What are high-strength bolts

What are high-strength bolts?

High-strength bolts made of high-strength steel, or bolts that require a large preload, are called high-strength bolts. High-strength bolts are mostly used for bridges, rails, and connections for high-pressure and ultra-high-pressure equipment. The fracture of these bolts is mostly brittle. High-strength bolts used in ultra-high-pressure equipment require a large pre-stress to ensure the sealing of the container.

Several concepts about high-strength bolts: The performance level of bolts above grade 8.8 is called high-strength bolts according to the regulations. The current national standard is only listed to M39, for large size specifications, especially the length of high-strength bolts greater than 10-15 times, domestic production is still a short line.

The difference between high-strength bolts and ordinary bolts

High-strength bolts are those that can withstand a greater load than ordinary bolts of the same size.
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High-strength external hexagon bolts

The material of ordinary bolts is made of Q235 (i.e. A3). High-strength bolts are made of 35# steel or other high-quality materials, and are heat-treated to improve their strength after they are made.
The difference between the two is the difference in material strength.
From raw materials
High-strength bolts are made of high-strength materials. High-strength bolts screw, nut and washer are made of high-strength steel, commonly used 45# steel, 40# boron steel, 20# manganese titanium boron steel, 35CrMoA, etc.. Ordinary bolts are commonly made of Q235 (equivalent to A3 in the past) steel.
From the strength level: high-strength bolts, increasingly widely used. Commonly used 8.8s and 10.9s two strength grades, of which 10.9 grade mostly. Ordinary bolts strength grade to lower, generally 4.4, 4.8, 5.6 and 8.8 grade.
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High-strength bolts
From the point of view of force characteristics: high-strength bolts apply pre-tension and rely on friction to transmit external force. Ordinary bolt connections rely on the bolt rod shear and hole wall pressure to transmit shear, tightening the nut produces little pre-pressure, the impact can be negligible, while high-strength bolts in addition to its high material strength, but also bolts to exert a lot of pre-pressure, so that the connection between the members of the squeezing pressure, so that there is a great deal of friction perpendicular to the direction of the screw, and pre-pressure, anti-slip coefficient and the type of steel have a direct impact on the high-strength bolts load-bearing capacity. According to the force characteristics of the compression type and friction type. The calculation methods are different for the two types. The minimum specification of high-strength bolts is M12, commonly used M16-M30, and the performance of oversized bolts is unstable and should be used carefully in design.
The difference between friction type and compression type connection of high strength bolts.
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High-strength bolts
High-strength bolts are connected by a large tightening pre-pressure within the bolt rod to clamp the plates of the connection plate, enough to generate a large friction force, thereby improving the integrity and stiffness of the connection, when subjected to shear, according to the different design and force requirements, can be divided into two types of high-strength bolts friction type connection and high-strength bolts compression type connection, the essential difference between the two is the limit state is different, although the same bolt, but in Although they are the same kind of bolts, there are great differences in calculation methods, requirements, and scope of application. In the shear design, high strength bolts friction type connection is the outer shear force reaches the maximum possible friction between the contact surface of the plate provided by the tightening force of the bolt as the limit state, that is, to ensure that the internal and external shear force does not exceed the maximum friction during the entire use of the connection. The plate will not be deformed by relative slip (the original amount of clearance between the screw and the hole wall is always maintained), and the connected plate is stressed in an elastic manner. In the shear design, high strength bolts pressure type connection allows the external shear force exceeds the maximum friction, when the relative slip deformation occurs between the connected plate until the bolt rod and hole wall contact, after which the connection will rely on the bolt rod body shear and hole wall pressure and friction between the contact surface of the plate joint force transfer, and finally the rod body shear or hole wall pressure damage as the limit state of the connected shear. In short, friction type high strength bolts and pressure type high strength bolts are actually the same kind of bolts, only whether the design considers slip. Friction type high strength bolts can never slide, the bolt does not bear shear, once slipped, the design is considered to reach the damage state, which is technically more mature; compression type high strength bolts can slide, the bolt also bears shear, and the final damage is equivalent to ordinary bolt damage (bolt shear bad or steel plate crush bad).
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High-strength bolts
From use
The bolted connections of the main members of the building structure are generally connected with 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 bolts are prestressed bolts, and the friction type applies the prescribed prestressing force with a torque wrench, and the compression type unscrews the plummer head. Ordinary bolts have poor shear resistance and can be used in secondary structural parts. Ordinary bolts can be tightened only.
Ordinary bolts are generally grade 4.4, 4.8, 5.6 and 8.8. High strength bolts are generally grade 8.8 and grade 10.9, with grade 10.9 being the majority.
Grade 8.8 is the same grade as 8.8S. The force performance and calculation methods of common bolts and high-strength bolts are different. High-strength bolts are subjected to external loads by first applying a pre-tension force P inside them and then generating frictional resistance on the contact surface between the connected parts, while ordinary bolts are subjected to external loads directly.
More specifically
High-strength bolt connection has the advantages of simple construction, good force performance, disassembly and replacement, fatigue resistance, and no loosening under power load, which is a promising connection method.
The high-strength bolt is tightened with a special wrench on the nut, so that the bolt produces a huge and controlled pre-tension, through the nut and pad, the same size of pre-pressure on the connected parts. Under the action of pre-pressure, a large friction force will be generated along the surface of the connected parts, obviously, as long as the axial force is less than this friction force, the member will not slip, the connection will not be damaged, which is the principle of high-strength bolting.
High-strength bolt connection is to rely on the friction between the contact surface of the connection to prevent its mutual slip, so that the contact surface has sufficient friction, it is necessary to improve the clamping force of the components and increase the friction coefficient of the contact surface of the components. The clamping force between the components is achieved by applying pre-tension to the bolt, so the bolt must be made of high-strength steel, which is also known as high-strength bolted connections.

In high-strength bolted joints, the size of the friction coefficient has a great influence on the load-bearing capacity. Tests have shown that the coefficient of friction is mainly affected by the form of the contact surface and the material of the member. In order to increase the friction coefficient of the contact surface, the construction is often used should be sandblasted, cleaned with wire brushes and other methods of contact surface of the members within the connection.

Processing of high strength bolts

Pre-plating process

Hot rolled coils → (cold dialing) → spheroidization (softening) annealing → mechanical phosphorus removal → pickling → cold dialing → cold forging forming → thread processing → heat treatment → inspection

Steel design

In fastener manufacturing, the correct choice of fastener materials is an important part, because the performance of the fastener and its material has a close relationship. If the material is not selected properly or incorrectly, it may cause the performance to meet the requirements, shorten the service life, and even accidents or processing difficulties, manufacturing costs, etc. Therefore, the selection of fastener materials is a very important link. Cold heading steel is the steel for fasteners with high interchangeability produced by cold heading process. Since it is formed by using metal plasticity processing at room temperature, the deformation of each part is very large and the deformation speed is also high, therefore, the performance requirements of cold heading steel raw materials are very strict. On the basis of long-term production practice and user use research, combined with GB/T6478-2001 “cold heading and cold extrusion with steel technical conditions” GB/T699-1999 “high-quality carbon structural steel” and the characteristics of the target JISG3507-1991 “carbon steel coil for cold heading steel” to 8.8 grade, 9.8 grade bolt and screw material requirements, the various chemical elements C content is too high, the cold forming performance will be reduced; too low can not meet the requirements of the mechanical properties of the parts, so set at 0.25%-0.55%; Mn can improve the permeability of steel, but adding too much will strengthen the matrix organization and affect the cold forming performance; in the parts of the tempering has the tendency to promote the growth of austenite grains, so in the international basis of the appropriate increase, set at 0.45%-0.55%. S.P. is an impurity element, their presence will produce segregation along the grain boundary, resulting in grain boundary embrittlement, damage to the mechanical properties of steel, should be reduced as much as possible, set at P less than or equal to 0.030%, S B. The maximum boron content is 0.005%, because although boron has a significant increase in steel permeability and other effects, but at the same time will lead to increased brittleness of steel. Boron content is too high for bolts, screws and studs such as the need for good overall mechanical properties of the workpiece is very unfavorable.

Spheroidal annealing

Countersunk head screws, hexagonal cylindrical head bolts using cold heading process, the original organization of the steel will directly affect the forming ability of the cold heading process. Plastic deformation in the local area of the cold heading process can reach 60%-80%, which requires steel must have good plasticity. When the chemical composition of steel is certain, the metallographic organization is the key factor to determine the plasticity, it is usually believed that the coarse lamellar pearlite is not conducive to cold heading forming, while the fine spherical pearlite can significantly improve the ability of steel plastic deformation. For high strength bolts with high dosage of medium carbon steel and medium carbon alloy steel, spheroidization (softening) annealing is carried out before cold heading in order to obtain uniform and meticulous spheroidized pearlite, so as to better meet the actual production needs. For medium carbon steel coil softening annealing, its heating temperature is mostly selected in the steel critical point up and down insulation, heating temperature generally can not be too high, otherwise it will produce three carburized body precipitation along the grain boundary, resulting in cold heading cracking, and for medium carbon alloy steel coil using isothermal spheroid annealing, in AC1 + (20-30%) after heating, furnace cooling to slightly below Ar1, the temperature of about 700 degrees Celsius isothermal for a period of time. Then the furnace is cooled to about 500 degrees Celsius and discharged from the furnace with air cooling. The metallographic organization of the steel is changed from coarse to fine, from flaky to spherical, and the cold heading cracking rate will be greatly reduced. 35\45\ML35\SWRCH35K steel softening and annealing temperature is generally 715-735 degrees Celsius; while SCM435\40Cr\SCR435 steel spheroidizing and annealing heating temperature is generally 740-770 degrees Celsius. -770 degrees Celsius, isothermal temperature 680-700 degrees Celsius.

Shelling and phosphorus removal

Cold heading steel coils to remove iron oxide plate process for stripping bright, phosphorus removal, there are two methods of mechanical phosphorus removal and chemical pickling. Mechanical phosphorus removal to replace the chemical pickling process of the coil, both to improve productivity and reduce environmental pollution. This phosphorus removal process includes bending method (commonly used with triangular groove round wheel reverse bending coil), spray nine method, etc., phosphorus removal effect is better, but can not make the residual iron phosphorus to net (iron oxide removal rate of 97%), especially when the iron oxide adhesion is very strong, therefore, mechanical phosphorus removal by iron thickness, structure and stress state, the use of low strength fasteners (less than or equal to 6.8 grade) with carbon steel Coil. High-strength bolts (greater than or equal to 8.8 grade) with the plate in the mechanical phosphorus removal, in order to remove all the iron oxide, and then by chemical pickling process that compound phosphorus removal. For low carbon steel coil, mechanical phosphorus removal residual iron is easy to cause uneven wear of the grain drawing die. When the grain drawing die hole due to the coil steel friction outside the temperature when adhering to the iron, so that the coil steel wire surface to produce longitudinal grain marks, coil steel wire cold upsetting flange bolts or cylindrical head screws, the head of the cause of micro-cracking, more than 95% of the steel wire surface in the drawing process caused by scratches. Therefore, mechanical phosphorus removal method is not suitable for high-speed drawing.

Drawing

Drawing process has two purposes, one is to reformulate the size of the raw material; second is to strengthen the role of fasteners through deformation to obtain the basic mechanical properties, for medium carbon steel, medium carbon alloy steel and a purpose, that is, to make the coil control cooling to get the sheet carburized body in the drawing process as cracked as possible, for the subsequent spheroidization (softening) annealing to get the granular carburized body ready, however, some manufacturers to reduce costs, the Arbitrarily reduce the number of draws, excessive surface reduction rate increases the processing hardening tendency of the coil steel wire, which directly affects the cold heading performance of the coil steel wire. If the distribution of the reduction rate of each channel is not appropriate, will also make the coil steel wire in the drawing process to produce torsional cracks, this longitudinal distribution along the wire, the cycle of certain cracks in the wire cold heading process exposed. In addition, the drawing process, such as poor lubrication, can also cause cold-drawn coil steel wire regularly appear transverse cracks. Coil steel wire out of the grain wire die mouth on the volume at the same time the tangential direction and drawing die is not the same center, will cause the wear and tear of the unilateral hole type of drawing die increased, so that the inner hole out of round, resulting in uneven drawing deformation of steel wire circumferential direction, so that the roundness of the steel wire super poor, in the cold heading process of steel wire cross-sectional stress is not uniform and affect the cold heading qualified rate. Coil steel wire drawing process, too large part of the rate of reduction of the surface quality of the wire deterioration, while too low rate of reduction is not conducive to the fragmentation of the lamellar carburized body, it is difficult to obtain as much as possible the granular carburized body, that is, the carburized body spheroidization rate is low, the cold heading performance of the wire is extremely unfavorable, using the production of drawn bar and coil steel wire, part of the rate of reduction of the surface straight control in the range of 10% – 15%. 15% of the range.

Cold forging

Usually, the bolt head forming using cold heading plastic processing, compared with the cutting process, metal fibers (metal stay line) along the shape of the product is continuous, no cut in the middle, thus improving the strength of the product, especially the excellent mechanical properties. Cold heading forming process includes cutting and forming, single-station click, double-strike cold heading and multi-station automatic cold heading. An automatic cold header performs multi-station processes such as stamping, upsetting, extruding and shrinking in several forming dies respectively. The processing characteristics of the original blank used in single or multi-station automatic cold heading machines are determined by the size of the material size 5-6 m long bar stock or 1900-2000 KG coiled steel wire, i.e. the process is characterized by the fact that cold heading is not performed with a pre-cut single blank, but with the automatic cold heading machine itself by The bar and coil wire are cut and upset (if necessary) from the blank. Prior to extruding the cavity, the blank must be shaped. The shaping results in a blank that meets the process requirements. Before upsetting, reducing and positive extrusion, the blanks do not need to be shaped. After the blank is cut, it is sent to the header shaping station. This station improves the quality of the blank, reduces the forming force of the next station by 15-17%, and extends the die life. The accuracy achieved by cold heading 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 condition, the accuracy of the tooling, the life and the degree of wear. The roughness of the working surface of the high alloy steel and carbide dies used in cold heading and extrusion should not be large Ra=0.2um, and the roughness of the working surface of these dies reaches Ra=0.025-0.050um when it has the highest life.

Thread processing

Bolt threads are generally cold processed so that thread blanks within a certain diameter range are passed through the thread rolling plate (die), and the threads are shaped by the pressure of the plate (die). The threaded part of the plastic flow line can be obtained without being cut off, and the strength is increased with high precision and uniform quality, so it is widely used. The diameter of the thread blanks required to produce the outside diameter of the final product varies, as it is limited by the thread accuracy, the material with or without coating, and other factors. Rolling is a process that uses plastic deformation to shape the threaded tooth. The thread is formed by using a rolling (thread rolling) die with the same pitch and thread shape as the thread to be processed, squeezing the cylindrical billet while rotating it, and finally transferring the thread shape from the rolling die to the billet. The common point of rolling (thread rolling) thread processing is that the number of rolling revolutions does not have to be too high; if it is too high, the efficiency is low and the threaded tooth surface is prone to separation or messy buckling. Conversely, if the number of revolutions is too low, the thread diameter will easily lose its roundness and the initial pressure of rolling will be abnormally high, resulting in a shortened die life. The common defects of thread rolling are: cracking or scratching on the surface of the thread part; messy buckling; and out-of-round thread part. If these defects occur in large numbers, they will be detected during the machining stage. If they occur in small quantities, the production process will not notice these defects and they will circulate to the user and cause trouble. Therefore, the key issues of processing conditions should be summarized and these key factors should be controlled in the production process.

Heat Treatment

High-strength fasteners are subject to tempering treatment according to technical requirements. Heat treatment tempering is to improve the comprehensive mechanical properties of fasteners to meet the tensile strength value and flexural strength ratio specified in the product. Heat treatment process has a vital impact on high-strength fasteners, especially its intrinsic quality, therefore, to produce high-quality high-strength fasteners, it is necessary to have advanced heat treatment technology and equipment. Due to the high production volume of high-strength bolts, low prices, and the threaded part is relatively fine and relatively precise structure, therefore, heat treatment equipment must be required to have a large production capacity, a high degree of automation, and good heat treatment quality. Into the 1990s with a protective atmosphere since the continuous heat treatment line has been dominant, shock bottom type, mesh belt furnace is particularly suitable for small and medium-sized fasteners heat treatment and tempering. Tempering line in addition to the furnace sealing performance, but also has an advanced atmosphere, temperature and process parameters computer control, equipment failure alarm and display function. High strength fasteners from feeding – cleaning – heating – quenching – cleaning – tempering – coloring to the next line, all automatic control operation, effectively ensure the quality of heat treatment. Decarburization of threads can cause fasteners to decarburize before they reach the required mechanical properties of resistance, making threaded fasteners fail and shortening their service life. Due to the decarburization of raw materials, if the annealing is improper, it will make the decarburization layer of raw materials deepen. The tempering heat treatment process will generally bring in some oxidation gas from outside the furnace. Bar steel wire rust or cold-drawn steel wire residue on the surface, heated into the furnace will also decompose, the reaction generates some oxidizing gas. For example, the surface rust of steel wire, which is composed of iron carbonate and hydroxide, will decompose into CO₂ and H₂O after heating, thus aggravating the decarburization. Research shows that the 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. As the adhesion on the surface of steel wire decomposes and synthesizes into carbon dioxide and water quickly under certain conditions, if the furnace gas of continuous mesh belt furnace is not properly controlled, it will also cause the screw to decarburize super poorly. High strength bolts when using cold heading forming, raw materials and annealing decarburization layer not only still exists, and is extruded to the top of the thread, for the need to quench the surface of the fastener, do not get the required hardness, its mechanical properties (especially strength and wear resistance) reduced. In addition, the surface decarburization of the steel wire, the surface layer and the internal organization are different and have different expansion coefficients, and there is a risk of surface cracking during quenching. For this reason, the quenching and heating to protect the top of the threads do not decarburize, but also the raw material has been decarburized fasteners moderate carbon overlay, the advantage of the protective atmosphere in the mesh belt furnace to the original carbon content of the parts and the carbon overlay is basically equal, so that the decarburized fasteners slowly return to the original carbon content, the carbon potential set at 0.42%-0.48% is appropriate, the carbon overlay temperature and Quenching heating the same, can not be carried out at high temperatures to avoid coarse grains, affecting the mechanical properties. Fasteners in the process of quenching and tempering quality problems may arise mainly: quenching state hardness is not enough; quenching state hardness is not uniform; quenching deformation super poor; quenching cracking. Such problems often appear on site with raw materials, quenching heating and quenching cooling, the correct development of heat treatment process, standardize the production process, often to avoid such quality accidents.

Inspection

In summary, the quality of high-strength fasteners affect the process factors are steel design, spheroid annealing, shelling and phosphorus removal, drawing, cold heading, thread processing, heat treatment and other aspects, sometimes the superposition of various factors.
The process of nickel-phosphorus plating of high-strength bolts consists of three parts:

  • The first part is the pre-treatment process, which includes precision and appearance inspection, manual oil removal, oil removal by immersion, pickling, electro-activation and flash nickel plating of high-strength bolts before plating.
  • The second part of the chemical nickel plating treatment process.
  • The third part is the post-treatment process, including processes such as hydrogen repellent heat treatment, polishing and finished product inspection.

As follows:

Bolt chemical composition inspection → bolt pre-plating accuracy and appearance inspection → manual oil removal → appearance inspection → oil removal by immersion → hot water washing → cold water washing → acid washing → cold water washing → electric activation → cold water washing → flash nickel plating → cold water washing → deionized water washing → chemical nickel plating → deionized water washing → cold water washing → hydrogen repelling → polishing → finished product inspection.

Key Processes
The pretreatment process is the key process to determine the quality of nickel-phosphorus plating of high-strength bolts. The purpose of this process is to remove the passivation layer on the bolt surface and prevent the regeneration of the passivation film. The execution of the process directly determines the degree of bonding between the substrate and the plating. Most of the quality accidents in production are caused by poor pre-treatment of bolts. Before plating, oil, rust and oxidation must be carefully removed from the surface of the bolt; the difference with electroplating is that the bolt should be inspected more carefully, and the bolt that is not treated cleanly should never be allowed to be plated.

  • ① Inspection of the bolt; visual inspection of the surface quality of the bolt requires that any burrs left by processing must be removed and sharp edges must be rounded.
  • ② Manual oil removal; ensure the surface of the substrate is free of oil stains.
  • ③ Soak to remove oil; put the bolt into alkaline water and boil it to remove surface oil.
  • ④ Acid washing; to prevent the alkaline degreasing solution from polluting the flash nickel plating bath, electro-activation treatment is carried out with the acid washing solution before flash nickel plating.
  • ⑤ Electrical activation; electro-activation treatment with acid solution.
  • ⑥ Flash nickel plating; flash nickel plating should be used for all low-alloy steel to increase the bonding strength between the plating and the substrate.

Post-process
Nickel phosphorus plating post-treatment, including hydrogen drive, polishing two main processes.

  • ① Hydrogen drive; according to the relevant standards, after plating hydrogen drive temperature of 200 ± 10 ℃, processing time of 2h. 200 ℃ is conducive to the elimination of hydrogen embrittlement, relaxation of internal stress, improve the bond between the plating and the substrate, improve the corrosion resistance of the plating.
  • ② Polishing; polished bolts have a bright appearance, but in order to better improve the quality of the plating, smooth out the tiny traces and get a bright mirror-like surface, the plating needs to be polished with a polishing machine.

Classification of high-strength bolts

Friction type high-strength bolts: They are suitable for steel frame structure beam and column connection, solid web beam connection, heavy crane beam connection in industrial plants, braking system and connection of important structures bearing dynamic load.
Pressure-bearing high-strength bolts: They can be used for shear connections in static-loaded structures that allow a small amount of sliding or in members that are indirectly subjected to dynamic loads.
Tensile high-strength bolts: When the bolts are under tension, the fatigue strength is low, and the bearing capacity is not easily exceeded 0.6P (P is the allowable axial force of the bolts) under dynamic load, so they are only suitable for use under static load, such as flange butt joints and T-joints of pressurized rods.

Process standards for high-strength bolts

Material and main machine tools

Bolts, nuts and washers should be accompanied by quality certificates and should conform to the design requirements and the provisions of national standards.
High-strength bolts should be stored in the warehouse according to the specifications, and protected from rain and moisture. In case of mismatched bolts, nuts, thread damage, shall not be used. Bolts, nuts, washers with rust, should be sampled to check the tightening axial force, meet the requirements before use.
Bolts and so on shall not be mud, oil and dirt sticky, keep clean, dry state. Must be used in the same batch according to the batch number, not mixed, mixed use.
Main tools: electric torque wrench and control instrument, manual torque wrench, hand wrench, wire brush, tool bag, etc.

Operating conditions

Friction surface treatment: the friction surface is treated by sandblasting, grinding wheel grinding, etc. The friction coefficient should meet the design requirements (a requirement of 0.45 or more for Q235 steel and 0.55 or more for 16 manganese steel). Friction surface wood allowed residual iron oxide, after treatment of the friction surface can be generated after the installation of bolts rust surface (generally open storage 10d or so), with sandblasting friction surface does not have to rust can be installed bolts. When using grinding wheel grinding, grinding range is not less than 4 times the diameter of the bolt, grinding direction and the direction of force perpendicular to the friction surface after grinding should not be obvious uneven. The friction surface should be prevented from being polluted by oil or paint, and should be thoroughly cleaned if polluted.
Check the bore size of the bolt hole, the burr must be removed from the edge of the hole.
The bolts, nuts and washers of the same batch number and specification should be packed together for use.
Electric wrenches and manual wrenches should be calibrated.

Technical requirements of high-strength bolts

Subject content and scope of application

The technical requirements specify the technical requirements for the manufacturing, installation and inspection of the steel structure of mobile machinery and equipment with high-strength bolts. This technical requirement does not specify the content, according to the relevant national standards.
The technical requirements apply to the need to apply high-strength bolts connected to the mobile machinery steel structure. This technical requirement applies to the quality control and construction methods in the manufacturing plant and on-site installation.

Bonding surface treatment

Friction-type high-strength bolt connection requires the joint surface to be close together and have sufficient friction coefficient. When the design drawings do not specify the processing requirements of the joint surface, the following provisions are processed: sandblasting or shot blasting of the joint surface of the high-strength bolts to remove rust, oil and other impurities on the surface to achieve Sa2.5 standard, roughness 50 ~ 75μm, the coefficient of friction shall not be less than 0.40. When the drawings are specified, the drawings shall be executed.
After treatment, the friction surface of the high-strength bolt joint should be protected from dirt and oil. It is strictly forbidden to make any mark on the friction surface of the high-strength bolt connection. In the factory storage, or in the transport, to the installation site storage to prevent pollution of the connection surface in particular. The installation unit should pay special attention to the characteristics of the friction surface to protect the cleanliness of the connection plate of the high-strength bolts and the connection surface of the parent body. It is not allowed to use a grinding machine to polish the connecting surface of the connecting plate and the connecting surface of the mother body at will.

High-strength bolts friction surface anti-slip coefficient test

Anti-slip coefficient of inspection should be a steel manufacturing batch as a unit to a single project every 2000t for a manufacturing batch, less than 2000t as a batch, a single project of the friction surface of the components choose two or more surface treatment process, then each surface treatment process are required to test. Each batch of three groups of test pieces. If the connection is diffused to the external enterprise, the corresponding each enterprise should do the anti-slip coefficient test.
Anti-slip coefficient test specimens should be processed by the plant or diffusion enterprises, the specimens and the representative steel members should be the same material, made in the same batch, using the same friction surface treatment process and have the same surface state, and apply the same batch of high-strength bolts with the same performance level of the connection, stored in the same environmental conditions. Slip resistance coefficient test according to GB50205 “steel construction quality acceptance specification” test method.
The minimum value of the anti-slip coefficient test must be equal to or greater than the value specified in the design. When not in line with the above-mentioned value, the friction surface of the member should be reprocessed. After processing the friction surface of the member re-inspection.

Installation of friction type high-strength bolts for steel structure

Preparation work before installation.
Select the bolts, nuts and washers that have passed the inspection. The guarantee period of the torque coefficient of the connection is six months from the date of delivery.
Bolts, nuts, washers have the following conditions for substandard products, prohibited.

  • a. The source (manufacturer) is unknown.
  • b. Mechanical properties are unknown.
  • c. Torque coefficient k is unknown.
  • d. Cracks, scars, burrs, bending, rust, thread wear, oil, water wetting or defects.
  • e. Not accompanied by a performance test report.
  • f. Bolts mixed with other batches.
  • g. Bolts with insufficient length, i.e., the head of the bolt does not show the end of the nut after tightening. Generally, the length of the bolt is 2 to 3 buckles of threads.
  • h. The torque coefficient of the connecting sub exceeds the guarantee period.

Special attention should be paid to waterproof in transportation and storage.
Large hexagonal head high-strength bolts before construction, the torque coefficient of high-strength bolts should be rechecked according to the factory batch of connection, each batch of 8 sets of recheck, the average value of 8 sets of torque coefficient should be within 0.110 ~ 0.150, and its standard deviation should be less than or equal to 0.010. The torque coefficient recheck method according to GB50205 “Specification for acceptance of steel construction quality”. The installation of high-strength bolts should be carried out in a shorter period of time after the test.

Quality precautions

  • (1) Surface floating rust, oil, bolt hole wall with burrs, weld tumors, etc. should be cleaned up.
  • (2) The contact friction surface should reach the required scratch resistance coefficient after treatment. The use of high-strength bolts should have matching nuts and washers, when used in accordance with the matching use, not interchangeable.
  • (3) the friction surface of the treated components are not allowed to get oil, dirt and other debris when installed.
  • (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) Hammering into the bolt is prohibited during installation to prevent damage to the bolt fillet.
  • (7) Test the electric wrench regularly when using to ensure the accuracy of the torque and operate in the correct torsion order.

Main safety technical measures

  • (1) The size of the wrench of the active wrench should match the size of the nut, and no casing should be added to the small wrench. High-altitude work should use dead wrenches, such as using live wrenches with a rope tethered, people should be tied up with safety belts.
  • (2) When assembling the bolts connecting the steel components, it is strictly forbidden to insert the connecting surface or probe the screw holes by hand, and when taking and putting the gasket plate, the fingers should be placed on both sides of the gasket plate.

Source: China High-strength Bolts Manufacturer – Yaang Pipe Industry (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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Summary
what are high strength bolts - What are high-strength bolts
Article Name
What are high-strength bolts
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The performance level of bolts above grade 8.8 is called high-strength bolts according to the regulations.
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