Sheet Metal · 2022年4月13日

High-strength Bolts-bolts That Require A Larger Pre-tightening Force

High-strength bolts can bear a larger load than ordinary bolts of the same specification.

The material of ordinary bolts is made of Q235 (that is, A3). The material of high-strength bolts is 35# steel or other high-quality materials. After being made, heat treatment is carried out to improve the strength.

The difference between the two is the difference in material strength.

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 all made of high-strength steel, commonly used 45 steel, 40 boron steel, 20 manganese titanium boron steel, 35CrMoA, etc. Ordinary bolts are usually made of Q235 (equivalent to A3 in the past) steel.

From the perspective of strength level: high-strength bolts are increasingly widely used. Two strength levels of 8.8s and 10.9s are commonly used, of which 10.9 is the majority. The strength grade of ordinary bolts is lower, generally 4.4, 4.8, 5.6 and 8.8.

High-strength bolts

From the point of view of the force characteristics: high-strength bolts exert pre-tension and transmit external force by friction. Ordinary bolt connections rely on bolt shear resistance and hole wall pressure to transmit shear force. The pre-tension force generated when the nut is tightened is very small, and its effect can be ignored. In addition to its high material strength, high-strength bolts also impose a large amount on the bolt. The pre-tension force produces squeezing force between the connecting components, so that there is a great friction force perpendicular to the screw direction, and the pre-tension force, the anti-slip coefficient and the type of steel directly affect the bearing capacity of the high-strength bolt. According to the characteristics of the force, it is divided into pressure type and friction type. The two calculation methods are different. The minimum specification of high-strength bolts is M12, and M16~M30 are commonly used. The performance of oversized bolts is unstable and should be used with caution in the design.

The difference between high-strength bolt friction type and pressure-bearing type connection:

High-strength bolt connection is to clamp the plate of the connecting plate through the large tightening pretension in the bolt rod, which is enough to generate a large friction force, thereby improving the integrity and rigidity of the connection. When subjected to shear force, according to the design and The difference in force requirements can be divided into high-strength bolt friction type connection and high-strength bolt pressure-bearing type connection. The essential difference between the two is that the limit state is different. Although they are the same type of bolts, they are calculated in terms of calculation methods, requirements, and scope of application. All are very different. In the shear design, the high-strength bolt friction type connection is the limit state when the external shear force reaches the maximum possible friction force provided by the bolt tightening force between the contact surfaces of the plate, which is to ensure that the internal and external shear force does not exceed the connection during the entire use period. Maximum friction. The plates will not undergo relative sliding deformation (the original gap between the screw and the hole wall is always maintained), and the connected plates will be stressed as a whole by elasticity. In the shear design, the external shear force is allowed to exceed the maximum friction force in the high-strength bolt pressure-bearing connection. At this time, relative sliding deformation occurs between the connected plates until the bolt rod contacts the hole wall, and then the connection depends on the bolt rod. Shaft shear and hole wall pressure and the friction between the contact surfaces of the plate jointly transmit force, and finally the shaft shear or hole wall pressure failure is regarded as the limit state of the connection under shear. In short, friction-type high-strength bolts and pressure-bearing high-strength bolts are actually the same kind of bolts, except that slippage is considered in the design. Friction-type high-strength bolts must not slide, and the bolts do not bear shearing force. Once slipped, the design is considered to have reached a failure state, which is technically mature; pressure-bearing high-strength bolts can slide, and the bolts also bear shearing force. The final damage is equivalent to ordinary Bolt damage (bolt shearing or steel plate crushing).

From the point of view of use:

High-strength bolts are generally used for the bolt connection of the main components of the building structure. 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 pre-stressed bolts. The friction type uses a torque wrench to apply the specified pre-stress, and the pressure-bearing type unscrews the torx head. Ordinary bolts have poor shear resistance and can be used in secondary structural parts. Ordinary bolts only need to be tightened.

Ordinary bolts are generally 4.4, 4.8, 5.6 and 8.8. High-strength bolts are generally 8.8 and 10.9, of which 10.9 is the majority.

Level 8.8 is the same level as 8.8S. The force performance and calculation methods of ordinary bolts and high-strength bolts are different. The force of high-strength bolts is first applied to the interior of the pretension P, and then friction resistance is generated on the contact surface between the connected parts to withstand the external load, while ordinary bolts directly bear the external load.

More specifically:

High-strength bolt connection has the advantages of simple construction, good mechanical performance, detachability, fatigue resistance, and no loosening under dynamic load, and it is a promising connection method.

High-strength bolts use a special wrench to tighten the nut, so that the bolt generates a huge and controlled pretension. The nut and the backing plate also produce the same amount of pretension on the connected parts. Under the action of pre-pressure, a relatively large friction force will be generated along the surface of the connected part. Obviously, as long as the axial force is less than this friction force, the component will not slip and the connection will not be damaged. This is a high-strength bolt connection. The principle.

High-strength bolt connections rely on the friction between the contact surfaces of the connecting parts to prevent them from sliding. In order to make the contact surfaces have sufficient friction, the clamping force of the components must be increased and the friction coefficient of the contact surfaces of the components must be increased. The clamping force between the components is achieved by applying pretension to the bolts, so the bolts must be made of high-strength steel, which is why it is called a high-strength bolt connection.

In the high-strength bolt connection, the friction coefficient has a great influence on the bearing capacity. Tests show that the friction coefficient is mainly affected by the form of the contact surface and the material of the component. In order to increase the friction coefficient of the contact surface, methods such as sandblasting and wire brush cleaning are often used to treat the contact surface of the components in the connection range.

Today's advanced manufacturing represented by large airplanes, large power generation equipment, automobiles, high-speed trains, large ships, and large complete sets of equipment has entered an important development direction. As a result, fasteners will enter an important stage of development. High-strength bolts are used for the connection of important machinery. Repeated disassembly or various installation torque methods require high-strength bolts. Therefore, the surface condition and thread accuracy will directly affect the service life and safety of the host. In order to improve the coefficient of friction and avoid rust, seizure or seizure during use, the technical requirements stipulate that the surface should be treated with nickel-phosphorus plating. The thickness of the coating is guaranteed to be within the range of 0.02~0.03mm, and the coating is uniform, dense and free of pinholes.

Bolt material: 18Cr2Ni4W, 25Cr2MoV steel; bolt specification: M27~M48. Since this type of steel is easy to form a passivation film on the surface, and this passivation film will prevent the bolt from obtaining a chemical nickel-phosphorus layer with good adhesion, special pretreatment measures must be taken to remove the film first, and measures should be taken To prevent it from regenerating, it is possible to ensure a good bonding force between the plated layer and the substrate after plating. At the same time, due to the large geometric size of the bolt, the quality inspection of the nickel-phosphorus plating treatment and the process increases the difficulty.

Processing technology

Hot-rolled wire rod-(cold drawing)-spheroidizing (softening) annealing-mechanical descaling-pickling-cold drawing-cold forging forming-thread processing-heat treatment-inspection.

1. Steel design

In fastener manufacturing, the correct selection of fastener materials is an important part, because the performance of fasteners is closely related to their materials. If the material is selected incorrectly or incorrectly, the performance may not meet the requirements, the service life may be shortened, accidents or processing difficulties may occur, and the manufacturing cost may be high. Therefore, the selection of fastener materials is a very important link. Cold heading steel is steel for fasteners with high interchangeability 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 it withstands is also high. Therefore, the performance requirements of the cold heading steel raw materials are very strict. On the basis of long-term production practice and user research, combined with GB/T6478-2001 "Technical Conditions of Cold Heading and Cold Extrusion Steel" GB/T699-1999 "High-quality Carbon Structural Steel" and the target JISG3507-1991 "Cold Heading" The characteristics of "Carbon Steel Wire Rod for Steel", take the material requirements of grade 8.8 and grade 9.8 bolts and screws as examples, and determine various chemical elements. If the C content is too high, the cold forming performance will be reduced; if it is too low, it will not meet the requirements of the mechanical properties of the parts, 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 has a tendency to promote the growth of austenite grains during quenching and tempering of parts, so it is appropriately increased on an international basis. It is 0.45%-0.80 %. Si can strengthen ferrite and promote the decrease of cold forming performance. The decrease of material elongation is defined as Si less than or equal to 0.30%. S.P. are impurity elements, their presence will cause segregation along the grain boundary, leading to embrittlement of the grain boundary and damage to the mechanical properties of the steel. They should be reduced as much as possible. P is less than or equal to 0.030%, and S is less than or equal to 0.035%. B. The maximum boron content is 0.005%, because although boron has the effect of significantly improving the permeability of the steel, it will also increase the brittleness of the steel. Excessive boron content is very unfavorable for 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, the original structure of the steel will directly affect the forming ability during cold heading. The plastic deformation of the local area in the cold heading process can reach 60%-80%. For this reason, the steel must have good plasticity. When the chemical composition of the steel is constant, the metallographic structure is the key factor in determining the plasticity. It is generally believed that the coarse 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 carried out before cold heading in order to obtain uniform and fine spheroidized pearlite to better meet the actual production needs. For the softening annealing of medium carbon steel wire rod, the heating temperature is usually selected at the upper and lower critical points of the steel. The heating temperature is generally not too high. Otherwise, tertiary cementite will precipitate along the grain boundary, causing cold heading cracking. The medium-carbon alloy steel wire rod is annealed by isothermal spheroidization. After heating in 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 air-cooled. The metallographic structure of the steel changes from coarse to fine, from flake to spherical, and the cold heading cracking rate will be greatly reduced. The general softening annealing temperature of 3545ML35SWRCH35K steel is 715-735 degrees Celsius; while the general heating temperature of SCM43540CrSCR435 steel spheroidizing annealing is 740-770 degrees Celsius, and the isothermal temperature is 680-700 degrees Celsius.

3. Peeling and descaling

The process of removing iron oxide plate from cold heading steel wire rod is stripping and descaling. There are two methods: mechanical descaling and chemical pickling. Using mechanical descaling to replace the chemical pickling process of wire rods not only improves productivity, but also reduces environmental pollution. This descaling process includes bending method (the round wheel with triangular groove is generally used to repeatedly bend the wire rod), spraying nine method, etc., the descaling effect is better, but the residual iron scale cannot be removed (the removal rate of oxide scale is 97% ), especially when the adhesion of the iron oxide scale is strong. Therefore, mechanical descaling is affected by the thickness, structure and stress state of the iron scale. It is used for carbon steel wire rods for low-strength fasteners (less than or equal to 6.8). After mechanical descaling for high-strength bolts (greater than or equal to 8.8), the wire rod is used to remove all the iron oxide scale, and then undergo a chemical pickling process that is compound descaling. For low-carbon steel wire rods, the iron sheet left by mechanical descaling is likely to cause uneven wear of the grain draft. When the grain draft hole adheres to the iron sheet due to the external temperature of the wire rod wire friction, the surface of the wire rod wire produces longitudinal grain marks. When the wire rod wire is cold-headed flange bolts or cylinder head screws, the cause of the micro cracks on the head, More than 95% is caused by scratches on the wire surface during the drawing process. Therefore, mechanical descaling is not suitable for high-speed drawing.

4. Drawing

The drawing process has two purposes. One is to change the size of the raw material; the other is to obtain 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 wire rod The flake cementite obtained after controlled cooling is cracked as much as possible during the drawing process to prepare for the subsequent spheroidization (softening) annealing to obtain granular cementite. However, some manufacturers arbitrarily reduce the drawing to reduce the cost. For passes, the excessive reduction in surface area increases the work hardening tendency of the wire rod wire and directly affects the cold heading performance of the wire rod wire. If the distribution of the reduction ratio of each pass is not appropriate, it will also cause torsion cracks in the wire rod wire during the drawing process. This kind of cracks distributed along the longitudinal direction of the wire and with a certain period are exposed during the cold heading process of the wire. In addition, if the lubrication is not good during the drawing process, it can also cause the cold drawn wire rod wire to regularly appear transverse cracks. The tangential direction of the wire rod wire exiting and winding die mouth is not concentric with the wire drawing die, which will increase the wear of the unilateral pass of the wire drawing die, make the inner hole out of round, and cause uneven drawing deformation in the circumferential direction of the wire. The roundness of the steel wire is too poor, and the cross-sectional stress of the steel wire is uneven during the cold heading process, which affects the cold heading qualification rate. During the drawing process of wire rod wire, the excessive surface reduction rate deteriorates the surface quality of the steel wire, while the excessively low surface reduction rate is not conducive to the crushing of flake cementite, and it is difficult to obtain as much granular cementite as possible. , That is, the low spheroidization rate of cementite is extremely unfavorable to the cold heading performance of the steel wire. The bar and wire rod steel wire produced by the drawing method has a partial surface reduction rate directly controlled within the range of 10%-15%.

5. Cold forging forming

Usually, the bolt head is formed by cold heading plastic processing. Compared with cutting processing, the metal fiber (metal wire) is continuous along the shape of the product without cutting in the middle, thus improving the strength of the product, especially the 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 feature of the original blank used by the single-station or multi-station automatic cold heading machine is that the material size is 5-6 meters long.

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