HSFG Bolts – What You Need to Know

What Are HSFG Bolts?

High-strength Friction Grip Bolt (HSFG Bolts). It can be seen that the high strength bolt said in construction is the abbreviation of the high strength friction pre-tightening bolt. In daily communication, because of the words “Friction” and “Grip” are abbreviated, which caused the basic definition of a high-strength bolt has been misunderstood by many engineers and technicians.

Misunderstanding 1: A bolt with a material grade exceeding 8.8 is a “high-strength bolt/ HSFG Bolts”?

The core difference between high-strength bolts/ HSFG Bolts and ordinary bolts is not the strength of the material used, but the form of force. The essence is whether to apply preload and use static friction force to resist shear.

In fact, the HSFG BOLT mentioned in the British standard and The American standard only has two kinds of grades 8.8 and 10.9 (BS EN 14399 / ASTM-A325&ASTM-490), while the ordinary bolt contains 4.6, 5.6, 8.8, 10.9, 12.9, etc. (BS 3692, Section 11, Table 2); This shows that the material strength is not the key to distinguish between high strength bolts and ordinary bolts.

Misunderstanding 2: The bearing capacity of high strength bolt is higher than that of the ordinary bolt, is it “high strength”?The calculation of a single bolt shows that the tensile strength and shear strength of high strength bolt is lower than that of an ordinary bolt.

HSFG Bolts manufactured by YIJIN Hardware

Its high-strength essence is:

When working normally, the node is not allowed to slip, that is, the elastic-plastic deformation is small and the node stiffness is large. As you see: In the case of a given design node load, the nodes designed with high strength bolts may not save the number of bolts, but they have small deformation, large stiffness, and high safety reserve. Suitable for the main beam, such as the requirements of the node stiffness of the position, in line with the “strong node, weak member bar” of the basic vibration resistance design principle. The strength of high-strength bolt/HSFG Bolts lies not in the design value of its bearing capacity, but in the stiffness of its design joints, high safety performance, and strong ability to resist damage.

What Is the Difference Between Normal Bolts and HSFG Bolts?

The high-strength bolt is the load that can bear is larger than the common bolt of the same specification.

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

In Terms of Raw Materials

High strength bolts/HSFG Bolts are made of high-strength materials. The screw, nut, and washer of high strength bolts/HSFG Bolts are made of high strength steel, commonly used 45 steel, 40 boron steel, 20 manganese titanium boron steel, 35CrMoA, and so on. Common bolts are usually made of Q235 (equivalent to A3 in the past) steel. High strength bolts/HSFG Bolts are generally made of high-strength steel (45 steel (8.8s), 20MmTiB(10.9s), prestressed bolts, friction type with a torque wrench to apply the specified prestress, pressure type screw off the head of the club. Common bolts are usually made of ordinary steel (Q235) and only need to be tightened.

In Terms of Grade of Strength

High-strength bolts are widely used. Two strength grades of 8.8s and 10.9s are commonly used, of which 10.9 is the majority. Ordinary bolt strength grade is low, generally 4.4, 4.8, 5.6, and 8.8. HSFG Bolts 8.8 is of the same grade as 8.8S. The mechanical properties and calculation methods of ordinary bolts and high-strength bolts are different. The force of the high-strength bolt is first applied in its internal pre-tension P, and then produces friction resistance on the contact surface between the connected parts to bear the external load, while the ordinary bolt is directly bearing the external load.

In Terms of Stress Characteristics

High-strength bolts apply pretension and transfer force by friction. Common bolt connection by shearing bolt rod and the pressure on the hole wall shear, tighten the nut when pressure is small, its influence can be neglected, and high strength bolts, besides its material strength is very high to the bolt exerted a lot of pressure, make connections between components to produce extrusion pressure, which is of great friction, perpendicular to the direction of the screw. Moreover, the pre-pressure, anti-slip coefficient, and steel type all directly affect the bearing capacity of high-strength bolts.

In Terms of Use

High strength bolts/HSFG Bolts are generally used to connect the main components of the building structure. Common bolts can be reused, but high strength bolts/HSFG Bolts cannot be reused. High strength bolts/HSFG Bolts are generally used for permanent connections.

High-strength bolts/HSFG Bolts are prestressed bolts, friction type with a torque wrench to apply the specified prestress, and pressure type screw off the head of the plum. Ordinary bolts have poor shear performance and can be used in secondary structural parts. Ordinary bolts only need to be tightened.

The screw holes of ordinary bolts are not necessarily larger than those of high-strength bolts. In fact, ordinary bolts have smaller holes.

Friction type high-strength bolts rely on friction to transfer load, so the screw and screw hole difference can reach 1.5 ~ 2.0mm.

The force transmission characteristic of bearing type high strength bolt is to ensure that the shear force does not exceed the friction force under normal use, and the friction type high strength bolt is the same. When the load capacity increases, there will be relative slippage between the connecting plates. The connection relies on the shear resistance of the screw and the bearing pressure of the hole wall to transmit the force, which is the same as the ordinary bolt, so the difference between the screw and the hole is slightly smaller, which is 1.0 ~ 1.5mm.

More Specifically

High strength bolt’s connection has the advantages of installation and maintenance simple, good mechanical performance, disassembling, fatigue resistance, and no loosening under dynamic load, which is a promising connection method.

The high-strength bolt is tightened with a special wrench, so that the bolt produces a huge and controlled pre-tension, through the nut and the pad, which also produces the same size of pre-pressure to the connector. Under the effect of pre-pressure, there will be greater friction along the surface of the connected parts. Obviously, as long as the axial force is less than this friction, the components will not slip and the connection will not be damaged. This is the principle of a high-strength bolt connection.

The high-strength bolt connection is prevented from sliding by the friction between the contact surfaces of the connectors. In order to have enough friction on the contact surfaces, the clamping force and friction coefficient of the contact surfaces of the components must be increased. The clamping force between the components is achieved by applying pre-tension to the bolts, so the bolts must be made of high-strength steel, which is why they are called high-strength bolted connections.

In 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 the contact surface and the material of the component. In order to increase the friction coefficient of the contact surface, sandblasting and wire brush cleaning methods are often used to deal with the contact surface of members within the connection range.

Ordinary bolts and high-strength bolts have great differences in construction inspection methods because of their different design stress principle.

The mechanical properties of ordinary bolts of the same grade are slightly higher than those of high-strength bolts, but high-strength bolts have more impact energy acceptance requirements than ordinary bolts.

HSFG Bolts Application Scope

At present, advanced manufacturing represented by large aircraft, large power generation equipment, automobiles, high-speed trains, large ships, and large complete sets of equipment has entered an important direction of development. Thus, fasteners will enter an important stage of development. HSFG bolts are used for important mechanical connections.

Friction type high strength bolt: It is suitable for steel frame structure beam and column connection, solid belly beam connection, heavy crane beam connection of industrial workshop, brake system, and essential structure connection bearing dynamic load.

Bearing type high strength bolt: Can be used for shear connections in static load structures that allow a small amount of sliding or in members that indirectly bear dynamic loads.

Tensile type high strength bolts: The fatigue strength is low when the bolt is under tension. Under dynamic load, the bearing capacity is not easy to exceed 0.6P(P is the allowable axial force of the bolt). Therefore, it is only suitable for use under static loads, such as the flange butt of the compression rod, T-joint, etc.

HSFG Bolts Materials

HSFG Bolts material: 18Cr2Ni4W, 25Cr2MoV steel; Bolt size: M27 ~ M48. Due to this kind of steel is easy to form a layer on the surface of the passivation membrane, and the passivation film will make the bolt can’t get good adhesion to the phosphorus chemical nickel layer, so we have to take special measures to remove membrane first, before and measures should be taken to prevent the regeneration, to guarantee the plating has good adhesion between coating and matrix. At the same time, because of the large size of the HSFG bolt, it is difficult to nickel – phosphorus plating and quality inspection.

HSFG Bolts Processing Technic

Pretreatment Process

Green Rod-Cold-Drawing-Spheroidizing(Soften) Annealing-Mechanical Dephosphorization-Pickling-Cold Drawing-Cold Forging Forming-Thread Processing-Heat Treatment-Examination

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 their materials. Such as improper or incorrect material selection, performance may not meet the requirements, lead to service life shortened, accidents or processing difficulties occur, or high manufacturing costs, so the selection of fastener materials is very important.

Cold forging steel is a kind of fastener steel with high interchangeability produced by the cold forging forming process. Because it is made of metal plastic processing under normal temperature, the deformation of each part is very large, the deformation speed is also high, so the performance of cold forging steel raw material is very strict. On the basis of long-term production practice and user investigation, combined with the characteristics of GB/T6478-2001 “Technical Conditions of Steel for Cold Forging and Cold Extrusion” GB/T699-1999 “High-quality Carbon Structural Steel” and jISG3507-1991 “Carbon Steel rod for Cold Forging Steel”, 9.8 bolt and screw material requirements, for example, determination of various chemical elements. When C content is too high, the cold forming property will decrease. Too low can not meet the mechanical properties of parts, so it is set as 0.25%-0.55%. Si can strengthen ferrite and reduce the cold forming property, and the elongation of the material decreases as Si is less than or equal to 0.30%. S.P. is the impurity element, their existence will produce segregation along grain boundary, resulting in grain boundary embrittlement, damage to the mechanical properties of steel should be reduced as far as possible, P is less than or equal to 0.030%, S is less than or equal to 0.035%. The maximum boron content is 0.005%, although boron can significantly improve the permeability of steel, it can also lead to increased brittleness of steel. High boron content is detrimental to bolts, screws, and studs, which require good comprehensive mechanical properties.

2. Spheroidizing Annealing

When countersunk head screw and hexagon socket head cap bolt are produced by the cold forging process, the original structure of steel will directly affect the forming ability during cold forging process. During cold forging, the plastic deformation of the local area can reach 60%-80%, so the steel must have good plasticity.

When the chemical composition of steel is constant, the microstructure is the key factor to determine the plasticity of steel. It is generally believed that the coarse flake pearlite is not conducive to cold forging forming, while the fine spherical pearlite can significantly improve the plastic deformation ability of steel.

3. Phosphorous Removal

The process of removing iron oxide plate from cold heading steel rod is stripping, dephosphorization, mechanical dephosphorization, and chemical pickling. In the chemical pickling procedure,  replace wire rod processing with mechanical dephosphorization, which increases productivity and reduces environmental pollution.  This phosphorus removal process includes bending method (commonly used with triangular groove round wheel repeatedly bending rod), sandblasting method, etc., which phosphorus removal effect is good, but can not make the residual iron and phosphorus to clean (iron oxide clearance rate is 97%), especially when the adhesion of iron oxide is strong, so the mechanical dephosphorization is affected by the thickness, structure and stress state of the iron sheet and is used for carbon steel rods for low strength fasteners (grade 6.8 or less). High strength bolts (greater than or equal to grade 8.8) with wire rod after mechanical dephosphorization, to remove all the oxide sheet, and then through chemical pickling process, namely compound dephosphorization. For low carbon steel wire rods, the residual iron sheet from mechanical dephosphorization is easy to causes uneven wear of grain draft. When the grain draft hole due to the wire rod friction temperature adhesion on the iron sheet, so that the surface of the wire rod longitudinal grain mark, wire cold heading flange bolt or cylindrical head screw, the head of the cause of micro cracks, more than 95 % is the wire surface in the process of drawing the scratch caused by. Therefore, the mechanical phosphorus removal method is not suitable for high-speed drawing.

 4. Cold Drawing

The wire drawing process has two purposes, one is to change the size of raw materials; The second is through the impact of deformation strengthening fasteners to obtain the basic mechanical properties, for carbon steel, carbon alloy steel has a purpose, that is, to make the strip cooling after the sheet cementite in the drawing process as far as possible to crack, for the subsequent spheroidization (softening) annealing to get granular cementite ready. However, in order to reduce the cost,  some manufacturers reduce arbitrarily pull out of the path times, which increased the work hardening tendency of wire and directly affected the cold heading performance of wire rods. If the reduction rate distribution of each pass is not appropriate, it will also cause torsion cracks in the drawing process of the wire rods, such as longitudinal distribution along the wire, and a certain period of cracks in the cold heading process exposed. In addition, if lubrication is not good in the drawing process, it can also cause the regular occurrence of transverse cracks in cold drawing wire. When the tangential direction of wire steel is different from that of wire drawing die, the wear of a single hole of wire drawing die is intensified, so that the inner hole is not round, and the deformation of wire circumferential direction is not uniform, so that the roundness of wire is out of order, and the cross section stress of wire is not uniform in the process of cold heading, and the qualification rate of cold heading is affected. Wire rod steel wire drawing process, the excessive part of the ratio of surface to make the surface of the steel wire quality deterioration, and a low ratio of the surface is not conducive to the lamellar cementite and broken, hard to get as much as possible of granular cementite, namely cementite spheroidization rate is low, is the performance of the cold heading steel wire, the drawing method of bar and wire rod steel wire production. Part of the surface reduction straightness control in the range of 10 %- 15 %.

5. Cold Forging Forming

Usually, the forming of the bolt head using cold heading plastic processing, compared with the cutting process, metal fiber (metal wire) along the shape of the product was continuous, no cut off in the middle, thus improving the strength of the product, especially excellent mechanical properties. Cold heading forming technology includes cutting and forming, single click, double-click cold heading, and multi-station automatic cold heading. An automatic cold heading machine carries out multi-station processes such as stamping, upsetting, extrusion, and diameter reduction in several forming dies. The processing characteristics of the raw blank used by the single-position or multi-position 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 is that the cold heading forming is not a pre-cut single blank. Instead, the blank is cut and upsetting (if necessary) by the automatic cold heading machine itself from rods and wire. The blank must be shaped before the core can be extruded. The blank can be obtained by shaping. The blanks need not be shaped before upsetting,  hole shrinkage, and forward extrusion. After the blank is cut off, it is sent to the upsetting shaping station. The station can improve the quality of the blank, can make the next station reduce the forming force by 15-17 %, and can prolong the life of the mold, and manufacturing bolts can be used for multiple diameter reduction. The precision of cold heading forming is also related to the selection of forming method and the working procedure used. In addition, it also depends on the structural characteristics of the equipment used, the process characteristics and its state, the precision of the tooling, service life, and wear degree. For high alloy steel used in cold heading forming and extrusion, the working surface roughness of hard alloy die should not be larger than Ra=0.2um. When the working surface roughness of this surface reaches Ra=0.025-0.050um, it has the highest service life.

6. Thread Processing

Bolt thread generally uses cold processing, so that the thread is blank within a certain diameter through the thread rolling die, by the thread rolling die pressure to shape the thread. Due to the thread part of the plastic streamline is not cut off, increased strength, high precision, and uniform quality of the product, so it is widely used. In order to produce the outer diameter of the thread for the final product, the blank diameter of the thread is required to be different because it is limited by factors such as thread precision and whether the material is coated or not. Rolling thread is a machining method that uses plastic deformation to form thread teeth. It is with the same pitch and thread processing thread rolling (screw plate) mold while extruding cylindrical screw billet while making the screw billet rotation, and finally rolling mold on the tooth transfer to screw billet, so that the thread forming. The common point of rolling pressing thread processing is that the number of rolling revolutions should not be too much. If too much, the efficiency is low, and the surface of thread teeth is easy to produce separation phenomenon or disorderly buckle phenomenon. On the contrary, if the number of revolutions is too little, the diameter of the thread is easy to lose the circle, and the initial pressure of rolling is abnormal, resulting in a shortened die life. Common defects of rolling thread: thread part surface crack or tear; Disorderly buckle; Thread part out of round. If these defects occur in large numbers, they will be discovered during the processing stage. If they occur in small numbers, these defects can flow unnoticed by the production process and cause problems for users. Therefore, it is necessary to summarize the key problems of processing conditions and control these key factors in the production process.

7. Processing

High-strength fasteners should 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 flexural strength ratio of the products.

The heat treatment process has a crucial influence on the inner quality of high-strength fasteners, especially it’s quality. Therefore, to produce superior quality high strength fasteners, it is necessary to have advanced heat treatment technology and equipment.

Due to the high strength bolts’ large production, low price, and the threaded part’s more subtle and relatively precise structure, therefore, the requirements of heat treatment equipment must have a large production capacity, a high degree of automation, and good heat treatment quality ability.

Since the 1990s, the continuous heat treatment production line with a protective atmosphere has been dominant. The vibrating bottom type and mesh belt furnace are especially suitable for heat treatment and tempering of small and medium-sized fasteners. In addition to good furnace sealing performance, the quenching and tempering line also has advanced computer control of atmosphere, temperature, and process parameters, equipment failure alarm, and display functions.

High strength fasteners from feeding – cleaning – heating – hardening – cleaning – tempering – coloring to offline, all automatic control operations, effectively ensure the quality of heat treatment. Thread decarbonization will lead to the fastener the failure to meet the mechanical properties of the resistance requirements of the first trip, the failure of thread fasteners, and shorten the service life. Because of the decarbonization of raw materials, if the annealing is not proper, it will deepen the decarbonization layer of raw materials. In the process of quenching and tempering heat treatment, some oxidizing gas is usually brought in from outside the furnace. The rust of bar steel wire or the residue on the surface of the wire rod after the cold drawing will decompose after heating 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 CO₂ and H₂O after heating, thus aggravating the decarbonization. Study shows that the degree of decarbonization of medium carbon alloy steel is more serious than carbon steel, and the fastest decarbonization temperature is between 700 and 800 degrees Celsius. Because the attachment on the surface of the steel wire decomposes and combines into carbon dioxide and water at a fast speed under certain conditions, if the gas control of the continuous mesh belt furnace is improper, it will also cause the screw decarbonization out of tolerance. When the high-strength bolt is formed by cold heading, the decarburization layer of raw material and annealing not only still exists, but is extruded to the top of the thread. For the fastener surface that needs quenching, the hardness required is not obtained, and its mechanical properties (especially strength and wear resistance) are reduced.

In addition, the surface decarbonization of steel wire, surface, and internal structure is different and has different expansion coefficients, hardening may produce surface cracks. Therefore, to protect the thread at the top of the decarburization in heat quenching, but also for raw materials that have been moderately coated carbon decarburization of fasteners, turn the advantage of the mesh belt furnace protective atmosphere in the basic equations to the original carbon content and carbon coating parts, already decarburization fasteners slowly back to the original carbon content, carbon potential is set in 0.42% 0.48% advisable, Carbon coating temperature is the same as quenching heating, can not be carried out at a high temperature, in order to avoid coarse grain, affect mechanical properties. The quality problems of fasteners during quenching and tempering are mainly as follows: Quenching hardness is insufficient; Uneven hardness in the quenched state; Quenching deformation out of tolerance; Quenching cracking. Such problems on site are often related to raw materials, quenching heating, and quenching cooling. Correctly formulating the heat treatment processes and standardizing the production operation processes can avoid such quality accidents.

8. Inspection

To sum up, the technological factors affecting the quality of high-strength fasteners include steel design, spheroidizing annealing, shell stripping, phosphorus removal, drawing, cold heading forming, thread processing, heat treatment, and other aspects, and sometimes it is the superposition of all kinds of factors.

HSFG Bolts Processing Notes

①Surface rust, oil, burr on the bolt hole wall, and welding flash should be cleaned up.

②The contact friction surface should meet the requirement of a scratch-resistant coefficient after treatment. The high-strength bolts should have matching nuts and washers, which should be used in accordance with matching and shall not be interchanged.

③The processed component friction surface is not allowed to be stained with oil, soil, and other sundries during installation.

④The friction surface of the component should be kept dry during installation and should not be operated in the rain.

⑤Strictly check and correct the deformation of the connected steel plate before installation.

⑥Do Do not hammer into the bolt during installation to prevent damage to the bolt screw.

⑦The electric spanner should be tested regularly to ensure the accuracy of torque, and according to the correct tightening sequence operation.

Main Safety Technical Measures

①The size of the opening of the adjustable wrench should be consistent with the size of the nut. Do not use a sleeve on the small wrench. A spanner should be used for high-altitude operations. For example, when using an adjustable wrench, use a rope to fasten it, and people must wear a seat belt.

②When assembling the connecting bolts of steel members, it is forbidden to insert the connecting surface or probe the screw hole by hand. When taking and placing the pad iron plate, fingers should be placed on both sides of the pad iron plate.

Conclusion

With the development of automobiles, machinery, energy, light industry, construction, and other industries, the material of all kinds of bolts is demanded, the material not only has a lower cost but also has lighter weight materials, higher strength, better toughness, excellent corrosion resistance, long life, and easy processing. So it has great significance to study the steel of high-strength bolts and to develop the high-strength bolts with better properties.

We can provide custom HSFG bolts and parts with diverse specifications. Pls get in touch with our experts via phone on  0755-23502273. Alternatively, you can send us an email with your requirements and queries at info@yijinsolution.com

Thank you for reading.

Leave a Reply

Get a quote
Email
Phone