High strength bolt manufacturing process
First, steel design
In fastener manufacturing, the correct selection of fastener materials is an important part, because the fastener's performance and its materials have a close relationship. If the material selection is improper or incorrect, it may cause the performance to meet the requirements, shorten the service life, even accidents or processing difficulties, and high manufacturing costs. Therefore, the selection of fastener materials is a very important link. Cold upsetting steel is a kind of steel with high interchangeability produced by cold upsetting molding technology. Since it is formed at room temperature using metal plastic processing, the deformation of each part is large and the deformation speed is also high. Therefore, the performance requirements for cold upsetting steel raw materials are very strict. Based on long-term production practices and user use research, Combining the characteristics of GB/T6478-2001 "Technical Conditions for Cold Uptake and Cold Extrusion Steel" GB/T699-1999 "Quality Carbon Structured Steel" and the target JIS G3507-1991 "Carbon Steel Plate for Cold Uptake Steel" with 8.8, 9.8 The material requirements for class bolt screws are examples of the determination of various chemical elements. C content is too high, cold forming performance will be reduced; Too low can not meet the requirements of the mechanical properties of the parts, so it is set to 0.25 <UNK> -0.55 <UNK>. Mn can improve the permeability of steel, but adding too much will strengthen the matrix structure and affect the cold forming performance; There is a tendency to promote the growth of austenite grains when the parts are tempered, so it is properly improved on an international basis and set at 0.45 <UNK> -0.80 <UNK>. Si can strengthen ferrite, promote the reduction of cold forming performance, and the material elongation rate is set to be less than or equal to 0.30 <UNK>. S.P. is an impurity element. Their presence will produce segregation along the grain boundary, resulting in brittle grain boundaries and damaging the mechanical properties of the steel. It should be reduced as much as possible to be less than or equal to 0.030 <UNK> and S less than or equal to 0.035 <UNK>. B. The maximum boron content is 0.005 <UNK>, because although the boron element has a significant effect on the permeability of steel, it also leads to an increase in the brittleness of steel. The boron content is too high and it is very unfavorable to the workpieces such as bolts, screws and nuts that require good comprehensive mechanical properties.
Second, spherical(softening) annealing
The original structure of steel will directly affect the forming ability of cold heading when the inner hexagonal cylindrical head bolt is manufactured by cold heading process. The plastic deformation of the local area during the cold heading process can reach 60 <UNK> -80 <UNK>. 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 the large bulbous pearlite is not conducive to the formation of cold heading, and the small spherical pearlite can significantly improve the plastic deformation ability of steel. For medium carbon steel and medium carbon alloy steel with high strength fasteners, spheroidization(softening) annealing is performed before cold heading to obtain a uniform and meticulous spheroidal pearlite to better meet the actual production needs. For the softening and annealing of the medium carbon steel plate, the heating temperature is often selected to heat up and down the steel critical point, and the heating temperature is generally not too high. Otherwise, three cementite deposits will occur along the crystal boundary, resulting in cold heading cracking. For the plate of Tanhejingang, isothermic annealing is used. After the AC1 +(20-30 %) is heated, the furnace is cold to slightly lower than Ar1, the temperature is about 700 degrees Celsius, and the temperature is equal to some time, and then the furnace is cold to 500 degrees Celsius. The air is cold. The metallographic structure of steel is thinned from sheet to spherical, and the cracking rate of cold heading will be greatly reduced. 3545 ML35 SWRCH35K steel softening annealing temperature general area of 715-735 degrees Celsius; The SCM43540 CrSCR435 steel spherical annealing and heating temperature generally has a temperature of 740-770 degrees Celsius and an isothermal temperature of 680-700 degrees Celsius.
Three, peel the shell to remove scales
The process of removing iron oxide plate from cold heading steel plate is peeling, scaling, mechanical scaling and chemical acid washing. The chemical pickling process of using mechanical scaling to replace the plate not only improves productivity but also reduces environmental pollution. This scale removal process includes bending method(commonly used repeated bending disk strips of round wheels with triangular grooves), spraying nine methods, etc.. The scale removal effect is better, but the residual iron scales can not be removed(the iron oxide removal rate is 97 <UNK>). In particular, when the adhesion of the oxidized iron sheet is very strong, the mechanical scale removal is affected by the thickness of the iron sheet, the structure and the stress state, and is used in carbon steel disc strips for low-strength fasteners(less than or equal to 6.8 levels). The high-strength fastener(greater than or equal to 8.8) uses a disc bar to remove all the oxidized iron sheets after mechanical scaling, and then compound scaling after chemical acid washing. For the low carbon steel strip, the residual iron sheet of mechanical scale removal can easily cause uneven wear of the pull-out die. When the granulated die hole is attached to the iron sheet due to the friction of the coil wire, the surface of the strip wire produces longitudinal grained marks. When the coil wire cold leads to the flange bolt or the cylindrical head screw, the cause of the micro-crack appears on the head. 95 above is caused by scratches on the surface of the wire during drawing. Therefore, mechanical scale removal is not suitable for high-speed pulling.
Pulling process has two purposes, one is to restructure the size of raw materials; The second is to obtain the basic mechanical properties of the fasteners through deformation and reinforcement. For medium carbon steel, Tanhejingang has another purpose, that is, to make the sheet carburetor obtained after cooling of the strip as much as possible during the pulling process. crack. Prepare for subsequent spheroidal(softened) annealing to obtain granular cementite. However, in order to reduce costs, some manufacturers arbitrarily reduce the number of pull-outs, and the excessive reduction rate increases the processing and hardening tendency of the wire, which directly affects the cold heading performance of the wire. If the subtraction rate of each channel is not suitable, it will also cause torsional cracks in the drawing process. This kind of longitudinal distribution of the steel wire, the periodic cracks are exposed during the cold upsetting process of the steel wire. In addition, in the process of drawing, if the lubrication is not good, it can also cause regular horizontal cracks in the cold drawing wire. Disc wire out of the same direction of the tangent of the grain wire mold roll at the same time is different from that of the drawing die, which will cause the wear of the unilateral hole type of the drawing die to intensify, causing the inner hole to lose round, resulting in uneven drawing deformation in the circular direction of the wire. The roundness of the wire is extremely poor. In the process of cold heading, the cross section stress of steel wire is not uniform and affects the passing rate of cold heading. In the process of drawing the wire, the surface quality of the steel wire is deteriorated by the excessive partial reduction rate, but the too low reduction rate is not conducive to the fragmentation of the sheet cementite, and it is difficult to obtain as many granular cementite as possible., that is, the spheroidization rate of the cementite is low, The cold upsetting performance of the steel wire is extremely unfavorable. The rod material and coil wire produced by pulling method are partially controlled in the range of 10 <UNK> -15 <UNK>.
Five, cold forging
In general, the forming of the bolt head adopts cold heading plastic processing. Compared with the cutting process, the metal fiber(metal leaving WIRE) is continuous along the shape of the product and there is no cutting in the middle, thus increasing the product strength, especially the good mechanical performance. The cold heading forming process includes cutting material and forming, single click, double-click cold heading and multi-task cooling heading. An automatic cold upsetting machine is used for stamping, upsetting forging, extrusion and indentation in several molds. The processing characteristics of the original rough used in the automatic cold upsetting machine for single work or multitask positions are determined by the size of the rod material with a length of 5-6 meters or the wire strip weighing 1900-2000KG. That is, the processing process is characterized by the cold heading molding is not the use of a pre-cut single piece of rough, but the use of automatic cold upsetting machine itself by rod material and coil wire cut and upsetting rough(if necessary) rough. Before the extrusion cavity, the blank must be shaped. Through plastic surgery can be obtained in accordance with the process requirements. No plastic work is required before upsetting forging, indentation and positive extrusion. After the rough cut, sent to upsetting rough plastic work. The workposition can improve the quality of the rough, reduce the forming force of the next workposition by 15-17 <UNK>, and can extend the die life. The bolting can be used for multiple indentation. 1. Cutting rough with a semi-enclosed cutting tool. The simplest method is to use a sleeve cutting tool; The angle of the incision should not be greater than 3 degrees; When using an open cutting tool, the oblique angle of the incision can reach 5-7 degrees. 2. Short-sized rough should be able to flip 180 degrees during the transfer from the previous working position to the next working position. This can give full play to the potential of the automatic cold upsetting machine, processing complex fasteners, and improving the accuracy of the parts. 3. Each molding position shall be equipped with a punching outlet device, and the concave die shall be equipped with a sleeve type top material device. 4. The number of molded jobs(excluding cutting jobs) should generally reach 3-4 jobs(more than 5 in special cases). 5. During the effective use period, the main slider guide rail and the structure of the process parts can ensure the positioning accuracy of the punch and concave die. 6. Terminal limit switches must be installed on the baffle that controls the material selection. Attention must be paid to the control of upsetting forging force. The unroundness of the cold dial wire used to manufacture high-strength fasteners on an automatic cold heading machine should be within the diameter tolerance range, while the unroundness of the more precise fasteners should be limited to the 1/2 diameter tolerance range., If the diameter of the wire does not reach the specified size, the head or head of the part will crack or form burrs. If the diameter is less than the size required by the process, the head will be incomplete, and the edges or thickened parts are not clear. The accuracy of cold heading molding is also related to the choice of forming method and the process used. In addition, it also depends on the structural characteristics of the equipment used, the process characteristics and its state, the precision of the die, its life and the degree of wear. The working surface roughness of the cemented carbide dies should not be large Ra = 0.2 um for cold heading molding and extrusion. When the working surface roughness of such dies reaches Ra = 0.025-0 .050 um, it has the highest life span.
Six, thread processing
The bolt threads are generally cold machined so that the threaded blank material within a certain diameter is formed by rubbing(rolling) the wire plate(die) pressure. The plastic streamline that can be obtained from the threaded part is not cut, the strength increases, the accuracy is high, and the quality is uniform, so it is widely used. In order to produce the thread outer diameter of the final product, the thread blank diameter required is different because it is limited by factors such as thread accuracy and whether the material is coated or not. Rolling(rubbing) thread refers to the process of forming a threaded tooth using plastic deformation. It uses a roll(screw) mold with the same pitch and tooth shape as the processed thread, while squeezing the cylindrical screw, while turning the screw, and finally transferring the tooth shape on the roll mold to the screw. On the billet, make the thread form. The common feature of rolling(rubbing) thread processing is that the number of rolling turns does not have to be too much. If there is too much, then the efficiency is low, and the surface of spiral teeth is prone to separation or cluttering. Conversely, if the number of turns is too small, the thread diameter is easy to lose round, and the initial pressure of rolling pressure increases abnormally, resulting in a shortened die life. Common defects of rolling threads: surface cracks or scratches on the threaded parts; Clutching; Thread part is rounded. If these defects occur in large numbers, they will be discovered during the processing phase. If the amount occurs less, the production process will not pay attention to these defects will circulate to the user, causing trouble. Therefore, the key problems of processing conditions should be summarized and these key factors should be controlled in the production process.
Seven, heat treatment.
High-strength fasteners are tempered according to technical requirements. The heat treatment quality adjustment is to improve the overall mechanical properties of the fastener to meet the product's specified tensile strength values and Quqiang ratio. The heat treatment process has a vital influence on the high strength fasteners, especially its internal quality. Therefore, to produce high quality high-strength fasteners, it is necessary to have advanced heat treatment technology equipment. Because the high strength bolts have large production capacity and low price, the threaded parts are relatively subtle and relatively precise structures. Therefore, the heat treatment equipment must have the ability to produce large production capacity, high degree of automation, and good heat treatment quality. Since the beginning of the 1990s, continuous heat treatment production lines with a protective atmosphere have occupied a dominant position. The shock bottom type and the network belt furnace are especially suitable for heat treatment and quality adjustment of small and medium-sized fasteners. In addition to the good sealing performance of the furnace, the conditioning wire also has advanced atmosphere, temperature and process parameters computer control, equipment failure alarm and display function. High-strength fasteners are operated automatically from top to top-cleaning, heating, quenching, cleaning, tempering, and colouring to the bottom line, effectively ensuring the quality of heat treatment. The decarbon of thread will cause the fastener to take off when it does not meet the resistance required for mechanical performance, which will invalidate the thread fastener and shorten its service life. Due to the decarbonization of raw materials, if the annealing is not appropriate, the decarbonization layer of raw materials will be deepened. In the process of tempering heat treatment, some oxidizing gas is generally brought in from the outside of the furnace. The rust of the rod wire or the residue on the surface of the coil wire after cold drawing will also decompose after heating and react to produce some oxidizing gases. For example, the surface rust of the steel wire, which is composed of iron carbonate and hydroxide, will decompose into CO2 and H2O after heating, thereby increasing decarbonization. Studies have shown that the degree of decarbonization of Tanhejingang is more serious than that of carbon steel, and the fastest decarbonization temperature is between 700-800 degrees Celsius. Since the adhesion of the steel wire surface decomposes to synthesize CO2 and H2O at a fast rate under certain conditions, if the gas control of the continuous mesh furnace is improper, screw decarbonization can also be extremely poor. When the high-strength fastener is formed using cold heading, the carbon layer of raw materials and annealing not only still exists, but is also squeezed to the top of the thread, and the required hardness is not obtained for the fastener surface that needs to be quenched. Its mechanical properties(especially strength and wear resistance) are reduced. In addition, the surface of the steel wire decarbonization, the surface and the internal structure are different and have different expansion coefficients, may produce surface cracks during quenching. For this reason, the top of the thread should be protected from decarbonization during quenching, and the fasteners of the raw materials that have been decarbonized should be moderately carbon coated, and the advantages of the protective atmosphere in the belt furnace should be adjusted to the original carbon content of the carbon coated parts. basically equal. The carbon potential is set to 0.42 <UNK> -0.48 <UNK>. The carbon cover temperature is the same as that of quenching and can not be carried out at high temperatures so that the grains are thick and affect mechanical properties. The quality problems of fasteners in quenching process are as follows: insufficient hardness of quenching state; The hardness of quenching state is uneven; Ultra-poor quenching deformation; Quenching cracking. Such problems in the field are often related to raw materials, quenching and quenching cooling. The correct formulation of heat treatment processes and the standardization of production and operation processes can often avoid such quality accidents.
In summary, the process factors that affect the quality of high-strength fasteners include steel design, spheroidal annealing, peeling and scaling, pulling, cold heading forming, thread processing, heat treatment, etc., and sometimes the superposition of various factors. We know that fastener defects are caused by the volatility of product quality characteristics, and only an accurate understanding of the process factors in the product manufacturing process produces a huge driving force for continuous quality improvement. Only through the continuous improvement of quality to obtain more profits and stronger competitiveness!