High strength bolt material selection and product processing technology

Bolt parts are an important standard part. They are used as connecting fasteners. They are widely used in various fields. According to their mechanical and physical properties, they are divided into 10 categories, among which the mechanical performance grade is greater than or equal to 8.8. We usually call it a high-strength bolt. The biggest difference between high-strength bolts and ordinary bolts is that they have high tensile strength, high surface hardness and good mechanical properties. The key lies in the choice of materials and heat treatment. High-strength bolt heat treatment generally refers to quenching and tempering treatment, that is, quenching + high temperature tempering. The hardness value can be used to estimate the approximate tensile strength of the material. You can refer to the relevant manual, check the bolt hardness value at ordinary times, and calculate the corresponding tensile strength value. It is also necessary to perform tensile tests on a regular basis to determine the tensile strength. There are many kinds of metal surface treatments, such as surface oxidation, Dacromet, galvanization and surface phosphating. Surface phosphating or surface oxidation is recommended for high strength bolt surface treatment. The close integration and cooperation between the smelting industry and the automotive industry will not only further promote the research of modern alloy technology, but also provide a material basis for the development of automotive high-strength fasteners and the improvement of competitiveness, which requires full attention from the whole industry.

1Structural characteristics and process analysis of high-strength bolts There are many types of high-strength bolts (see), the shapes are different, and the external dimensions are ever-changing, but the overall structure and overall external shape have certain similarities. Based on these similarities, we divide it into three main parts: the head, the stem, and the threaded portion. High-strength bolt machining generally does not require special machine tools with extremely high precision, and can be processed on ordinary equipment. According to its three main parts, its machining process can be divided into three parts: head machining, rod machining and thread machining. Each part of the processing technology is divided into several types according to its size and shape and technical requirements. Different processing methods are used; although it is divided into three parts, the three parts of processing are complementary and interrelated, and may coexist in the same Processes may also coexist in the same step.

The main function of the head is to apply a reverse moment when the nut is engaged with the bolt to ensure that the nut has sufficient tightening torque. There are many types of forms, mainly square, semi-circular, and hexagonal. In addition, some non-standard parts high-strength bolt head forms are specially designed by the designer according to the assembly needs. The shape of the bolt head directly determines the product blank form. Generally speaking, the square head bolt blank can be selected from cold drawn square steel, the hexagonal head bolt blank can be selected from cold drawn hexagonal steel, and the semicircular head bolt blank should be selected from forging blank; the head shape specially designed bolt should be analyzed according to the specific shape. In order to avoid increasing the head processing process, it is recommended to use the forging blank if the technical requirements permit; the maximum size of the head and the outer diameter of the rod are larger or the overall length is larger, in order to reduce material waste and reduce For processing man-hours, it is recommended to use forging blanks. Blank machining allowance: For steel blanks, the reserved machining allowance is mainly the length direction. Under normal circumstances, the length of the length of 4mm can be reserved, while ensuring the utilization of the blanking, if the length of a single blank is small, consider a blank to make multiple parts. For forging blanks, the head shape is preferably forged directly under the premise of technical requirements, and the inner end of the head is reserved for a margin of 1.5 mm. The head forming process can be completed on a conventional lathe. Some high-strength bolt inner end faces and rod centerline have end face runout and perpendicularity requirements, generally between 0.04~0.10mm. At this time, the head width is generally reserved with a machining allowance of 0.2mm during roughing, and the finishing part is finished. The precision of the outer circle is high, and the machine tool relies on the accuracy of the machine tool to ensure the tolerance of the end face shape of the head. In order to ensure the tensile strength of high-strength bolts, the value of chamfering at the inner end face is generally ±0.2. The main part of the shank is guiding, especially the guide bolt, which bears a certain radial shear force after assembly, and requires small holes. The clearance fit requires strict precision and roughness of the outer circumference of the rod. Some bolts that are only subjected to the axial tensile force after assembly are not very strict with the requirements of the rod, and the outer ring has a large tolerance. For high-strength bolts, the contact between the rod and the head requires a certain rounded corner to avoid breakage of the part when subjected to a large tensile force, and avoid cracks during heat treatment cooling, which is a key factor for processing.

The outer circumference of the rod is reserved for processing with a margin of 1.5mm. For the slender bolt of the rod to avoid large deformation during heat treatment, a margin of 2mm can be reserved, or the blank can be directly tempered to the required hardness, but the hardness is not easy. High, generally below HRC32. Forging blank technical requirements stipulate the surface defect layer, head and rod coaxiality requirements, the specific value depends on the product requirements, the general value is not more than 0.3mm. If there is no special requirement after forging, the forging should be normalized to reduce the hardness , to adapt to subsequent machining. The processing of the rod is mainly the machining of the outer surface, and turning and grinding are the main processing methods. Turning the outer circle, when the outer diameter dimension and surface roughness of the bolt part are not high, the final size and precision of the outer circle can be obtained by turning. Generally, the roughing accuracy can reach IT12~mi, and the surface roughness Ra value is about 50~ 12.5pm, generally adopts large depth of cut, large feed rate and low cutting speed; semi-finishing precision can reach IT10~IT9, surface roughness Ra is about 6.3~3.2pm, depth of cut and advance The amount is smaller than the rougher car. When turning the outer circle, the head of the bolt is the clamping part, and the width of the head is small. The other end face is required to assist the clamping position with the center hole. This requires turning the center hole of the front end face of the outer circular surface, depending on the size of the bolt and the type of material. When the surface dimensional accuracy and surface roughness of the outer circle are required to be high, other processes need to be added after the outer circle of the car, mainly referring to grinding, and the balance of 0.2~0.45mm is reserved bilaterally. The diameter of the rod is relatively large or requires multiple grinding. For processing, the remaining amount takes a large value.

The threaded portion is the main part of the bolt. It can be divided into effective thread part, finishing part (retracting part) and thread end three parts; three main elements of thread: pitch, tooth half angle and pitch, directly affect the thread matching precision, is also the key factor of processing. Grinding the outer circle generally adopts centerless cylindrical grinding, which has high production efficiency and simple and convenient operation. However, it is more troublesome to adjust the machine tool. Grinding of the grinding wheel also requires a certain level of technology, especially the bolt with the jumping and vertical requirements of the inner end surface of the head. The shape tolerance is guaranteed by the precision of the grinding wheel, and the grinding wheel must be strictly trimmed. There are many thread processing methods, such as turning, milling, grinding and rolling. For high-strength bolts, rolling thread is the best choice. Rolling thread is a chipless machining process. The thread is plastically deformed on the surface of the blank. This processing technology has high productivity, the precision can reach 4h, the surface roughness can reach Ra0.2pm, and the workpiece material fiber is rolled. Not only has it not been cut, but it has been further strengthened. The rolling thread can also improve the fatigue strength due to work hardening and low surface roughness. Since the thread is extruded, the diameter of the rolled thread is smaller than the diameter of the blank of the cutting thread. Therefore, it can save 16%~25%. However, the rolling thread has higher requirements on the precision of the blank diameter. The machining of the blank diameter can be processed by a grinding machine or a general lathe. A certain amount of retracting space should be reserved at the end of the thread end, about 2~3mm. 2 The high-strength bolt steel is used in the manufacture of fasteners. The correct selection of fastener materials is an important part because of the performance of the fasteners and its The material has a close relationship. 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 cold heading and cold extrusion steel technical conditions GB/T699-1999 high quality carbon structural steel and target ISG3507-1991 cold heading steel carbon steel The characteristics of the wire rod are determined by the material requirements of the 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%. The microalloyed steel is to change the steel by adding a small amount of certain elements in the steel. Performance to improve the working strength of the fastener. Because each element has its own characteristics. These elements can be used separately or in combination depending on the content of other elements in the steel, the production process and the requirements for use. For a long time, 8.8 grade bolts are usually made of ML35 steel after quenching and tempering. There are certain problems in production and use, such as cold cracking, easy quenching and decarburization during heat treatment. In the process of assembly and disassembly and service, there are slip buckles, deformation (elongation, necking, bending), fracture, Hexagon head rounding and other issues. CH35ACR cold heading steel is a larger bolt material that replaces ML35 steel to make body M14. Compared with ML35 steel, CH35 steel has different Si and Mn contents. The former adds Ci element and reduces P and S content. The quenching critical diameter of cooling in oil is increased by 18-20 mm, and the hardness difference after the same high temperature tempering is relatively large, and the anti-tempering stability is strong. In recent years, the cold heading alloying technology of steel has been greatly developed. On the one hand, alloying has been greatly developed. On the other hand, a small amount of <0.1% of carbide elements, such as Ti, V, Ci, and low carbon are added. In C-Mn steel.

The use of non-tempered steel to manufacture bolts can omit the spheroidizing annealing before bolt cold drawing and the quenching and tempering after bolt forming, and can also reduce the decarburization tendency of the thread cusps and improve the bolt yield, and the economic effect is very obvious. Since the hardness of the non-tempered steel wire during cold working is higher than usual, the life of the processing die is reduced. Therefore, the bolts made of non-tempered steel are mainly grade 8.8, and the 10.9 grade studs with less processing capacity can also be made of non-quality steel, and the amount is gradually expanding. Currently used non-tempered steel, the structure is low carbon and manganese ferrite + pearlite type and bainite type. In terms of production, the use of refining outside the furnace to reduce inclusions and control the composition in a narrow range, by controlling rolling and controlling cooling, refine the structure to improve toughness and precipitation strengthening. ML08Mn2Si duplex cold heading steel is developed for the replacement of ML35 steel. It has high strength and excellent cold forming performance, which is mainly manifested in improving the coldness of steel and increasing the utilization rate of steel. It achieves the strength requirement of the bolt by cold drawing deformation strengthening, thus simplifying the hot processing process of the fastener, saving energy consumption, and having high economic value and social benefit.

Generally, the strength and plasticity of medium carbon (alloy) structural steel after a conventional heat treatment is a contradiction between each other. In order to pursue high plasticity and toughness, measures such as quenching and high temperature tempering (tempering and conditioning) are inevitable. Strength, if you want to maintain high strength level, use quenching, low temperature tempering, and it appears that plasticity and toughness are insufficient. The microstructure of the low carbon (alloy) structural steel after quenching is low carbon martensite + lath phase boundary retained austenite and fine and dispersed carbide during inclusion. This structure is intergranular dislocation structure with high Strength (hardness 45~50HRC), yield strength 1000~1300MPa, good plasticity body 10%, Z body 40% and toughness Ah body 59, as well as good cold workability, weldability and heat treatment distortion, etc. Research and development of low carbon martensitic structural steel has important theoretical and practical significance. Important bolts on automobiles, such as connecting rod bolts, cylinder head bolts, and semi-axle bolts, used to be made of 40Cr or 35CrM steel. Due to their poor cold heading performance, they often produce a large amount of waste due to cold cracking and U-turn. An accident occurred due to an indeterminate quality hazard of the bolt.

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 the basis of the national standard. It is 0.45%~0.80%. Si can strengthen ferrite, which promotes the cold forming performance, and the material elongation decreases to Si less than or equal to 0.30% S. P is an impurity element, and their existence will cause segregation along the grain boundary, resulting in crystal Boundary embrittlement, damage to the mechanical properties of steel, should be reduced as much as possible, P is less than or equal to 0.030%, S is less than or equal to 0.035% B, the maximum content is 0.005%, because boron has a significant effect on the permeability of steel, But at the same time it will lead to increased brittleness of the steel. Excessive boron content is highly detrimental to bolts, screws and studs that require good overall mechanical properties. The development of Mn-VB cold heading steel can guarantee the complete martensite structure below M20. The quenched state and 200 tempering state are typical dislocation lath martensite, which can be used to replace 40Cr quenching and tempering treatment. Has excellent comprehensive mechanical properties. It has both high strength and good toughness and low cold-brittle transition temperature.

Connecting rod bolts, cylinder head bolts, 20Mn-VB steel production CA488 engine bolt static strength is increased by about 35% compared with 40Cr bolts, which increases the bolt load capacity by 45%~70%. Existing low carbon (alloy) steel, For example, the 8.8-grade and 9.8-level riveted bolts used for the front and rear blocks of the automobile, and the SWRCH22A or 20Mn for the internal three-point welding are quenched by different quenching media to obtain low-carbon martensite, which can meet the service requirements of the bolts. In addition, the use of low-carbon (alloy) steel cold drawn cold 镦 is not easy to crack, cold drawing die, cold boring die, crepe plate, rolling wheel, etc. are not easily damaged, so that the process performance of the bolt can be significantly improved. The development of low-carbon martensitic steels has added optional new steel grades for fastener manufacturing. It is characterized by good toughness matching and basically avoids the defects easily caused by the bolts made of medium carbon steel.

The quenching and tempering hardness of multi-steel species is obviously improved. Therefore, in the aspect of automobile fasteners, boron is used to replace precious rare elements, and good economic benefits can be obtained. In the case of boron alone, there is no obvious influence on the cold working of steel, but it is obvious in heat treatment. Carbon boron steel has the characteristics of high toughness, strong plasticity, low tempering temperature and high strength, and is very suitable for producing high-strength fastening. Pieces. In order to further improve the cold workability and omit the spheroidizing annealing treatment, a low-cost low-medium carbon high-strength boron steel was developed. The basic principle of its composition design is to reduce the carbon content, improve the cold deformation ability of steel, add a trace of boron to compensate for the loss of strength and hardenability caused by carbon reduction. In addition, an appropriate amount of alloying elements such as Cr and Mn may be added as needed to further improve the hardenability. Since a small amount of boron replaces a large amount of alloying elements, the cost of steel is lowered, the content of carbon and alloying alloy is low, and the cold working property is good, and the rolled material can be directly drawn and cold-rolled, and no prior spheroidizing annealing treatment is required, thereby saving the manufacturing cost of the bolt. Usually 8.8 grade bolts use 40B, 40MnB, while grade 9.8 and 10.9 grade bolts use MnB123H steel. The steel is developed by Kobe Steel Co., Ltd. in Japan. In order to obtain a strength of more than 1000 MPa when tempering at temperature above 425, the carbon content is controlled at about 0.25%, while controlling the content of Mn and B, and reducing the content of impurity elements P and S. Therefore, the delayed fracture resistance in the strength range of 900 to 1100 MPa is equivalent to or better than that of SCM435 steel. It is common for Japan to produce high-strength bolts using low-carbon boron steel. Carbon-boron steel has been widely used in high-strength bolts such as automobiles and tractors. The use of boron can reduce the use of other alloying elements. In the past few years, the excellent properties of carbon-boron steel have not attracted people's attention in some aspects, which is caused by some inappropriate production links and other misuse. The production of carbon boron steel needs to be strictly controlled in the steel smelting process for the production of the fastener industry.

3 The process of cold head forming and thread processing of high-strength threaded fasteners for the production of high-strength threaded fasteners is the processing of raw materials, refrigeration, forming, thread processing (rolling or twisting), heat treatment, surface treatment sorting packaging, grade 10.9 or higher. Generally, a rolling process after heat treatment is employed. The quality of threaded fasteners In addition to materials, forming equipment and threading equipment and molds (production processes and equipment) are key factors in ensuring their quality. Especially in the large-volume and multi-variety supply state, how to ensure the consistency of the products and the prevention of defects are one of the problems faced by fastener production. Common defects include dimensional and geometric tolerances, head folding, thread flow breaks, tooth wrinkles and cracks. At present, domestic fastener factories are limited to funds or other reasons. Domestically produced equipment and Taiwan equipment are used to produce automotive fasteners. To ensure the dimensional tolerance and shape tolerance of large-scale production of high-end fastener products, it should be increased. Online monitoring means and mold making level. Eliminate the non-conforming products in production, thus ensuring the assembly quality of the OEM and the OEM.

Generally, the forming of the bolt head is performed by cold head plastic working. Compared with the cutting process, the metal fiber is continuous along the shape of the product without cutting in the middle, thereby improving the product strength, especially the mechanical properties. The cold heading forming process includes cutting and forming, single-station click, double-click 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 material with a material length of 5~6m or the wire rod weight of 1900~2000kg, that is, the processing technology is characterized by Cold heading is not a pre-cut single piece of blank, but an automatic cold heading machine that itself cuts and upsets (if necessary) from the bars and wire rods. 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% to 17%, and prolong the life of the mold.

The simplest method of cutting a blank with a semi-closed cutting tool is to use a sleeve-type cutting tool; the angle of the slit should not be greater than 3°; and when using an open-cut tool, the angle of the slit can be 5-7 °. The short-size blank should be able to be turned 180° during the transfer from the previous station to the next forming station, which can realize the potential of the automatic cold heading machine, process complex fasteners and improve the precision of the parts. A punch returning device should be installed at each forming station, and the die should have a sleeve type topping device. The number of forming stations should generally reach 3 to 4 stations. During the effective use period, the structure of the main slider rail and the process components can ensure the positioning accuracy of the punch and the die. The end 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-strip wire used in the manufacture of high-strength fasteners on automatic cold-twisting machines should be within the diameter tolerances. For more precise fasteners, the out-of-roundness of the steel wire should be limited to 1/2 diameter tolerance. If the wire diameter does not reach the specified size, cracks or burrs may form on the upset or head of the part. If the diameter is smaller than the size required by the process, the head will be incomplete and the edges or bulges will be unclear. The precision that can be achieved by cold heading is also related to the choice of molding method and the process used. In addition, it depends on the structural characteristics, process characteristics and state of the equipment used, tooling accuracy, life and wear.

The wire rod 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 the deformation strengthening effect, and the purpose of the medium carbon steel is to control the wire rod. The flaky cementite obtained afterwards is cracked as much as possible during the drawing process, and is prepared for the subsequent spheroidization (softening) annealing to obtain granular cementite. If the lubrication is not good during the drawing process, the cold-drawn wire rod wire may have a transverse crack regularly. The tangential direction of the coiled wire of the wire rod is not the same as that of the drawing die, which will cause the wear of the single-sided 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, so that the steel wire The roundness of the roundness 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 the cold head. 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%c.

The 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), and the plastic flow line of the threaded portion can be obtained without being cut and strength. Increased, high-precision, uniform quality products are 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 (搓) 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 (搓) 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.

4 heat treatment heat treatment of high-strength threaded fasteners is to improve the overall mechanical properties of the fasteners, to meet the tensile strength and yield ratio specified by the product. The quenching and tempering heat treatment process has strict requirements on raw materials, furnace temperature control, furnace atmosphere control, and quenching medium. The main control defects are carbon segregation of the core of the material, surface decarburization during material and annealing, cold cracking, quenching cracking and deformation in quenching and tempering. 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 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. About 80% of China's fastener enterprises have heat treatment equipment, and most of them use Taiwan's heat treatment process line; the process line equipment is a continuous belt belt furnace with atmosphere protection, and the atmosphere, temperature and process parameters are controlled by computer. The problems are that the quenching medium lacks the cooling performance measurement, the carbon potential control is unstable, and the furnace temperature check cycle is too long, which may cause heat treatment defects.

Since the 1990s, continuous heat treatment production lines with protective atmosphere have dominated. The shock-bottom and mesh belt furnaces are 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 computer control of atmosphere, temperature and process parameters, equipment fault alarm and display function. 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 iron pound of the bar steel wire or the residue on the surface of the wire rod after cold drawing, the human furnace will also decompose after heating, and the reaction generates some oxidizing gas. For example, the surface of the steel wire is iron pound, its composition is iron carbonate and hydroxide, which will decompose into C2 and H2O after heating, thereby 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 and 800. Since the adhesion of the surface of the steel wire to the synthesis of CO2 and H2O under certain conditions is fast, if the furnace gas of the continuous mesh belt furnace is improperly controlled, the decarburization of the screw may 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 essentially equal, allowing the decarburized fasteners to slowly return to their original carbon content.

The carbon potential is set at 0.42%~0.48%, and the carbon coating temperature is the same as the quenching heating. It cannot be carried out at high temperature to avoid coarse grains and affect mechanical properties. During the quenching and quenching process of fasteners, the main quality problems may be: 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.

The results of static tensile, skewed tensile, impact toughness, fatigue strength and delayed fracture strength of low carbon martensitic steel and medium carbon quenched and tempered steel show that low carbon Markov is compared with medium carbon quenched and tempered steel. The strength of the body steel is increased by more than 1/3, while maintaining high plasticity and toughness, the bearing capacity of the bolt is increased by 45% c~70%c, and the sensitivity of the notch deflection is not significantly increased; the fatigue strength of the bolt and the medium Carbon steel tempered bolts are generally the same; low carbon martensitic steels have less delayed fracture sensitivity than 40Cr steels of the same strength level and are insensitive to delayed fracture in brine and water. Therefore, the use of low-carbon martensitic steel as a high-strength bolt material not only has many advantages in the direction of comprehensive mechanical properties, but also its excellent process performance is incomparable to medium carbon steel.

When the countersunk head screw and the hexagon socket head bolt are produced by the cold heading process, the original structure of the steel directly affects the forming ability during cold heading. The plastic deformation of the local area during cold heading can reach 60%~80%. For this reason, the steel must have good plasticity. 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 finely 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 adopts isothermal spheroidizing annealing. After heating by ACi+ (20%~30%), the furnace is cooled to slightly lower than An, the temperature is about 700 isothermal for a period of time, and then the furnace is cooled to about 500 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 5/45/ML35/SWRCH35K steel is generally 740~770 for the heating temperature. The process of removing the iron oxide from the isothermal temperature cold-rolled steel wire rod is stripping, descaling, mechanical descaling and chemical pickling. Ways. The chemical pickling process that replaces the wire rod by mechanical descaling improves productivity and 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), shot peening, 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. 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.

5 Future trends in high-strength bolt product processing technology With the development of the automobile, motorcycle and machinery industries, more and more demands are placed on various fasteners. For example, the high performance and light weight of automobiles and motorcycles impose new functional requirements on steel, which may not reduce the cost of the parts themselves, but may reduce the total cost of the machine or components. Practice has shown that the cost of adding element materials such as Ni, Cr, Mo, etc. is bound to increase, but the resistance to delayed fracture of bolts is also improved. Compared with the original, since the bolt diameter is reduced, the bolt mounting hole is correspondingly reduced, and the size of the member to be fastened is also correspondingly reduced, thereby achieving the overall cost reduction. According to the World Metallurgical Report, Beijing Iron and Steel Research Institute successfully developed on the basis of 42CrM. steel by reducing the content of S, P, Si and Mn, adding trace alloying elements V and Nb, and increasing Mo content. A high-strength bolt steel of 1300MPa grade, 42CrMoVNbAPFl, has a comprehensive improvement in its comprehensive mechanical properties. Most of the high-strength bolt steels are medium-carbon steel and medium-carbon alloy steel, which are used in quenching and quenching high-temperature tempering. Therefore, steel for mechanical manufacturing, alloy structural steel and quenched and tempered steel are often called together. At present, the annual output of such steel in China has exceeded 15 million tons, and only medium carbon steel is about 10 million tons/year. After quenching and tempering, the microstructure is tempered martensite carbide. By super-refining the austenite before quenching, it is proved that the mechanical properties can be improved. Compared with the traditional fine-grained heat treatment austenite grain size of 42CrM. steel, it is about 8~30m, which is about 20~30m. When the refinement is below 10pm, all the mechanical properties, plasticity and toughness are obviously improved. .

The design and manufacture of the blank not only affects the manufacturing cost of the blank, but also affects the economic and environmental impact of the subsequent processes. In combination, as far as possible, the use of high-precision processing such as precision forging can greatly reduce the amount of machining and make full use of resources; combined with specific production conditions, consider the possibility of external cooperation, and realize the specialized production of blanks, so that Adopt new processes and new technologies in the mass production mode.

The same is true for the blanks of high-strength bolts. With the development of forging technology, many rough blanks now use the advanced precision forging technology to produce blanks that can directly forge the shape of high-strength bolts and reduce the length of the axles. Resources and energy consumption; with the development of future forging technology, the blank of high-strength bolts will be further developed, the diameter of the thread and the diameter of the rod can be forged, and the processing can be simplified to the processing and heat treatment of the thread, which will greatly Reduce processing costs and reduce resource and energy consumption.

Cutting fluids are often used in modern cutting operations, but the cutting fluids used today often contain toxic substances, which are harmful to the environment, increase the environmental burden, and are not conducive to the health of workers. Based on this, in the process of high-strength bolt processing, the future possible method is dry cutting. Dry cutting is an effective way to eliminate cutting fluid contamination and achieve clean production, but dry cutting relies to a large extent on the development and application of new tools.

Ceramic knives are very suitable for dry cutting due to their high heat resistance and good chemical stability. However, the inherent physical properties of ceramic materials such as high brittleness and poor strength and toughness limit the application of ceramic tools in dry cutting. In order to solve this problem, new ceramic material cutters that reduce the size of ceramic grains and improve the purity of materials are generally used; the design of reasonable nano-coating can significantly increase the hardness and toughness of the tool, making it excellent in wear resistance and self-resistance. Lubrication performance.

6 Conclusion Common bolt parts, including high-strength bolts, are relatively simple to process and have a variety of processing methods. Different products and different manufacturing companies and different operators adapt to different processing techniques.而影响高强度紧固件品质的工艺因素有钢材设计，球化退火，剥壳除鳞，拉拨，冷镦成形，螺纹加工，热处理等方面，有时则是诸种因素的叠加。众所周知，紧固件缺陷正是由于产品质量特征的波动性造成的，只有对产品制造流程中的工艺因素准确了解，由此产生持续改进品质的巨大原动力，才能通过质量的不断提升获得更多的利润和更强的竞争力。高强度螺栓材料的性能与热处理有着相互依存的关系，材料的性能取决于材料的内部组织结构，而内部组织结构又随钢材牌号和热处理及其他加工工艺的不同而变化。根据我国国情和企业实际工艺水平，应该向国外企业学习，通过应用和研究不断优化筛选，使高强度螺栓的选材集中到少数几个钢种。这样不但有利于提高原材料质量，还能使紧固件制造的各工艺过程易于控制。这样，在紧固件出口逐年增加的同时，力争在品种和质量上达到世界先进水平。

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