High-strength Bolts VS Ordinary Bolts, The Strictest Manufacturing Process

Nowadays, people can enjoy the convenience brought by industrial products everywhere, and bolts have become a common part in daily life. We are used to seeing bolts of different sizes and lengths, and we have no sense of novelty about them. I feel that all the bolts are the same, and simply play a role of fixing and connecting.

This is not the case. Although they are all bolts, there are also "conventional forces" and "special forces". "Conventional forces" are the most common ordinary bolts in our daily life, whether it is household appliances, vehicles, or even a small radio, you can see it; and "special forces" are different from ordinary bolts. High-strength bolts are more commonly found in the connection of some steel structure projects, bridges, rails, high-pressure and ultra-high-pressure equipment.

The difference between high-strength bolts and ordinary bolts

High-strength bolts are divided into torsional shear high-strength bolts and large hexagonal high-strength bolts. Large hexagonal high-strength bolts belong to the high-strength level of ordinary screws, while torsional shear-type high-strength bolts are improved types of large hexagonal high-strength bolts. Both of them have similarities with ordinary bolts. The obvious difference is that high-strength bolts can withstand a larger load than ordinary bolts of the same specification.

 1. In terms of raw materials, high-strength bolts are made of high-strength materials. The screw, nut and washer of high-strength bolts are all made of high-strength steel. In China, such as No. 45 steel, 40 boron steel, and 20 manganese-titanium-boron steel. After they are made, they are heat-treated to improve their strength. Ordinary bolts are usually made of Q235 steel.

2. High-strength bolts are widely used. In terms of strength levels, two strength levels of 8.8s and 10.9s are commonly used, of which 10.9 is the majority. Ordinary bolts have lower strength grades, generally 4.4, 4.8, 5.6 and 8.8.

3. From the perspective of force characteristics: high-strength bolts apply pre-tension and transmit external forces by friction. Ordinary bolt connections rely on the shear resistance of the bolt rod and the pressure bearing of the hole wall to transmit shear force. When the nut is tightened, the preload is very small, and its influence can be ignored. In addition to its high material strength, the high-strength bolt also exerts a large The preload produces extrusion force between the connected components, which causes great friction perpendicular to the direction of the screw, and the preload, anti-slip coefficient and steel type all directly affect the bearing capacity of high-strength bolts. According to the force characteristics, it is divided into pressure type and friction type. The calculation methods of the two are different. The minimum size of high-strength bolts is M12, and M16~M30 is commonly used. The performance of oversized bolts is unstable, so they should be used with caution in design.

4. From the point of view of use: the bolt connection of the main components of the building structure is generally connected by high-strength bolts. Ordinary bolts can be reused, but high-strength bolts cannot be reused. High-strength bolts are generally used for permanent connections.

High-strength bolt connection has the advantages of simple construction, good mechanical performance, fatigue resistance, and no loosening under dynamic load, and is a promising connection method. However, due to the important role of high-strength bolts in it, relevant inspections must be done when using them.

The difference between high strength bolt friction type and pressure type connection

The high-strength bolt connection is to clamp the plates of the connecting plate through a large tightening pre-pressure in the bolt rod, which is enough to generate a large friction force, thereby improving the integrity and rigidity of the connection. According to different force requirements, it can be divided into high-strength bolt friction connection and high-strength bolt pressure-bearing connection. The essential difference between the two is that the limit state is different. Both are very different. In the shear design, the high-strength bolt friction connection is the limit state where the external shear force reaches the possible maximum friction force provided by the bolt tightening force between the contact surfaces of the plates, that is, to ensure that the internal and external shear force of the connection does not exceed maximum friction. There will be no relative sliding deformation of the plates (the original gap between the screw and the hole wall is always maintained), and the connected plates are stressed as a whole according to elasticity. In the shear design, the allowable external shear force exceeds the maximum friction force in the high-strength bolt pressure-bearing connection.

At this time, the relative sliding deformation occurs between the connected plates until the bolt rod contacts the hole wall. After that, the connection depends on the bolt rod. The shearing of the shaft, the pressure of the hole wall and the friction between the contact surfaces of the plates jointly transmit the force, and finally the shearing of the shaft or the failure of the pressure of the hole wall is taken as the limit state of the connection shear. In short, friction-type high-strength bolts and pressure-type high-strength bolts are actually the same kind of bolts, but whether the design considers slippage or not. Friction-type high-strength bolts must not slide, and the bolts do not bear shear force. Once they slip, the design considers that they have reached a destructive state, which is relatively mature in technology; pressure-bearing high-strength bolts can slide, and the bolts also bear shear force, and the final failure is equivalent to that of ordinary bolts. Bolt damage (bolt shearing or steel plate crushing).

1. Steel design

In the manufacture of fasteners, the correct selection of fastener materials is an important part, because the performance of fasteners is closely related to its materials. If the material is not selected properly or correctly, the performance may not meet the requirements, the service life will be shortened, and even accidents or processing difficulties will occur, and the manufacturing cost will be high. Therefore, the selection of fastener materials is a very important link. Cold heading steel is a highly interchangeable fastener steel 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 also high.

Therefore, the performance requirements for cold heading steel raw materials are very strict. On the basis of long-term production practice and user use research, combined with GB/T6478-2001 "Technical Conditions of Steel for Cold Heading and Cold Extrusion" GB/T699-1999 "High Quality Carbon Structural Steel" and the target JISG3507-1991 "Cold Heading The characteristics of carbon steel wire rods for steel use, taking the material requirements of grade 8.8 and grade 9.8 bolts and screws as an example, the determination of various chemical elements. If the C content is too high, the cold forming performance will be reduced; if it is too low, the mechanical properties of the parts cannot be met, so it is set at 0.25%-0.55%. Mn can improve the permeability of steel, but adding too much will strengthen the matrix structure and affect the cold forming performance; it tends to promote the growth of austenite grains during quenching and tempering of parts, so it should be increased appropriately on the basis of international standards. 0.45% - 0.80%.

Si can strengthen ferrite and reduce the cold forming performance, and the reduction of material elongation is set to Si less than or equal to 0.30%. S.P. is an impurity element. Their existence will cause segregation along the grain boundary, cause grain boundary embrittlement, and damage the mechanical properties of the steel. It should be reduced as much as possible. B. The maximum value of boron content is 0.005%, because although boron can significantly improve the permeability of steel, it will also increase the brittleness of steel. Excessive boron content is very unfavorable to workpieces such as bolts, screws and studs that require good comprehensive mechanical properties.

2. Spheroidizing annealing

When 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 process. During the cold heading process, the plastic deformation of the local area can reach 60%-80%, so the steel must have good plasticity. When the chemical composition of the steel is constant, the metallographic structure is the key factor that determines the plasticity. It is generally believed that the thick flaky pearlite is not conducive to cold heading, while the fine spherical pearlite can significantly improve the plastic deformation ability of the steel. For medium-carbon steel and medium-carbon alloy steel with a large amount of high-strength bolts, spheroidizing (softening) annealing is performed before cold heading to obtain uniform and fine spheroidized pearlite to better meet actual production needs.

For the softening annealing of medium carbon steel wire rod, the heating temperature is mostly chosen to keep warm above and below the critical point of the steel, and the heating temperature should not be too high, otherwise, three cementites will be precipitated along the grain boundary, resulting in cold heading cracking, while for The wire rod of medium carbon alloy steel adopts isothermal spheroidizing annealing. After heating at AC1+ (20-30%), the furnace is cooled to slightly lower than Ar1, and the temperature is about 700 degrees Celsius for a period of time, and then the furnace is cooled to about 500 degrees Celsius and then air-cooled. The metallographic structure of the steel changes from coarse to fine, from flake to spherical, and the cracking rate of cold heading will be greatly reduced. 35\45\ML35\SWRCH35K steel softening annealing temperature generally ranges from 715 to 735 degrees Celsius; while SCM435\40Cr\SCR435 steel spheroidizing annealing heating temperature generally ranges from 740 to 770 degrees Celsius, and isothermal temperature is 680 to 700 degrees Celsius.

3. Shelling and phosphorus removal

The process of removing iron oxide plate from cold heading steel wire rod is stripping and dephosphorization. There are two methods of mechanical dephosphorization and chemical pickling. Replacing the chemical pickling process of wire rods with mechanical phosphorus removal not only improves productivity, but also reduces environmental pollution. This phosphorus removal process includes bending method (round wheel with triangular grooves is commonly used to bend the wire rod repeatedly), spraying nine methods, etc. The phosphorus removal effect is good, but the residual iron and phosphorus cannot be removed (the oxide scale removal rate is 97% ), especially when the scale adhesion is strong, therefore, mechanical dephosphorization is affected by the thickness, structure and stress state of the scale, and it is used for carbon steel wire rods for low-strength fasteners (less than or equal to 6.8 grades).

High-strength bolts (greater than or equal to grade 8.8) use wire rods to remove all the iron scale after mechanical dephosphorization, and then undergo a chemical pickling process to dephosphorize compositely. For low-carbon steel wire rod, the iron sheet left by mechanical dephosphorization is easy to cause uneven wear of grain draft. When the grain draft hole adheres to the iron sheet due to the friction of the wire rod against the external temperature, longitudinal grain marks appear on the surface of the wire rod. More than 95% are caused by scratches on the surface of the steel wire during the drawing process. Therefore, the mechanical phosphorus removal method is not suitable for high-speed drawing.

4. Pulling

The drawing process has two purposes, one is to change the size of the raw material; the other is to obtain the basic mechanical properties of the fastener through deformation strengthening. For medium carbon steel and medium carbon alloy steel, there is another purpose, that is, to make the wire rod The flaky cementite obtained after controlled cooling is cracked as much as possible during the drawing process to prepare for the subsequent spheroidizing (softening) annealing to obtain granular cementite. However, some manufacturers arbitrarily reduce the drawing process to reduce costs. Passes, too large area reduction rate increases the work hardening tendency of the wire rod, which directly affects the cold heading performance of the wire rod. If the area reduction rate distribution of each pass is not appropriate, torsional cracks will also occur in the wire rod during the drawing process. This kind of cracks distributed along the longitudinal direction of the steel wire and with a certain period of time will be exposed during the cold heading process of the steel wire. In addition, if the lubrication is not good during the drawing process, it can also cause regular transverse cracks in the cold-drawn wire rod.

The tangent direction of the wire rod and the wire drawing die is not concentric with the wire drawing die mouth, which will cause the wear of the unilateral pass of the wire drawing die to intensify, make the inner hole out of round, and cause the drawing deformation of the steel wire in the circumferential direction to be uneven. The roundness of the steel wire is out of tolerance, and the cross-sectional stress of the steel wire is not uniform during the cold heading process, which affects the qualified rate of cold heading. In the drawing process of wire rod, too large area reduction rate will deteriorate the surface quality of steel wire, while too low area reduction rate is not conducive to the crushing of flaky cementite, and it is difficult to obtain as much granular cementite as possible. , That is, the spheroidization rate of cementite is low, which is extremely unfavorable to the cold heading performance of the steel wire. For the bar and wire rod steel wire produced by drawing, the partial surface reduction rate is directly controlled within the range of 10%-15%.

5. Cold forging

Usually, cold heading plastic processing is used for the forming of the bolt head. Compared with cutting processing, the metal fiber (metal wire) is continuous along the product shape without cutting in the middle, thus improving the product strength, especially excellent mechanical properties. The cold heading forming process includes cutting and forming, single-station single-click, double-click cold heading and multi-station automatic cold heading. An automatic cold heading machine performs 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 5-6 meters or the size of the wire rod with a weight of 1900-2000KG, that is, the characteristics of the processing technology. The cold heading is not a pre-cut single-piece blank, but a blank that is cut and upset (when necessary) by the automatic cold heading machine itself from the bar and wire rod. Before the cavity is extruded, the blank must be shaped. A blank that meets the process requirements can be obtained through shaping. Before upsetting, reducing and extruding, the blank does not need to be shaped. After the blank is cut off, it is sent to the upsetting and shaping station.

This station can improve the quality of the blank, can reduce the forming force of the next station by 15-17%, and can prolong the life of the mold, and can use multiple reductions in the manufacture of bolts. The precision that cold heading can achieve 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 characteristics of the process and its state, the precision of the tool, the life and the degree of wear. For high-alloy steel used in cold heading and extrusion, the roughness of the working surface of the cemented carbide mold should not be greater than Ra=0.2um. When the roughness of the working surface of this type of mold reaches Ra=0.025-0.050um, it has the highest service life.

6. Thread processing

Bolt threads are generally cold processed, so that the thread blank within a certain diameter range is rubbed (rolled) through the wire plate (die), and the thread is formed by the pressure of the wire plate (rolling die). The plastic streamline of the threaded part can be obtained without being cut off, the strength is increased, the precision is high, and the product with uniform quality is thus widely used. In order to make the outer diameter of the thread of the final product, the required thread blank diameter is different, because it is limited by factors such as thread accuracy, whether the material is coated or not. Rolling (rubbing) threading refers to a processing method that uses plastic deformation to form thread teeth. It uses a rolling (rolling board) die with the same pitch and tooth shape as the thread to be processed. While extruding the cylindrical screw blank, the screw blank is rotated, and finally the tooth shape on the rolling mold is transferred to the On the screw blank, the thread is formed.

The common point of rolling (rubbing) thread processing is that the number of rolling revolutions does not need to be too much. If it is too many, the efficiency will be low, and the surface of the thread teeth is prone to separation or random buckling. Conversely, if the number of revolutions is too small, the diameter of the thread is likely to be out of round, and the pressure at the initial stage of rolling is abnormally high, resulting in shortened die life. Common defects of rolled threads: surface cracks or scratches on the threaded part; chaotic buckling; out-of-round threaded part. If these defects occur in large numbers, they will be discovered during the processing stage. If the number of occurrences is small, these defects will be circulated to users without being noticed by the production process, causing trouble. Therefore, the key issues of processing conditions should be summarized, and these key factors should be controlled in the production process.

7. Processing

High-strength fasteners must be quenched and tempered according to technical requirements. The purpose of heat treatment and tempering is to improve the comprehensive mechanical properties of fasteners to meet the specified tensile strength and yield ratio of the product. The heat treatment process has a crucial influence on high-strength fasteners, especially its intrinsic quality. Therefore, in order to produce high-quality high-strength fasteners, advanced heat treatment technology and equipment are necessary. Due to the large production volume and low price of high-strength bolts, and the threaded part is a relatively fine and relatively precise structure, it is required that the heat treatment equipment must have a large production capacity, a high degree of automation, and a good heat treatment quality.

Since the 1990s, the continuous heat treatment production line with a protective atmosphere has been dominant, and the vibration bottom type and mesh belt furnace are especially suitable for heat treatment and tempering of small and medium-sized fasteners. In addition to the good sealing performance of the furnace, the quenching and tempering line also has advanced computer control of atmosphere, temperature and process parameters, and equipment failure alarm and display functions. High-strength fasteners are automatically controlled from feeding-cleaning-heating-quenching-cleaning-tempering-coloring to off-line, which effectively guarantees the quality of heat treatment. The decarburization of the thread will cause the fastener to trip first when the resistance force required by the mechanical properties is not met, which will make the threaded fastener invalid and shorten the service life.

Due to the decarburization of the raw material, if the annealing is not proper, the decarburization layer of the raw material will be deepened. During quenching and tempering heat treatment, some oxidizing gas is generally brought in from outside the furnace. The rust of the bar steel wire or the residue on the surface of the coil steel wire after cold drawing will also decompose after being heated in the furnace, and the reaction will generate some oxidizing gases. For example, the rust on the surface of the steel wire is composed of iron carbonate and hydroxide, which will be decomposed into CO2 and H2O after heating, thus aggravating decarburization. Studies have shown that the degree of decarburization of medium-carbon alloy steel is more serious than that of carbon steel, and the fastest decarburization temperature is between 700-800 degrees Celsius. Since the attachment on the surface of the steel wire decomposes and synthesizes carbon dioxide and water very quickly under certain conditions, if the furnace gas of the continuous mesh belt furnace is not properly controlled, it will also cause the decarburization of the screw to be out of tolerance.

When high-strength bolts are formed by cold heading, the decarburized layer of raw materials and annealing not only still exists, but also is extruded to the top of the thread. For the surface of the fastener that needs to be quenched, the required hardness cannot be obtained, and its mechanical properties (especially strength and wear resistance) decreased. In addition, the surface of the steel wire is decarburized, the surface layer and the internal structure are different and have different expansion coefficients, and surface cracks may occur during quenching. For this reason, it is necessary to protect the top of the thread from decarburization during quenching and heating, and moderately carbon-coat the fasteners whose raw materials have been decarburized, and adjust the advantages of the protective atmosphere in the mesh belt furnace to the original carbon-coated parts. The carbon content is basically equal, so that the decarburized fasteners will slowly return to the original carbon content. The carbon potential should be set at 0.42%-0.48%. , so as not to coarsen the grains and affect the mechanical properties. The quality problems that may occur during the quenching and quenching process of fasteners mainly include: insufficient hardness in the quenched state; uneven hardness in the quenched state; out-of-tolerance quenching deformation; quenching cracking. Such problems that occur 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.

8. Inspection

In summary, the process factors affecting the quality of high-strength fasteners include steel design, spheroidizing annealing, peeling and dephosphorization, drawing, cold heading, thread processing, heat treatment, etc., and sometimes it is the superposition of various factors .

Process flow

The process flow of nickel-phosphorus plating for high-strength bolts consists of three parts:

The first part is the pre-treatment process, including precision and appearance inspection of high-strength bolts before plating, manual degreasing, soaking degreasing, pickling, electro activation and flash nickel plating;

The second part of electroless nickel plating process;

The third part is the post-processing process, including processes such as hydrogen-dispelling heat treatment, polishing and finished product inspection. as follows:

Bolt chemical composition inspection→bolt pre-plating accuracy, appearance inspection→manual degreasing→appearance inspection→soaking degreasing→hot water washing→cold water washing→pickling→cold water washing→electro activation→cold water washing→nickel flash plating→cold water washing→removal Ionized water washing→electroless nickel plating→deionized water washing→cold water washing→hydrogen drive→polishing→finished product inspection.

Key processes

The pretreatment process is a key process to determine the quality of high-strength bolt nickel-phosphorus plating. The purpose of this process is to remove the passivation layer on the surface of the bolt and prevent the regeneration of the passivation film. The execution status of this process directly determines the quality of the combination of the substrate and the coating. Most of the quality accidents in production are caused by poor pre-treatment of bolts. Before plating, the oil stains, rust and scale attached to the bolt surface must be carefully removed; the difference from electroplating is that it should be inspected more carefully, and plating is absolutely not allowed for unclean bolts.

① Inspection of bolts: Visual inspection of the surface quality of bolts requires that any burrs left by processing must be removed, and sharp edges and corners must be rounded.

② Manual degreasing; ensure that the surface of the substrate is free of oil stains.

③Soak to remove oil; boil the bolts in alkaline water to remove surface oil.

④Pickling: In order to prevent the alkaline degreasing solution from contaminating the flash nickel plating tank, electro-activation treatment is performed with pickling solution before flash nickel plating.

⑤Electric activation: use acid solution for electrical activation treatment.

⑥ Flash nickel plating: Nickel flash plating should be used for low alloy steel to increase the bonding strength between the coating and the substrate.

Post process

Nickel-phosphorus post-plating treatment includes two main processes: hydrogen drive and polishing.

①Hydrogen removal: According to relevant standards, the hydrogen removal temperature after plating is 200±10°C, and the treatment time is 2h. 200°C is beneficial to eliminate hydrogen embrittlement, relax internal stress, improve the bonding force between the coating and the substrate, and improve the corrosion resistance of the coating.

②Polishing: The polished bolts are bright in appearance, but in order to better improve the quality of the coating, smooth the tiny traces, and obtain a bright mirror-like surface, the coating needs to be polished with a polishing machine.

Process standard

Materials and main equipment

1. Bolts, nuts and washers should be accompanied by quality certificates, and should meet the design requirements and national standards.

2. High-strength bolts should be stored in categories according to specifications, and should be protected from rain and moisture. Do not use if the bolts and nuts are not matched or the threads are damaged. If the bolts, nuts and washers are corroded, the fastening axial force should be checked by sampling, and they can be used only after meeting the requirements.

Bolts, etc. shall not be stained by mud and oil, and shall be kept clean and dry. It must be used in the same batch according to the batch number, and must not be mixed and used.

3. Main tools: electric torque wrench and controller, manual torque wrench, manual wrench, wire brush, tool bag, etc.

Operating conditions

1.  Friction surface treatment: The friction surface is treated by sandblasting, grinding wheel grinding, etc., and the friction coefficient should meet the design requirements (one requirement is 0.45 or more for Q235 steel, and 0.55 or more for 16 manganese steel). The wood on the friction surface is allowed to have residual oxide scale, and the treated friction surface can form a red rust surface before installing bolts (generally stored in the open air for about 10 days), and the sandblasted friction surface can be installed without rust. When using a grinding wheel for grinding, the grinding range shall not be less than 4 times the diameter of the bolt, the grinding direction shall be perpendicular to the direction of the force, and the friction surface after grinding shall have no obvious unevenness. The friction surface should be prevented from being polluted by oil or paint, etc., if polluted, it should be cleaned thoroughly.

2. Check the hole size of the bolt hole, and the burrs on the edge of the hole must be removed.

3. Bolts, nuts and washers of the same batch number and specification should be packaged in boxes for use.

4. The electric wrench and manual wrench should be calibrated.

Skills requirement

Subject content and scope of application

This technical requirement specifies the technical requirements for the steel structure high-strength bolt pair connectors in the manufacturing, installation and inspection process of mobile machinery and equipment. The contents not stipulated in this technical requirement shall be implemented according to relevant national standards.

This technical requirement applies to mobile machinery steel structures that require high-strength bolt connections. This technical requirement applies to quality control and construction methods in the manufacturing plant and on-site installation.

Joint surface treatment

1. Friction-type high-strength bolt connections require that the bonding surfaces at the joints be in close contact and have sufficient friction coefficients. When the design drawings do not specify the treatment requirements for the joint surface, the treatment shall be carried out according to the following regulations: Sandblasting or shot blasting treatment shall be carried out on the joint surface of high-strength bolts to remove impurities such as rust and oil stains on the surface, and meet the Sa2.5 standard , roughness 50 ~ 75μm, the friction coefficient shall not be lower than 0.40. When there are regulations on the drawings, the regulations on the drawings shall be followed.

2. Protective measures should be taken for the friction surface of the treated high-strength bolt joints to prevent contamination with dirt and oil. It is strictly forbidden to make any marks on the friction surface of the high-strength bolt connection. When storing in the factory, or during transportation, and storage at the installation site, special precautions should be taken to prevent contamination of the connection surface. The installation unit should pay special attention to protecting the cleanliness of the connecting plate of the high-strength bolt and the connecting surface of the mother body, and the characteristics of the friction surface. It is not allowed to use a grinder to grind the connecting surface of the connecting plate and the connecting surface of the mother body.

Anti-slip coefficient inspection of high-strength bolt friction surface

The inspection of the anti-slip coefficient shall be based on the steel structure manufacturing batch, and every 2000t of a single project shall be regarded as a manufacturing batch, and those less than 2000t shall be regarded as a batch. Each surface treatment process needs to be inspected. Three sets of test pieces per batch. If the connection is diffused to external enterprises, each corresponding enterprise shall conduct anti-slip coefficient inspection.

1. The test piece used for the anti-slip coefficient test should be processed by the factory or the diffusion enterprise. The test piece and the steel structure member represented should be of the same material, produced in the same batch, adopt the same friction surface treatment process, and have the same surface state. And use the same batch of high-strength bolt connection pairs of the same performance level, and store them under the same environmental conditions. The anti-slip coefficient test is carried out according to the test method of GB50205 "Code for Acceptance of Construction Quality of Steel Structure Engineering".

2. The minimum value of the anti-slip coefficient inspection must be equal to or greater than the design specified value. When the above specified value is not met, the friction surface of the component should be treated again. After the treatment, the friction surface of the component is re-inspected.

Connection and installation of friction-type high-strength bolts for steel structures

1. Preparations before installation

2. Select qualified bolts, nuts and washers. The guarantee period of the coupling torque coefficient is six months from the date of delivery.

3. Bolts, nuts and washers are unqualified if the following conditions exist, and their use is prohibited.

a. The source (manufacturer) is unknown;

b. Those whose mechanical properties are unknown;

c. The torque coefficient k is unknown;

d. Cracks, scars, burrs, bends, rust, thread wear, oil stains, wet or defective;

e. Those without a performance test report;

f. Mixed with other batches of bolts;

g. Bolts that are not long enough, that is, the head of the bolt does not expose the end face of the nut after tightening. Generally, the length protruding from the end face of the nut is preferably 2 to 3 threads.

h. The torque coefficient of the connecting pair exceeds the guarantee period.

Pay special attention to waterproofing during transportation and storage.

4. Before the construction of high-strength bolts with large hexagonal heads, the torque coefficient of the high-strength bolt connection pair should be re-inspected according to the factory batch, and 8 sets of each batch should be re-inspected. Less than or equal to 0.010. The re-inspection method of the torque coefficient is carried out according to the provisions of GB50205 "Code for Acceptance of Construction Quality of Steel Structure Engineering". The installation of high-strength bolts should be carried out within a short period of time after the test.

Quality Considerations

(1) Rust floating on the surface, oil stains, burrs on the walls of bolt holes, and welding tumors should be cleaned up.

(2) After the contact friction surface is treated, it must meet the specified anti-scratch coefficient requirements. The high-strength bolts used should have matching nuts and washers, and should be used as a set when used, and should not be interchanged.

(3) The friction surface of the treated components is not allowed to be stained with oil, mud and other sundries during installation.

(4) The friction surface of the components should be kept dry during installation, and should not be operated in the rain.

(5) Strictly check and correct the deformation of the connected steel plates before installation.

(6) It is forbidden to hammer into the bolts during installation to prevent damage to the bolt threads.

(7) The electric wrench that is regularly tested during use ensures the accuracy of the torque and operates in the correct tightening sequence.

Main security technical measures

(1) The size of the wrench of the adjustable wrench should be consistent with the size of the nut, and a small wrench should not be used to add a sleeve. A dead wrench should be used for high-altitude operations. If an adjustable wrench is used, it must be fastened with a rope, and people must fasten their seat belts.

(2) When assembling the connecting bolts of steel components, it is strictly forbidden to insert the connecting surface or touch the screw holes by hand. When taking and placing the iron plate, fingers should be placed on both sides of the iron plate.

Use of high strength bolts

Ultra-high-strength bolts can reduce weight and increase installation space by reducing their size under the same clamping force, so the function and volume of the connected parts can be optimized, so that the overall weight reduction and performance optimization of the equipment can be achieved. There is an urgent demand for long-life ultra-high-strength bolts for fasteners used in the manufacture of major national equipment such as automobiles, high-speed rail, aviation, aerospace, national defense, and wind power.

1. Automobile bolts

In automobile manufacturing, bolts can be applied to almost every subsystem, such as: engine, wheel suspension system, chassis system, airbag, automatic anti-lock braking system, etc. The engine is the core of the car, and its service conditions are very harsh, especially the cylinder head bolts, crankshaft main bearing cap bolts, crankshaft pulley bolts, connecting rod bolts and flywheel bolts of the engine are the five most critical fasteners. To the performance stability and safety of the car.

There are many kinds of automobile bolts, which are mainly divided into four categories, including standard fasteners, non-standard fasteners, standard mechanical components and non-standard mechanical components, among which non-standard fasteners have the highest quality requirements. Among non-standard fasteners, such as automobile engine connecting rod bolts, flywheel bolts, wheel bolts, suspension bolts, etc., due to their high quality and good stability, my country still cannot meet the localization requirements, and most of them rely on imports. (Click here to learn more about the uses of automotive fasteners)

2. Wind power bolts

Bolts are an important and indispensable part of wind power generation. According to relevant data, a set of wind power equipment needs thousands of fasteners. From the figure below, we can see that some main fasteners are used, including wind power The tower bolts and anchor bolt components in the machine tower, the main engine bolts in the nacelle, the embedded screw sleeves in the blades and the blade stud bolts, etc., and a wind power blade needs about 100 embedded screw sleeves to be tightly connected with the main engine. The special "embedded screw sleeve" fastening system components for wind power blades are the key core precision components for the progress of large-scale wind power blade manufacturing technology.

picture

The high-strength fasteners used in wind power have long-term field service, harsh environment, and poor maintenance conditions, so the stability of the wind turbine is required. Under normal continuous working conditions, wind power fasteners must have a service life of more than 15 years. Most of the wind power high-strength bolts are grade 10.9, and a few are grade 8.8. Grade 10.9 high-strength bolts have a hardness value of 32-39HRC, tensile strength Rm≥1040MPa, elongation after fracture A≥9%, reduction of area Z≥48%, and low-temperature impact strength (-45°C) KV2≥27J.

Wind turbines use high-strength bolts as the connecting parts of the main engine, blades, towers and gearboxes, and together with the main engine, they undertake various requirements for basic performance and basic functions of components. There are mainly the following varieties.

a. Tower bolts: the bolts used on the wind turbine tower base, mainly used are GB/T 1228~1231, DIN 6914~6916 and DAST and other large hexagonal head steel structure connection auxiliary bolts.

b. Bolts of the whole machine: the bolts used on wind turbines, mainly used are GB/T 5782, GB/T 5783, GB/T 70.1, GB/T 6170, GB/T 97 and other hexagonal head bolts and inner hexagonal bolts , Hex nuts and washers.

c. Blade bolts: the bolts connecting the wind turbine blades, mainly some newly designed non-standard stud bolts and T-shaped round nut products.

3. Aerospace bolts

There are many types of bolts used in aerospace structures, such as solid rivets, threaded pins, bolts, nuts and one-sided fasteners, and of course other common fasteners, such as cotter pins, quick release locks, washers and so on. (Click here to learn more about the uses of aerospace fasteners)

Application fields and uses of aerospace fasteners

4. High-speed iron bolts

Fasteners are the "joints" of high-speed rail, and any part of a fast-moving high-speed rail may cause huge losses. High-speed trains and rails are constantly in contact, resulting in very large vibrations. The strength of some ordinary fasteners cannot adapt to the vibration of high-speed rails, so high-speed rails require high-strength fasteners. It is understood that an average of 1,210 fasteners are used in each high-speed rail car. These include 155 bolts, 121 nuts, 900 rivets, and 34 other fasteners. In order to ensure the connection of the power branch frame of the high-speed rail, that is, the EMUs, stainless steel bolts and stainless steel self-locking nuts are mainly used to fasten the links, and more than 10,000 sets are used per kilometer. 

Summarize

The bolt industry in developing countries is generally at the low-end level, and high-end bolts rely on imports. However, Shenzhen Tuofa, China has continuously made breakthroughs in high-end fasteners, which will promote China's bolts to the world and promote the comprehensive development of bolt manufacturers. Participating in international cooperation and competition is of great significance.

As a leading bolt manufacturer, Tuofa CNC Machining strives to ensure that each of our customers can receive satisfactory service and provide customers with comprehensive and complete solutions. Our rapid prototyping and low-volume production includes complex and precision optical parts, automotive parts, medical devices or aerospace parts. No matter how complex your project is, we can do our best to meet your needs.