (die). It is widely used because it can obtain products with high precision and uniform quality, in which the plastic streamline of the threaded part is not cut off, and the strength is increased. In order to produce 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 the accuracy of the thread, whether the material has a coating, etc. Rolling (rubbing) thread refers to the processing method of forming thread teeth by plastic deformation. It uses a rolling (thread rolling plate) die with the same pitch and tooth shape as the processed thread. While extruding the cylindrical screw blank, it rotates the screw blank, and finally transfers the tooth shape on the rolling die to the screw blank to form the thread. The common point of thread rolling (rubbing) and pressing processing is that the number of rolling revolutions does not need to be too much. If it is too much, the efficiency is low, and the surface of thread teeth is prone to separation or disorderly threading. On the contrary, if the number of revolutions is too small, the thread diameter is easy to be out of round, and the pressure increases abnormally at the initial stage of rolling, resulting in the shortening of the service life of the die. Common defects of rolled threads: cracks or scratches on the surface of thread parts; Disorderly buckle; The thread is out of round. If these defects occur in large numbers, they will be found in the processing stage. If the number is small, these defects will flow to users and cause trouble if they are not noticed in the production process. Therefore, we should summarize the key problems of processing conditions and control these key factors in the production process

VII. Heat treated high-strength fasteners shall be subject to quenching and tempering treatment according to technical requirements. Heat treatment and tempering is to improve the comprehensive mechanical properties of fasteners to meet the specified tensile strength and yield of products