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What is a bolt

What is a bolt?

Bolt: mechanical parts, cylindrical threaded fasteners with nuts. A kind of fastener composed of head and screw (cylinder with external thread), which is used to connect two parts with through hole. This kind of connection is called bolt connection. The bolt can be separated from the connecting part by screwing down.

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Classification of bolts

According to the stress mode of connection: common and reamed. According to the head shape: hexagonal head, round head, square head, countersunk head and so on. The hexagonal head is the most commonly used. Generally, countersunk head is used where connection is required.
The English name of riding bolt is U-bolt. It is a non-standard part, and its shape is U-shaped, so it is also called U-bolt. There are threads at both ends, which can be combined with nuts. It is mainly used to fix tubular objects, such as water pipes or flakes, such as leaf springs of cars. Because its way of fixing objects is like riding on a horse, it is called riding bolt. According to the thread length, it can be divided into full thread and non full thread.
According to the thread profile, it can be divided into coarse thread and fine thread. The coarse thread profile is not shown in the mark of bolt. According to the performance grade, bolts are divided into 8 grades: 3.6, 4.8, 5.6, 6.8, 8.8, 9.8, 10.9 and 12.9. Among them, bolts above grade 8.8 (including grade 8.8) are made of low carbon alloy steel or medium carbon steel and heat treated (quenching + tempering), which are generally called high strength bolts, while bolts below grade 8.8 (excluding grade 8.8) are generally called ordinary bolts.
According to the manufacturing accuracy, ordinary bolts can be divided into three grades A, B and C. grade A and B are refined bolts, and grade C is rough bolts. For connecting bolts used in steel structure, unless otherwise specified, they are generally common rough C-grade bolts. There are differences in the processing methods of different levels, and the corresponding processing methods are usually as follows:

  • ① the bolt rod of Grade A and B bolts is machined by lathe, with smooth surface and accurate size, and its material performance grade is 8.8, which is difficult to manufacture and install, and the price is high, so it is rarely used;
  • ② grade C bolts are made of unprocessed round steel, and the size is not accurate enough, and its material performance grade is 4.6 or 4.8. Shear connection has large deformation, but it is easy to install and has low production cost. It is mostly used for tensile connection or temporary fixation during installation.

Application of bolt

There are many names for bolts. Everyone may call them different. Some people call them screws, some people call them bolts and some people call them fasteners. Although there are so many names, but the meaning is the same, are bolts. Bolt is a common term for fastener. Bolt is a tool that uses the physical and mathematical principles of oblique circular rotation and friction force of objects to fasten the mechanical parts step by step.
Bolts are indispensable in daily life and industrial production. Bolts are also known as industrial rice. It can be seen that bolts are widely used. The application scope of bolts are: electronic products, mechanical products, digital products, electrical equipment, mechanical and electrical products. Bolts are also used in ships, vehicles, hydraulic engineering, and even chemical experiments. Anyway, bolts are used in many places. Such as precision bolts used in digital products. DVD, cameras, glasses, clocks, electronics and other micro bolts; television, electrical products, musical instruments, furniture and other general bolts; as for engineering, construction, bridges use large bolts, nuts; transportation equipment, aircraft, trams, cars and other large and small bolts. Bolts have important tasks in industry. As long as there is industry on the earth, the function of bolts will always be important.

Shape of bolt

Generally, the countersunk head is used in the place where the surface after connection is required to be smooth without protrusion, because the countersunk head can be screwed into the parts. The round head can also be screwed into the part. The tightening force of square head can be larger, but the size is large. In addition, in order to meet the needs of locking after installation, there are holes in the head and in the rod, which can prevent the bolts from loosening when they are subject to vibration. Some bolts have no thread of light rod to do thin, called thin waist bolt. This kind of bolt is beneficial to the connection under variable force. There are special high-strength bolts on the steel structure, the head will be larger, and the size will also change. In addition, there are special uses: T-shaped slot bolt, most of which are used in machine tool fixture, with special shape, and both sides of the head should be cut off. Anchor bolts are used to connect the machine and the ground. There are many shapes. U-bolt, as previously mentioned. wait. There are also special studs for welding, one with thread and the other without thread, which can be welded on the parts, and the other side can directly screw the nut.

Stress mode of bolt

Common and reamed. Ordinary bearing mainly axial force, can also bear the requirements of the lateral force is not high. The bolt used for reaming should match the size of the hole and be used when subjected to transverse force.

Grade meaning of bolts

According to relevant standards, the performance grades of carbon steel and alloy steel bolts 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), which are generally called high strength bolts, and the rest are called ordinary bolts. There are two parts in the performance grade of bolt, which are the nominal tensile strength value and yield strength ratio of bolt material. Stainless steel bolts can be divided into A1-50, A1-70, A1-80, A2-50, A2-70, A2-80, A3-50, A3-70, A3-80, A4-50, A4-70, A4-80, A5-50, A5-70, A5-80, C1-50, C1-70, C1-110, C4-50, C4-70, C3-80, F1-45, F1-60 according to the performance grade of steel. The first letter and number represent the stainless steel group, and the second and third numbers represent 1 / 10 of the tensile strength.

Inspection of bolts

Bolt detection is divided into manual and machine. Manual is the most primitive and the most common way of consistent detection. In order to reduce the outflow of defective products as much as possible, the general production enterprise personnel visually inspect the packaged or delivered products to eliminate the defective products (including tooth injury, mixing, rust, etc.).
Another way is automatic machine testing, mainly magnetic particle testing. Magnetic particle inspection is based on the interaction of magnetic flux leakage field and magnetic powder at the defect of bolt. In view of the difference between the permeability of the defect (such as crack, slag inclusion, mixture, etc.) and the permeability of steel, the magnetic field at the discontinuity of these materials will change after magnetization, forming part of the magnetic flux leakage field on the workpiece surface, thus attracting the magnetic powder to form the magnetic powder pile at the defect Under proper illumination conditions, the position and shape of the defects can be seen. By observing and explaining the accumulation of these magnetic particles, the purpose of eliminating defective products has been achieved.

How to make high quality bolt?

To make a bolt, we need to design a bolt first.

If you want to do a good job, you should first define the characteristic requirements. Here is the technical requirements. How to define the technical requirements is really very difficult. You must know the relationship between the functional requirements and the technical requirements. The technical requirements come from the functional requirements, and the functional requirements come from the requirements of the users or the use environment or other external conditions. Functional requirements analysis can be carried out by using quality function deployment (also known as house of quality) technology, which is not introduced here.
How can functional requirements be transformed into technical requirements? This requires a certain amount of technical accumulation. To give two simple examples, if you want to make the fatigue strength of bolts better, you need to reasonably design the transition fillet of bolt head, the fillet of bolt root, the transition of thread and screw, and then reflect these designed information on the design drawings or technical requirements. For another example, in practical application, bolts are required not to be corroded for a long time in a slightly corrosive environment, so you need to select the appropriate surface treatment or bolt material according to the corrosion environment and corresponding standards to meet the requirements of the application environment.
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In addition, the bolt performance grade, thread tolerance grade, dimensional accuracy grade, surface treatment method are all factors to be considered in the design, and even the sequence of some processes also need to be defined in the design (such as thread rolling and quenching and tempering, the reverse sequence has a certain impact on the fatigue strength of the thread).

In order to make the bolt meet the application needs, the material should be selected.

In fact, when designing bolts, the materials should have been selected. Here, I will say it separately because the author thinks that the materials are really important. Most bolts are made of ordinary carbon steel or alloy steel, which is a common case. Bolts may be used in highly corrosive environment or environment with large temperature variation, or extreme temperature environment, so ordinary carbon steel or alloy steel is not suitable, For example, ordinary carbon steel materials will become brittle at extremely low temperature, which is easy to produce brittle fracture without warning. If it is used in high temperature environment, ordinary carbon steel will produce creep, that is to say, when the thread is not loose, the bolt preload will be reduced, which is easy to cause loosening or fatigue failure.
In addition, some bolts also require high creep resistance, good stress corrosion resistance, oxidation resistance and low notch sensitivity.

The design material is ready and the process sequence is very important.

How is the bolt made? Generally, in mass production, the whole coil rod is used, and then it is straightened and cut to the required length. Some processes can be changed according to the production equipment layout of the manufacturer, but some processes cannot be adjusted. If the process is adjusted, the product quality may be affected or the product performance may be reduced.
An important basis for determining process sequence is to improve production efficiency and reduce production cost without affecting product performance or ensuring product quality.
Which process sequence adjustment will affect the product quality, such as heat treatment and rolling thread. If the bolt has no special anti fatigue performance requirements, rolling thread after heat treatment or rolling thread before heat treatment is no problem, but if the fatigue strength requirements are high, rolling thread after heat treatment is necessary.

  • Disk element – straightening – annealing – peeling oxide skin – pier head – heat treatment – thread rolling – surface treatment – Lubrication – packaging

Upsetting bolt head

Cold heading process is the preferred process for fastener head forming. Cold heading process has the characteristics of high productivity, high material utilization, high surface quality and internal quality. When the steel bar is cut to the required length, it can be heading. The heading can be cold forming or hot forming. Generally, the smaller bolts are formed by cold upsetting, and the larger bolts are formed by hot upsetting.
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The forged bolt head has continuous metal fiber, high strength and better fatigue resistance.

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Heat treatment of bolts

Heat treatment is one of the important processes in the manufacture of high-strength bolts. Its purpose is to improve the comprehensive mechanical properties of fasteners. Heat treatment process has a very important impact on the internal quality of fasteners, especially high-strength bolts.
With the development of the times, the protective continuous heat treatment production line has occupied a dominant position since 1990s. The device not only has excellent sealing performance, but also can accurately control the temperature and process parameters by computer, and has the function of equipment fault alarm and display. In the whole process of quenching and tempering heat treatment, some oxidation gas will be produced outside the furnace. Through a series of chemical changes and corrosion on the surface of high strength bolt, the material of high strength bolt will decarbonize itself. According to technicians, decarburization of medium carbon alloy steel is more serious than that of carbon steel, and the fastest decarburization temperature is between 700 ℃ and 800 ℃.
Decarburization is one of the failure factors that need to be prevented in the thermal care stage of bolts. Decarburization will cause local strength reduction, and micro cracks are easy to occur in the stress of bolts, and then develop into fatigue failure.
Carburization on the surface of the thread will increase the strength and decrease the plasticity, resulting in delayed crack or fracture.
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If the gas is not well controlled, the screw decarbonization will be out of tolerance. In the cold upsetting process of high strength bolt, the decarburization layer of raw material not only exists, but also extrudes to the top of high strength bolt thread. At this time, the surface of high-strength bolt that needs quenching does not need the pre-designed hardness, and the mechanical properties of high-strength bolt will be greatly reduced.
In a broad sense, the carbon potential of furnace gas refers to the equilibrium state of chemical reaction between furnace atmosphere and steel with certain carbon content at a certain temperature. The carbon potential of the furnace gas depends on the composition of the furnace gas, the temperature of the furnace gas, and the conditions of catalysis and catalyst in the furnace. For the shielding gas, the carbon potential of the furnace gas should be equal to or slightly higher than that of the steel itself, so that the steel will not be oxidized and decarburized during heating.
In order to make the carbon potential have a favorable comprehensive effect on the carburizing process, the carbon potential of furnace gas can be controlled by stages, that is, the carbon potential can be increased as much as possible at the beginning stage, because the carbon absorption capacity of the workpiece is very strong at this time, it is not easy to produce carbon black, and the high carbon potential can make the concentration gradient on the surface of the carburizing layer larger, which is conducive to diffusion and improve the carburizing rate. In the second stage, the carbon potential should be appropriately reduced, because the surface layer has established a higher concentration gradient and reached a certain depth of carburizing layer, at this time, the carbon absorption capacity of the workpiece surface has begun to decline, so the carbon potential should not be too high to avoid carbon black. The third stage is the diffusion stage. The carbon potential in the furnace can be further reduced to make the carbon concentration gradient and the depth of the surface layer meet the technological requirements.

Rolling thread of bolts

Before rolling the thread, the part of the thread to be rolled should be turned to the pitch diameter of the thread.
The thread is formed by the pressure of the screw plate (rolling die). The thread formed by rolling process has the advantage that the plastic metal fiber streamline of the thread part is not cut off, so the bolt strength is increased, the precision is high, and the quality is uniform, so the rolling process is widely used.
In order to make the thread outer diameter of the final product, the required thread blank diameter is different, because it is limited by the thread precision, whether the material has coating and other factors. For example, if the thread needs to be hot-dip galvanized surface treatment in the future, the influence of surface treatment on the final size of the thread should be considered in the thread processing, and the rolling thread needs to be advanced.
Rolling (rolling) and pressing thread is a kind of processing method which uses plastic deformation to form thread. It uses a rolling die with the same pitch and profile as the thread to be processed to extrude the cylindrical screw blank and make the screw blank rotate at the same time. Finally, the profile on the rolling die is transferred to the screw blank to form the thread.
The common point of thread rolling (rolling) and pressing is that the number of rolling revolutions does not have to be too much. If it is too much, the efficiency is low, and the thread surface is easy to produce separation phenomenon or random thread phenomenon. On the contrary, if the number of turns is too small, the thread diameter is easy to be out of round, and the initial rolling pressure is abnormally increased, resulting in shortened die life.
Common defects of rolling thread: surface crack or scratch of thread part; disorderly thread; out of roundness of thread part. If a large number of these defects occur, they will be found in the processing stage. If the number of defects is small and the production process does not pay attention to these defects, they will flow to users and cause trouble. Therefore, we should sum up the key problems of processing conditions and control these key factors in the production process.
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Surface treatment of bolts

There are many materials used in the production of bolts, such as carbon steel, alloy steel, and stainless steel. But the most common and economical material is carbon steel. Carbon steel itself has no anti-corrosion ability, so it needs to be properly treated on its surface to meet the requirements of anti-corrosion. Different surface treatment methods have different anti-corrosion grades, and the characteristics of anti-corrosion coatings are also different, and the service temperature of different coatings are also different. Here are several common surface treatment methods for bolts.

  • 1. Trivalent white zinc plating, that is white zinc plating. White zinc plating refers to the process of plating a layer of white zinc on the surface of the screw to achieve beautiful and antirust effect.
  • 2. Blue zinc plating is trivalent blue zinc plating. Blue zinc plating refers to the process of plating a layer of blue zinc on the surface of the screw to achieve the effect of blue beauty and rust prevention.
  • 3. Trivalent color zinc plating, that is, color zinc plating. Color zinc plating refers to the process of plating a layer of color zinc on the surface of the screw to achieve beautiful and antirust effect.
  • 4. Trivalent black zinc plating, that is, black zinc plating. Black zinc plating refers to the process of plating a layer of black zinc on the surface of the screw to achieve beautiful and antirust effect.
  • 5. Nylok anti dropping treatment, plating anti dropping, is a kind of loosening technology. The anti loosing technology of anti dropping screw is the first developed successfully in the world by the United States. It makes special engineering resin stick to the screw teeth permanently with special technology. By using the rebound property of engineering resin material, it makes the bolt and nut achieve absolute resistance to vibration and impact through extrusion in the locking process, and completely solves the problem of screw teeth The looseness of the wire.
  • 6. Imitation gold treatment is a kind of surface treatment method to obtain 14-18k gold color on the surface of parts. It is mainly used for electroplating in construction, hardware, lighting, jewelry and other industries.
  • 7. Gilding, gilding, is a kind of decoration technology, is also one of the commonly used words. At first, it refers to plating a thin layer of gold on the surface of objects.
  • 8. Copper plating treatment, through technical treatment, makes the screw surface into a bronze color.
  • 9. Dacromet plating technology was born in North America. It is a new type of anticorrosive coating mainly composed of zinc powder, aluminum powder, chromic acid and deionized water.
  • 10. High temperature black pot, is a high temperature black pot screw treatment.
  • 11. Copper plating, copper plating is usually divided into electroless copper plating and electroplating copper. Electroless copper plating is on the surface of palladium and other catalytic active substances, through the effect of formaldehyde and other restoration agents, so that copper ions can be recovered and separated.
  • 12. Nickel plating, after electrolysis or chemical method in metal or some non-metal plating on a layer of nickel method, known as nickel plating. Nickel plating includes electroless nickel plating and electroless nickel plating. Electroless nickel plating is a kind of electrolyte composed of nickel salt (called main salt), conductive salt, pH buffer and wetting agent. The anode is made of metal nickel, and the cathode is made of plated parts. With direct current, a uniform and fine nickel coating is deposited on the cathode (plated parts).
  • 13. Copper plated with nickel.
  • 14. Silver plating, the coating is used to avoid screw corrosion, add screw conductivity, reflective and beautiful. Widely used in electrical appliances, instruments, appearance and lighting appliances manufacturing industry. In order to avoid the discoloration of silver coating, it is generally treated after plating, mainly by immersion, chemical and electrochemical passivation, precious or rare metal plating or coating.
  • 15. Ordinary black pot is a normal temperature black pot screw processing method.
  • 16. Head spray paint, a kind of artificial spray paint, made of lead nitrate, vitamin, resin, pigment, solvent, etc. Usually spray evenly on the surface of screw with spray gun, which is water-resistant, oil resistant and can dry quickly. The substance is toxic and has a certain impact on the body. Different brands of spray paint have different toxicity due to different content of ingredients. Pay special attention to safety when using, avoid inhalation and skin contact.
  • 17. Surface dyeing, chemical coloring of screw surface, simple process, simple operation, high efficiency, low cost, bright color.

In addition, there are 18 kinds of coating, 19 kinds of phosphating and 20 kinds of oxidation on the surface of screws.

lubrication of bolts

The friction coefficient of different lubricating materials on the same thread surface is also different, even if the same lubricating material is coated on the thread surface with different surface treatment, the friction coefficient will be different. And the working temperature of different lubricating materials is not the same, beyond the working temperature range, it will lose the lubricating effect.
General grease can play the role of lubrication, but also has a certain anti-corrosion effect, but if the ambient temperature is higher than 120 ℃, grease will play a role. Grease cannot be used in vacuum. Graphite is a good thread lubricant, but graphite must be mixed with fluid medium. The working temperature of graphite is affected by the working temperature of medium.
Molybdenum disulfide is the most commonly used lubricant. The effect of low-temperature wear reduction of molybdenum disulfide is very good. When the temperature is higher than 390 ℃, it becomes trisulfide, which will play the role of increasing wear. The following is the possible range of friction coefficient when different lubricating materials are matched with different thread surface treatments.


Surface treatment system

Lubrication / aftertreatment

Approval system (enterprises providing surface treatment or



Technical requirement


High strength connector (bolt and nut) FK ≥ 8.8/8

Bolts with strength grade of 12 / 12.9 are not allowed to be treated by electroplating



Zinc based aluminum salt flake coating without surface paint.

General: with lubricating oil coating.

If micropores are required to be closed, they cannot be lubricated.

Geomet 321

Dacrolub 11B

The maximum surface coating thickness shall not affect the tightening capacity of bolts.

Zinc sheet only for ≥ M6.

VDA 235-104.40

ZnNi tape sealing

ZnNi (Hillebrand)

ZnNi tape sealing


Zinc based aluminum salt scale coating (Duroc).

Centralized lubrication

If it is necessary to meet the requirements of friction coefficient according to section 6.7, apply lubricant.

Geomet 500

(+ Dacrolub 11B)

The maximum surface coating thickness shall not affect.

Self tapping ability of self tapping bolt.
Zinc sheet only for ≥ M6.

VDA 235-104.40


Zinc based aluminum salt scale coating (Duroc).

If it is necessary to meet the requirements of friction coefficient according to section 6.7, apply lubricant.

(general) Delta tone / delta seal.

Delta PROTEKT vh301gz ZnNi tape sealing.


Connector in visible range.

VDA 235-104.50

Electroplating surface treatment with surface layer.


Surface phosphating treatment.

All post-treatment of the surface is done with silicon-free anticorrosive oil.

All surfaces are treated with an emulsion containing lubricating oil (e.g. parkerol).


It is only used inside the instrument or when the coating completely covers the workpiece.

It is not allowed to clean the emulsion with lubricating property.

DIN 50942

DIN EN 12476V

DA 235-104.10

Therefore, only when the design, material selection, heat treatment, rolling thread, surface treatment, lubrication and other important processes are well done, can the bolts with good quality be made.

Source: China Bolts Supplier:

(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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