What are the differences between high strength bolts and common bolts?
What is a common bolt?
Ordinary bolt, we generally refer to the bolt with low strength level requirements, and in general, it is the ordinary bolt with 4.8 level. Bolts with low hardness and strength are generally produced with ordinary screw wires. The hardness, strength, shoulder tension and torque of ordinary bolt materials are not very high, not very high. High strength bolt generally refers to the bolt of high strength grade. Its screw material, bolt material and bolt material are relatively good, and its hardness is also relatively high. Moreover, after the production and manufacturing, the bolt will be hardened. Make the bolt reach the grade strength requirement of high strength bolt.
The performance grades of bolts for steel structure connection are divided into more than 10 grades, such as 3.6, 4.6, 4.8, 5.6, 6.8, 8.8, 9.8, 10.9 and 12.9. The bolts of grade 8.8 and above are made of low-carbon alloy steel or medium carbon steel and heat treated (quenched and tempered). They are generally called high-strength bolts and the rest are generally called ordinary bolts. The bolt performance grade mark consists of two parts of numbers, which respectively represent the nominal tensile strength value and yield strength ratio of bolt material. For example, a bolt of property class 4.6 means:
- 1. The nominal tensile strength of bolt material reaches 400MPa;
- 2. The yield strength ratio of bolt material is 0.6;
- 3. The nominal yield strength of bolt material is 400 × 0.6 = 240mpa.
The performance grade is 10.9 high-strength bolt, and the material can reach:
- 1. The nominal tensile strength of bolt material is up to 1000MPa;
- 2. The yield strength ratio of bolt material is 0.9.
Bolt processing technology
Pre plating process
Hot rolled wire rod (cold drawing) – spheroidizing (softening) annealing – mechanical descaling – Pickling – cold drawing – cold forging – threading – heat treatment – Inspection
In fastener manufacturing, it is important to select fastener material correctly, because the performance of fastener is closely related to its material. If the materials are not selected properly or correctly, the performance may not meet the requirements, the service life may be shortened, even accidents or processing difficulties may occur, and the manufacturing cost is high, so the selection of fastener materials is a very important link. Cold heading steel is a kind of fastener steel with high interchangeability produced by cold heading forming process. Because it is formed by metal plastic processing at room temperature, each part has a large amount of deformation and bears a high deformation speed. Therefore, the performance requirements of cold heading steel raw materials are very strict. On the basis of long-term production practice and user investigation, combined with the characteristics of GB / t6478-2001 “technical conditions for cold heading and cold extrusion steel” GB / t699-1999 “high quality carbon structural steel” and jisg3507-1991 “carbon steel wire rod for cold heading steel”, the material requirements of grade 8.8 and 9.8 bolts and screws are taken as examples to determine various chemical elements. If the content of C is too high, the cold forming property will be reduced; if it is too low, the mechanical property of the part cannot be satisfied, so it is set as 0.25% – 0.55%. Mn can improve the permeability of steel, but adding too much Mn can strengthen the matrix structure and affect the cold formability; it has the tendency to promote the growth of austenite grain during the quenching and tempering of parts, so it is set as 0.45% – 0.80% on the international basis. Si can strengthen the ferrite and reduce the cold formability. The elongation of the material is determined to be less than or equal to 0.30%. S、 P. as impurity elements, their existence will produce segregation along the grain boundary, leading to grain boundary embrittlement and damaging the mechanical properties of the steel. It should be reduced as much as possible, with P less than or equal to 0.030% and S less than or equal to 0.035%. B. The maximum value of boron content is 0.005%, because although boron element can significantly improve the permeability of steel, it will also lead to the increase of brittleness of steel. The boron content is too high, which is very disadvantageous to the work pieces such as bolts, screws and studs that need good comprehensive mechanical properties.
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 of cold heading process. In the process of cold heading, the plastic deformation of local area can reach 60% – 80%, so the steel must have good plasticity. When the chemical composition of the steel is certain, the metallographic structure is the key factor to determine the plasticity. Generally, it is considered that the coarse large pearlite is not conducive to cold upsetting, while the small spherical pearlite can significantly improve the plastic deformation ability of the steel. For medium carbon steel and medium carbon alloy steel with more high strength bolts, spheroidizing (softening) annealing is carried out before cold heading to obtain uniform and fine spheroidized pearlite, so as to better meet the actual production needs. For the softening annealing of medium carbon steel wire rod, the heating temperature is usually kept at the critical point of the steel. Generally, the heating temperature should not be too high. Otherwise, three cementite precipitates along the grain boundary, causing cold heading cracking. For medium carbon alloy steel wire rod, isothermal spheroidizing annealing is adopted. After heating with AC1+ (20-30%), the furnace cooling is slightly lower than Ar1, and the temperature is about 700 The temperature is isothermal for a period of time, and then the furnace is cooled to about 500 ℃ and air cooled. The metallographic structure of steel changes from coarse to fine, from flake to ball, and the cracking rate of cold heading will be greatly reduced. The softening and annealing temperature of 35 \ 45 \ ML35 \ swrch35k steel is 715-735 ℃, while that of SCM435 \ 40Cr \ scr435 steel is 740-770 ℃, and the isothermal temperature is 680-700 ℃.
Shelling and descaling
In the process of removing iron oxide plate from cold heading steel wire rod, there are two methods: peeling, descaling, mechanical descaling and chemical pickling. Replacing the chemical pickling process of wire rod with mechanical descaling not only improves productivity, but also reduces environmental pollution. The descaling process includes bending method (the round wheel with triangular groove is commonly used to repeatedly bend the wire rod), spraying nine methods, etc. the descaling effect is good, but the residual iron scale can not be removed (the removal rate of oxide scale is 97 %), especially when the scale adhesion is very strong, therefore, mechanical descaling is affected by the thickness, structure and stress state of the scale, which is used for carbon steel wire rod for low-strength fasteners (up to grade 6.8). After mechanical descaling, high-strength bolts (8.8 grade and above) are descaled with wire rod. In order to remove all oxide scales, they are descaled by chemical pickling process. For low-carbon steel wire rod, the residual iron sheet of mechanical descaling is easy to cause uneven wear of grain draft. When the core drawing hole adheres to the iron sheet due to the friction of the wire rod and the external temperature, the longitudinal grain mark will appear on the wire rod surface. When the wire rod is cold heading flange bolt or cylindrical head screw, the micro crack will appear on the head. More than 95% of the causes are caused by the scratch on the wire surface during the drawing process. Therefore, mechanical descaling is not suitable for high speed drawing.
There are two purposes in the drawing process, one is to change the size of raw materials; the other is to obtain basic mechanical properties of fasteners through deformation strengthening. For medium carbon steel, medium carbon alloy steel, there is another purpose, that is, to crack the lamellar cementite obtained after controlled cooling of wire rod as much as possible in the drawing process, so as to prepare for the subsequent spheroidizing (softening) annealing to obtain granular cementite, However, in order to reduce the cost, some manufacturers arbitrarily reduce the drawing passes, and the excessive surface reduction rate increases the work hardening tendency of wire rod, which directly affects the cold heading performance of wire rod. If the distribution of the reduction rate of each pass is not appropriate, it will also cause torsional cracks in the drawing process of wire rod. The cracks distributed along the longitudinal direction of the wire and with a certain period will be exposed in the cold heading process of the wire. In addition, if the lubrication is not good during the drawing process, the cold drawn wire rod can also have regular transverse cracks. At the same time, the tangent direction of the wire drawing die is not concentric with the drawing die, which will cause the wear of the single side pass of the drawing die, make the inner hole out of round, cause the drawing deformation of the wire in the circumferential direction to be uneven, and make the roundness of the wire out of tolerance. During the cold heading process, the cross section stress of the wire is not uniform, which will affect the qualified rate of cold heading. In the drawing process of wire rod, the surface quality of wire is deteriorated by too large part of the reduction rate, while the too low reduction rate is not conducive to the fragmentation of lamellar cementite, and it is difficult to obtain as many granular cementite as possible, that is, the low spheroidization rate of cementite, which is very bad for the cold heading performance of wire rod. For the rod and wire rod produced by drawing, part of the reduction rate is controlled within 10% – Within 15%.
Generally, the bolt head is formed by cold heading plastic processing. Compared with the cutting process, the metal fiber (metal wire) is continuous along the product shape, without cutting in the middle, so the product strength is improved, especially the mechanical properties are excellent. Cold heading forming process includes cutting and forming, single station click, double-click cold heading and multi station automatic cold heading. An automatic cold heading machine carries out stamping, upsetting, extruding and reducing in several forming dies. The processing characteristics of the original blank used in the single station or multi station automatic cold heading machine are that the size of the material is 5-6 meters long or the weight is 1900-2000 kg The characteristics of the processing technology is that the cold upsetting forming is not the use of pre cut single blank, but the use of automatic cold upsetting machine itself by the bar and wire cutting and upsetting (when necessary) blank. Before extruding the cavity, the blank must be shaped. Through shaping, we can get the blanks that meet the technological requirements. Before upsetting, reducing and forward extrusion, the blank does not need to be reshaped. After cutting the blank, it is sent to the upsetting and shaping station. The working position can improve the quality of the blank, reduce the forming force of the next working position by 15-17%, and prolong the service life of the die. The manufacturing bolt can adopt multiple reduction. The accuracy of cold upsetting is also related to the selection of forming method and the process used. In addition, it also depends on the structural characteristics of the equipment used, process characteristics and their states, die accuracy, service life and wear degree. For the high alloy steel used in cold upsetting and extrusion, the working surface roughness of cemented carbide die shall not be larger than RA = 0.2um. When the working surface roughness of this kind of die reaches RA = 0.025-0.050um, it has the highest service life.
The bolt thread is generally cold-worked, so that the thread blank within a certain diameter range can be formed by the screw plate (rolling die) pressure through the screw plate (rolling die). The plastic streamline of the thread part can be obtained without being cut off, the strength is increased, the accuracy is high, and the quality is uniform, so it is widely used. In order to produce the outer diameter of the thread of the final product, the required blank diameter of the thread is different, because it is limited by the accuracy of the thread, whether the material is coated or not. Rolling (rubbing) thread refers to the processing method of forming thread teeth by plastic deformation. It is a rolling (rolling) die with the same pitch and profile as the thread being processed, which extrudes the cylindrical billet while making the billet rotate, and finally transfers the profile on the rolling die to the billet to form the thread. The common point of rolling (rubbing) thread processing is that the number of rolling turns does not need to be too much, if it is too much, the efficiency is low, and the surface of thread teeth is easy to produce the phenomenon of separation or disorderly buckling. On the contrary, if the number of turns is too small, the thread diameter is easy to be out of round, and the pressure at the initial rolling stage is abnormally increased, resulting in the shortening of die life. Common defects of rolling thread: surface crack or scratch of thread part; disordered thread; out of round thread part. If these defects occur in large quantities, they will be found in the processing stage. If the number of occurrence is small, the production process will not notice these defects will flow to users, causing trouble. Therefore, we should summarize the key problems of processing conditions and control these key factors in the production process.
High strength fasteners shall be 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 strength ratio of products. Heat treatment technology has a vital influence on high-strength fastener, especially its internal quality. Therefore, in order to produce high-quality high-strength fastener, we must have advanced heat treatment technology and equipment. Due to the large production of high-strength bolts, low price, and relatively fine and relatively precise screw structure, heat treatment equipment is required to have the ability of large production capacity, high degree of automation, and good heat treatment quality. Since the 1990s, the continuous heat treatment production line with protective atmosphere has occupied a leading position. The shock bottom type and mesh belt furnace are especially suitable for the 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, equipment fault alarm and display functions. The high-strength fasteners can be automatically controlled from charging, cleaning, heating, quenching, cleaning, tempering and coloring to the production line, which effectively guarantees the quality of heat treatment. Decarburization of thread will cause fastener to trip first when it does not meet the resistance required by mechanical properties, which will make threaded fastener invalid and shorten its service life. Due to the decarburization of raw materials, if the annealing is not proper, the decarburization layer of raw materials will be deepened. In the process of quenching and tempering heat treatment, some oxidation gases are usually brought in from outside the furnace. The rust of bar steel wire or the residue on the surface of wire rod after cold drawing will also decompose after being heated in the furnace and react to generate some oxidizing gas. For example, the surface rust of steel wire, which is composed of iron carbonate and hydroxide, will decompose into CO 2 and H 2 O after heating, thus aggravating the decarburization. The results show that the decarburization degree of medium carbon alloy steel is more serious than that of carbon steel, and the fastest decarburization temperature is between 700-800 ℃. Due to the fast decomposition of the attachments on the steel wire surface into carbon dioxide and water under certain conditions, if the continuous mesh belt furnace gas is not properly controlled, the screw decarburization will also be out of tolerance. When the high-strength bolt is formed by cold upsetting, the decarburization layer of raw material and annealing not only still exists, but also is extruded to the top of the thread. For the fastener surface that needs quenching, the required hardness is not obtained, and its mechanical properties (especially the strength and wear resistance) are reduced. In addition, the steel wire surface decarburization, surface and internal structure have different expansion coefficient, which may produce surface cracks during quenching. Therefore, it is necessary to protect the top of the thread from decarburization during quenching and heating, and to properly carbon coat the decarburized fasteners of the raw materials. The advantage of the protective atmosphere in the mesh belt furnace should be adjusted to be basically equal to the original carbon content of the carbon coated parts, so that the decarburized fasteners can slowly recover to the original carbon content, and the carbon potential is set at 0.42% – 0.48% %Therefore, the temperature of carbon coating is the same as that of quenching and heating, and it can not be carried out at high temperature, so as to avoid the influence of coarse grains on mechanical properties. The quality problems that may occur in the process of quenching, tempering and quenching of fasteners are as follows: insufficient hardness in the quenched state; uneven hardness in the quenched state; out of tolerance quenching deformation; quenching cracking. This kind of problems in the field are often related to raw materials, quenching heating and quenching cooling. The correct formulation of heat treatment process and standardization of production and operation process can often avoid such quality accidents.
In conclusion, the technological factors affecting the quality of high-strength fasteners include steel design, spheroidizing annealing, shelling and descaling, drawing, cold heading, thread processing, heat treatment, etc., sometimes the superposition of various factors.
The process of Ni-P plating for high-strength bolts consists of three parts:
- The first part is the pretreatment process, including precision and appearance inspection, manual oil removal, immersion oil removal, pickling, electric activation and flash nickel plating before high-strength bolt plating;
- The second part is the process of electroless nickel plating;
- The third part is the post-treatment process, including hydrogen drive heat treatment, polishing and finished product inspection.
As follows: chemical composition inspection of bolt → accuracy and appearance inspection before bolt plating → 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 drive → polishing → finished product inspection.
Pretreatment process is the key process to determine the quality of Ni-P plating for 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 implementation of this process directly determines the bonding degree of substrate and coating. Most of the quality accidents in production are caused by poor pre-treatment of bolts. Before plating, the oil stain, rust and oxide skin on the bolt surface must be removed carefully; the difference with plating is that it should be inspected more carefully, and plating is not allowed for the bolts that are not cleaned.
- ① Check the bolt; check the surface quality of the bolt by visual inspection. Any burr left by processing must be removed, and the sharp edge must be rounded.
- ② Remove oil by hand to ensure no oil stain on the surface of the substrate.
- ③ Soak the bolts to remove oil; boil the bolts in alkaline water to remove oil stains on the surface.
- ④ Acid cleaning: in order to prevent the alkaline degreasing solution from polluting the flash nickel plating bath, the acid wash solution is used for the electric activation treatment before the flash nickel plating.
- ⑤ Electroactivation; electroactivation with acid solution.
- ⑥ Flash nickel plating should be used for low alloy steel to increase the bonding strength between the coating and the substrate.
The post-treatment of Ni-P plating includes two main processes: hydrogen drive and polishing.
- ① According to the regulations of relevant standards, the temperature of hydrogen drive after plating is 200 ± 10 ℃, and the treatment time is 2H. 200 ℃ is beneficial to eliminate hydrogen embrittlement, relax internal stress, improve the adhesion between coating and substrate, and improve the corrosion resistance of coating.
- ② Polishing: the polished bolt has a bright appearance, but in order to improve the coating quality, level the tiny traces and get a bright mirror like surface, a polishing machine is needed to polish the coating.
Classification of high strength bolts
Friction type high strength bolt: it is applicable to the connection of steel frame structure beam and column, solid web beam, heavy crane beam of industrial plant, brake system and important structure bearing dynamic load.
Bearing type high strength bolt: it can be used for shear connection in static load structure that allows a small amount of sliding or in components that indirectly bear dynamic load.
Tensile high-strength bolt: when the bolt is under tension, the fatigue strength is low. Under the action of dynamic load, its bearing capacity is not easy to exceed 0.6p (P is the allowable axial force of the bolt). Therefore, it is only suitable for the use under static load, such as flange butt joint of compression member, T-joint, etc.
What are the differences between high strength bolts and common bolts?
From the perspective of raw materials:
High strength bolts are made of high strength materials. The screws, nuts and washers of high-strength bolts are made of high-strength steel, commonly used No. 45 steel, 40 boron steel and 20 manganese titanium boron steel. Ordinary bolts are usually made of Q235 steel.
In terms of strength grade:
High strength bolt is widely used. The strength grades of 8.8s and 10.9s are commonly used, of which 10.9 is the most.
The strength grade of ordinary bolts is lower, generally 4.4, 4.8, 5.6 and 8.8.
According to the stress characteristics:
High strength bolts exert pretension and transmit external force by friction. The shear force of ordinary bolt connection is transmitted by the shear resistance of bolt rod and the pressure of hole wall. The pretension produced when tightening the nut is very small, and its influence can be ignored. In addition to the high material strength of high-strength bolt, a great pretension is also exerted on the bolt, so that the extrusion pressure is produced between the connecting members, so that there is a great friction force perpendicular to the screw direction, and the pretension, anti slip coefficient and steel All kinds of materials directly affect the bearing capacity of high-strength bolts.
According to the stress characteristics, it can be divided into pressure bearing type and friction type. The minimum specification of high strength bolt is M12, and M16 ~ M30 is commonly used. The performance of oversized bolt is unstable, so it should be used carefully in design.
Inspection requirements for high strength bolt connection before and after installation:
- 1. Before construction, the torque coefficient of high-strength large hexagon head bolt connection pair shall be rechecked according to the factory batch number, and its average value and standard deviation shall comply with the provisions of the current national standard code for design, construction and acceptance of high-strength bolt connection of steel structure; the pre stress of torsional shear type high-strength bolt connection pair shall be rechecked according to the factory batch number, and its average value and standard deviation shall comply with the current national standard <Code for design, construction and acceptance of high strength bolt connection of steel structure>.
- 2. The surface hardness test shall be carried out for the high-strength bolts connected to the steel grid structure with bolt ball joints whose safety level is level I and whose span is more than 40m.
- 3. The manufacturer and the installation unit shall carry out the anti sliding coefficient test with the steel structure manufacturing batch as the unit.
- 4. After the high-strength bolt connection and installation, the torque inspection of the connection pair shall be carried out, which can be divided into torque method inspection and angle method inspection. The torque inspection shall be completed within 48h after 1h.
- 5. For the inspection after the final tightening of high-strength bolts, the torsional shear type high-strength bolts can be visually inspected to see whether the plum head at the end of the bolt is screwed off; the high-strength large hexagon head bolts can be inspected one by one by “small hammer knocking method”. Methods use finger to press the corresponding side of the nut (as close to the washer as possible), and use a small hammer of 0.3-0.5kg weight to knock the corresponding other side of the nut. If the finger feels slight vibration, it is qualified. If the vibration is large, it is under tightened and missed tightened. If the finger does not vibrate at all, it is over tightened. Since the bolt may have plastic deformation after overtightening, in order to avoid delayed fracture of the bolt, the overtightening bolt should be replaced.
The difference between friction type and pressure type connection of high strength bolt:
The high-strength bolt connection is to clamp the plates of the connecting plate through a large tightening pre tension in the bolt rod, which is enough to produce a great friction force, so as to improve the integrity and rigidity of the connection. When subjected to shear force, according to the different design and stress requirements, it can be divided into two types: high-strength bolt friction type connection and high-strength bolt pressure type connection. The essential difference between the two is that the ultimate state is different However, it is the same bolt, but there are great differences in calculation method, requirements, scope of application, etc.
In the shear design, the friction type connection of high-strength bolt is a limit state when the external shear force reaches the maximum friction provided by the bolt tightening force between the contact surfaces of plates, that is to ensure that the internal and external shear force of the connection does not exceed the maximum friction force during the whole service period. The plate will not have relative sliding deformation (the original gap between the screw and the hole wall is always maintained), and the connected plate will be stressed as an elastic whole.
In shear design, the allowable external shear force exceeds the maximum friction force in the high-strength bolt bearing connection. The relative slip deformation between the connecting plates occurs until the bolt rod contacts the hole wall. After that, the joint is transferred by the shear of the bolt body and the friction between the hole wall and the contact surface of the plate, and finally the simultaneous interpreting of the shear stress is carried out by the shear of the shaft or the failure of the hole wall.
In a word, friction type high-strength bolt and pressure type high-strength bolt are actually the same kind of bolt, only whether sliding is considered in the design. Friction type high-strength bolt shall not slide, and the bolt shall not bear shear force. Once it slips, the design is considered to be in a state of failure, which is relatively mature in technology; pressure type high-strength bolt can slide, and the bolt also bears shear force, and the final failure is equivalent to that of ordinary bolt (bolt shear or steel plate compression).
High strength bolts are generally used to connect the main components of building structures. Common bolts can be reused, and high-strength bolts can not be reused. High strength bolts are generally used for permanent connections.
The high-strength bolt is a prestressed bolt. The friction type uses a torque wrench to apply the specified prestress, and the pressure type screws off the plum head. Ordinary bolts have poor shear resistance, which can be used in secondary structure. Ordinary bolts only need to be tightened.
High strength bolt connection has the advantages of simple construction, good mechanical performance, detachable and replaceable, fatigue resistance, and no looseness under dynamic load. It is a very promising connection method.
The high-strength bolt is to tighten the nut with a special wrench, so that the bolt produces a huge and controlled pretension. Through the nut and the base plate, the same amount of preload is generated on the connected parts. Under the pre pressure, there will be great friction along the surface of the connected parts. Obviously, as long as the axial force is less than this friction, the components will not slide and the connection will not be damaged. This is the principle of high-strength bolt connection.
High strength bolt connection relies on the friction between the contact surfaces of the connectors to prevent them from sliding. In order to make the contact surfaces have enough friction, it is necessary to increase the clamping force and the friction coefficient of the contact surfaces of the components. The clamping force between components is achieved by applying pretension to the bolt, so the bolt must be made of high-strength steel, which is called high-strength bolt connection.
In the high strength bolt connection, the friction coefficient has a great influence on the bearing capacity. The test shows that the friction coefficient is mainly affected by the form of contact surface and the material of components. In order to increase the friction coefficient of the contact surface, sand blasting and wire brush cleaning are often used to treat the contact surface of components within the connection range. In fact, there are two types of high-strength bolts: friction type and pressure type.
The criterion of shear force of friction type high strength bolt is that the shear force caused by design load does not exceed the friction force. The design criterion of pressure bearing high strength bolt is that the rod body is not sheared or the plate is not crushed.
Source: China Fasteners 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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