Development trend of fasteners and fasteners

1 fastener industry overview

Fasteners such as screws and bolts are fastening mechanical parts that are easy to install and disassemble in many industries. They are widely used, ranging from small products such as watches and clocks to mobile phones to buildings such as houses and buildings. fastener. So fasteners are the basic, versatile parts of all industries. Therefore, the fastener market situation is greatly affected by the changes in the use of screws and bolts in automobiles, machinery, construction, etc., and is also a reflection of the production status of these industries.

Not long ago, the world fastener market was equally divided by Asia, the United States, and Europe. In recent years, due to the increase in China's production, Asian fastener production accounted for more than half of world production. In the past, Japan has maintained the status of fastener manufacturing in the world's largest country. In recent years, China's fastener production has increased significantly. In 2005, its output exceeded Japan's number one in the world.

2 development trend of fastener technology

2.1 types and uses of fasteners

The size of the fasteners is generally metric thread, which is divided into coarse ribs and fine ribs according to the diameter and pitch. Because fasteners come in a variety of sizes, precisions, strengths, and materials, these elements combine to make hundreds of thousands and millions of fasteners. Roughly speaking, fasteners can be divided into bolt-type male rib products and nut-like rib products. According to the shape of the rotating portion of the fastener head, there are a cross groove head, a single head groove, a hexagon head, and the like. In addition, fasteners have additional characteristics for different environments and uses, so the classification of fasteners is quite complex.

Regarding the indication method of the strength of screws and bolts, the first figure is 1/100 of the nominal tensile strength in MPa, and the second number is 10 times the ratio of the nominal lower yield point to the nominal strength, for example "10.9". , indicating a bolt with a nominal tensile strength of 1000 MPa and a 0.2% yield strength of 900 MPa. The strength of the nut is expressed in terms of the strength level number of the highest strength bolt used in combination therewith. The strength of stainless steel fasteners is expressed in a combination of steel grade and tensile strength grade numbers, such as "A2-50". These representations are internationally uniform methods of representing fastener strength. Fasteners are the best products standardized in industrial products, and the detailed regulations can be found in ISO standards.

The fasteners are made of carbon steel, alloy steel, stainless steel and other non-steel materials such as copper and aluminum. Only ordinary carbon steel, alloy steel and stainless steel fasteners have many varieties.

2.2 Technical development of fasteners

1) Fastening of fasteners

Increasing the strength of the fasteners can reduce the number of fasteners used, reduce the diameter, and reduce weight. However, it is necessary to solve the problem of delayed fracture and fatigue fracture of bolts having a tensile strength of 1200 MPa and a yield ratio of more than 90%. Therefore, the development of high-strength bolt steel is important, and the development of high-strength bolt manufacturing technology must be carried out simultaneously.

2) Small and lightweight fasteners

Development of small and lightweight fasteners without sacrificing fastening performance. For this purpose, new materials and fastener shapes for fasteners have been studied, and methods for processing difficult-to-machine materials have also been developed.

3) Utilization of new materials

We have developed fastener materials and improved fastening properties for aircraft and medical equipment, and developed new material fasteners such as Ti alloys and Mg alloys.

4) Fastening technology and anti-relaxation technology

In order to securely fasten the fastener to the component and prevent accidents due to slack, the fastening technique and the anti-relaxation technique were studied. Since the failure of the fasteners is not carefully installed, resulting in a large accident causing a large accident, it cannot be taken lightly. It should be recognized that the easy installation and safe use of fasteners are two completely different concepts. The key to the fastening of the fasteners is the correct installation work, so the fastening design and the tightening work cannot be stopped. Development of construction methods.

3 automotive fasteners

A large number of fasteners are used in automobiles. There are at least 1,000 in number. In addition, there are fasteners in the purchased assembly parts, so the number of fasteners actually used in automobiles is more. The problem that automotive fasteners need to study is high strength and improved quality. The high strength of automotive fasteners can reduce the weight of fasteners and the weight of the corresponding components, which will be the eternal topic of hybrid vehicles and pure electric vehicles in the future. The most important problem to be solved in the quality of automotive fasteners is to prevent delayed fracture of inexpensive high-strength fasteners.

3.1 The strength level of automotive fasteners and the use of steel grades

The standard bolts used in various automobile factories are 8.8 bolts, mostly boron steel products. The price of boron steel bolts is lower because of the low boron content, which makes the carbon equivalent of steel very low, and the spheroidizing annealing before processing can be omitted. 10.9 grade bolts are alloy steel and boron steel products. The difference from grade 8.8 boron steel bolts is that the content of harmful elements such as P and S which cause delayed fracture is reduced, and the non-phosphorus lubricating coating is used in the wire drawing. 12.9 grade automobile bolts are alloy steel products, and 14.9 grade automobile bolts are special alloy steel products. The content of impurities in steel is low, and the addition of carbides such as V precipitates elements to capture hydrogen in the steel to improve the resistance to delayed fracture. The bolt of the highest strength level used in mass production of automobiles is 16T. Its material is the eutectoid steel for wire drawing. The vanadium carbide in the steel captures hydrogen, making it harmless, and the tightening ability of the bolt is further improved. Can be used to manufacture 16.9 grade ultra high strength bolts.

3.2 High strength of automobile bolts

The high strength of automotive bolts allows parts to be lightweight and compact, so the demand for high-strength bolts will continue to increase. In particular, the lightweight and compactness of the bolts around the automobile engine, the transmission system and the electric motor has a great effect on the weight reduction and performance improvement of the surrounding components. Therefore, high strength bolts are used in these parts. For example, the 16T high-strength bolt M9 is used to replace the traditional material bolt M12, each bolt has a weight reduction of 80g, the entire connecting rod has a weight loss of 240g, and the overall weight reduction and miniaturization of the engine.

3.3 Delayed fracture of automotive bolts

Delayed breakage of automotive bolts is the most worrying quality issue. Delayed bolt breaks often occur several years after the bolt is manufactured due to design and manufacturing issues. The manufacturing reasons are due to high surface hardness or high internal hardness materials, poor heat treatment, the root of the nut, the R of the bottom of the rib is too small, the shape factor of the steel fiber in the bolt is not suitable, etc. These problems are enhanced by process management. Can be solved. The problem is the design error, that is, the setting of the delayed fracture safety factor is wrong. In addition to delayed fracture, the fracture of the bolt also has fracture forms such as fatigue fracture, impact fracture and ductile fracture. It is necessary to set an appropriate safety factor through strength calculation and experiment. For fatigue fracture and impact fracture, the safety factor can be determined with high precision because there is sufficient theoretical knowledge of these fracture modes and experiments have been carried out for several months. However, for delayed fracture, the mechanism of diffuse hydrogen-induced void formation has been gradually recognized since more than ten years ago, and hydrogen capture technology is used to produce delayed-resistance steel, but the safety factor of the supply bolts in the market cannot be set.

Comparing fatigue fracture with delayed fracture, it is known that there is a fatigue limit value for fatigue fracture, but there is no limit value for the delayed fracture. In solving the setting of delayed fracture safety factor, it is important for steel companies to study the delayed fracture mechanism and the development of new materials, but the user information can not be ignored. It is not uncommon for the bolt to break during use, but the information obtained from the broken bolt is very important for the study of delayed fracture. It is very important information about how many sales of broken bolts, how long it takes to drive, how many kilometers after driving, etc. And information on various factors such as surface hardness, internal hardness, and phosphorus infiltration can be obtained from the broken bolt. Therefore, in the future research on delayed fracture evaluation methods and benchmark value setting, attention should be paid to the combination of theoretical research, laboratory research and use information analysis.

4 engineering machinery fasteners

4.1 Bolts for bulldozers and excavators

1) Strength level of the bolt

JIS specifies a total of 10 bolt strength levels of 3.6-12.9, but 99% of the bolts for bulldozers and excavators are 10.9 and 12.9, and a very small number of bolts of 9.8 and below. The total amount of bolts for a 65t class bulldozer is about 6,000, of which 12.9 is 8.5%, 10.9 is 8.5%, and 9.8 is below 1.1%. The total amount of bolts for a 20t hydraulic excavator is about 2,000, of which 12.9 is 12.2%, 10.9 is 87%, and 9.8 is 0.8%. The above ratio of bolt usage has not changed for more than 10 years. The reason is that the working environment of the construction machinery is irregular ground and is subject to wind and rain. In such heavy loads and harsh environments, high-strength bolts are required to fasten the components with high axial forces. On the other hand, bolts with a strength exceeding 12.9 are expensive, and there is a problem in resisting delayed fracture. Therefore, the bolt usage ratio has not changed for many years.

2) Size and shape of the bolt

In addition to special bolts, the nominal diameter of bolts is 15 types of M4-M36, but most of them are M10-M20. The nominal length of the bolt is 12-95mm. A total of 250 types of bolts are used in bulldozers, and about 130 types of bolts are used in hydraulic excavators. Most of the bolts used are hexagonal bolts, in addition to hexagon socket bolts, flange bolts, eye bolts, etc. There are no bolts of special shape. In the past, bolts and corresponding gaskets were separately managed. Later, the combined bolts developed were flanged bolts combined with gaskets. The combination bolts can be used to manage bolts and gaskets by one part number. The part number is made and the gasket is miniaturized, and the problem of the gasket being leaked can also be prevented. However, at present, the combination bolts have only small diameter bolts of M14 or less, and it is expected that the specifications of the combination bolts will be expanded.

3) Material and heat treatment of bolts

JIS does not specify the material of the bolt. The upper and lower limits of the carbon content are specified for carbon steel. For the low alloy steel for high strength bolts, only B, Mn, and Cr are used as reference addition elements. That is to say, as long as the mechanical properties meet the standard requirements, what materials can be used, so the bolts for construction machinery vary from manufacturer to manufacturer.

Bolts with strengths above 10.9 and above are quenched and tempered to achieve the required mechanical properties. In the conventional process, the bolts heated by the continuous heating furnace are oil-quenched under an air atmosphere, and then immediately tempered. However, for large-diameter bolts of class 12.9, due to the relatively long heating time, a protective atmosphere is required to prevent abnormal decarburization at the ribbed teeth and the bottom of the rib. At this time, it is necessary to pay special attention to the fact that it is not too worried about decarburization, but instead causes carburization.

4) Surface treatment of bolts

To prevent corrosion and sticking of the bolts, the bolts should be surface treated. In the past, when galvanizing bolts, a hexavalent chromium-based passivation treatment liquid was used, but after the harmful effects of hexavalent chromium on the human body were ascertained, the use was completely stopped. Now, all of the harmless trivalent chromium-based passivation treatment liquids have been used. In order to enhance the anti-rust effect, the thickness of the bolt plating is larger than that of JIS.

4.2 Measures to improve the bolting effect of bulldozers and excavators

Equipment accidents due to bolts are mainly loose and delayed breakage of the bolts. The non-rotational relaxation caused by the elastic attenuation of the bolt contact surface is a form of bolt slack, but the main form of relaxation is still the rotational slack caused by insufficient torsional force. To prevent looseness of the rotation, the method is to use ultra-high strength bolts and greater torsional forces for fastening. For this purpose, ultra-high-strength bolts with strengths exceeding the JIS strength level have been developed. In general, the higher the strength of the bolt, the greater the risk of delayed fracture. The tightening method of the crawler bolt with the strength of 12.9 is the plastic rotary fastening method, first tightening with the pre-torque, and then further tightening the nut according to the predetermined rotation angle for fastening. To connect a track, use 4 bolts and tighten the 4 bolts with a fully automatic nut tightener. At present, research is being conducted to increase the tightening force by increasing the angle of rotation of the nut and changing the hydraulic nut tightener to the electric tightener.

The crawler as a bulldozer and excavator moving device is a consumable part that is subjected to intense friction in contact with earth and stone, and is repeatedly removed and replaced during use. At this time, the crawler bolt must be removed, and since the head of the crawler bolt is also worn, it is often the case that the wrench cannot catch the bolt head. In particular, bulldozers move frequently, and this is more common. To this end, the bulldozer crawler bolt head is subjected to induction hardening to increase its hardness. In addition, the super large bulldozer track bolt head is 5mm larger than the traditional bolt head to increase wear margin.

4.3 Development Trends of Bolts for Construction Machinery

As mentioned above, there have been no significant changes in the bolts for construction machinery for more than a decade. And the construction machinery also hardly uses other fastening methods instead of bolted connections. The reason is that the fastening of the bolts is highly reliable and the bolts that have been tightened are also easier to disassemble, even during replacement and repair. Therefore, improving the performance and cost performance of the bolt is more meaningful than trying to reduce or replace the bolt.

The absence of slack and easy disassembly after bolt tightening are two opposite characteristics. In order to improve these two characteristics, the bolts should not be rusted and bonded. For this reason, a lot of work has been carried out over the years in terms of improving the rib precision, applying the coating and surface treatment techniques, and improving the fastening method.

In terms of bolt cost, material costs account for the largest proportion, followed by processing and heat treatment costs. Therefore, in order to achieve higher cost performance, it is necessary to increase the yield of bolt materials. For this reason, in the manufacture of small bolts, the flashless die forging of the bolt head and the rolling forming of the rib are realized, and it is expected that these techniques will be extended to the manufacture of large bolts. In the case of steel for bolts, large-size high-strength bolts for construction machinery have become a bottleneck affecting the expansion of use due to the problem of delayed fracture and hardenability.

In terms of hardenability, high-priced alloying elements are replaced by B, which is inexpensive and good, and it is expected that heat-treating technology and non-tempered steel high-strength bolts that can be sufficiently cooled after heating with low-hardenability steel will be developed in the future. In terms of resistance to delayed fracture, steel mills have developed ultra-high strength bolt steels and are technically feasible, but prices are still high. For conventional 12.9 bolts, the tempering temperature will decrease after reducing the alloying element content, which will increase the risk of delayed fracture. Therefore, it is expected to develop a new low-strength bolt with low cost and high reliability. steel.

5 Building and bridge fasteners

5.1 High-strength bolts for buildings and bridges

In the past, the connection method of steel structures was mostly riveting. Later, the United States developed a more reasonable friction type connection method. After the introduction of the friction type connection method in Japan, the use of high-strength bolts was rapidly spread. The hexagonal high-strength bolts were originally used, but nowadays it is mainly used for torsional shear-type high-strength bolts that are more convenient for on-site construction.

The function of the high-strength bolt is to ensure the safety of the structure by ensuring the rigidity and strength of the steel structure. Specifically, it is important to tighten the axial force management. In order to reduce the tightening of the axial force, the high-strength bolt standard is established as the standard of the high-strength bolting joint, and the tightening axial force is specified in consideration of the construction conditions on the site, so that the bolt is in the temperature range of 0-60 °C. The fastening effect can be exerted inside. In actual construction, the high-strength bolts must not only meet the requirements of the standard, but also carry out on-site inspections during the tightening axial force inspection and installation before construction, and confirm the actual tightening axial force through frequent inspections. In addition, in order to ensure the strength of the actual joint, the sliding test is often performed before construction to confirm that the strength of the bolted joint meets the requirements. Therefore, ensuring the safety of friction-connected structures is a very delicate task.

5.2 Types of high-strength bolts

Classified by shape, high-strength bolts have hexagonal high-strength bolts and torsion-cut high-strength bolts. High-strength bolts can be hot dip galvanized into hot-dip galvanized high-strength bolts The torsion-shear high-strength bolt stabilizes the tightening torque and the axial force by friction management of the mating portion, and the tightening torque coefficient of the hexagonal high-strength bolt and the hot-dip galvanized high-strength bolt has been included in the standard.

1) High-strength bolt size for buildings and bridges

JIS1186 incorporates the M12-M30 high-strength bolt into the standard, and the M30 high-strength bolt is certified to be manufactured. The high-strength bolts used in wind turbine towers are certified M48 high-strength bolts.

2) High-strength bolt strength for buildings and bridges

In the past, high-strength bolts of 11T and above were used. Due to the delayed fracture, 10T bolts are mainly used. Hot-dip galvanized high-strength bolts have the possibility of hydrogen infiltration due to pickling treatment, so 8T bolts are the main products. In recent years, the problem of delayed fracture has been solved by the improvement of the shape of steel and bolts for bolts, and 14T ultra-high-strength bolts have been developed, and the amount thereof is gradually increased. In addition, since the structure is required to have high deformation energy when it is broken, the standard specifies that the elongation of the 10T high-strength bolt should be equal to or greater than 14%.

5.3 Steel for high strength bolts for buildings and bridges

1) Weather resistant high strength bolt

In order to omit the rust-proof coating of the bridge, a weather-resistant steel is used to manufacture the bridge, and the weathering steel contains elements such as Cu, Cr, and Ni, and a special rust layer is formed on the surface of the steel to protect the steel. The fastener bolts of structures such as bridges made of weather-resistant steel are also made of weather-resistant high-strength bolts. In the offshore area, in order to prevent salt corrosion, general weather-resistant steels cannot be used, and newly developed Ni-based weathering steels that suppress chlorine corrosion are used. Therefore, high-strength bolts made of Ni-based weathering steels are also used.

2) Ultra high strength bolt

Fine-crystallization treatment of 14T ultra-high-strength bolt steel to improve toughness and precipitation of fine carbides in steel to suppress hydrogen diffusion, and the use of such high-performance steel to produce ultra-high-strength bolts.

3) Stainless steel high strength bolt

Stainless steel high-strength bolts are used to fasten stainless steel parts. The material used is precipitation hardened SUS630.

5.4 Surface treatment of high-strength bolts for buildings and bridges

The high-strength bolt surface treatment methods for buildings and bridges are generally hot-dip galvanized, but hot-dip galvanized high-strength bolts are not suitable for JISB1186, and all manufacturers need to pass the certification for production. When used in harsh environments, the bolt plating is HDZ55 thick plating. The strength of hot-dip galvanized high-strength bolts is generally F8T. In addition, F12T ultra-high strength hot-dip galvanized bolts are also partially used in practical applications. The method of fastening the hot-dip galvanized high-strength bolt is a method of fastening the plastic zone of the rotating nut. In the actual on-site construction, the fastening operator of the hot-dip galvanized high-strength bolt is required to have the construction technical qualification.

In addition to hot-dip galvanizing, high-strength bolt surface treatment methods for buildings and bridges are also treated with brine. The high-strength bolts are washed with salt water for 100h to provide temporary rust prevention. The bolts will not rust after installation on site and until they are painted, thus omitting the rust removal operation. This method is often used in bridge construction, but it has also recently been used in the construction of the Tokyo Sky Tree Radio Tower.

Nowadays, there is a trend of maintenance-free bridges. In order to adapt to this new trend, bolts with special surface treatment by spraying molten Al-Mg have appeared and are being put into practical use. In addition, hot-plated Al-Zn high-strength bolts for building structures with salt resistance have been developed. There will be many surface-coated bolts in the future, but it is more important to develop a fastening method so that the axial force remains stable after the bolts are tightened.

5.5 Improve construction

Special high-strength bolts that improve workability are also used in special applications. For example, a unilateral bolt that can be fastened from one side at a narrow construction site or a high-strength bolt that can be unilaterally fastened when a bridge plate is attached without a scaffolding.

6 fasteners for home appliances and office equipment

At present, ordinary household appliances and office equipment can be produced in many countries, but high-performance, high-quality home appliances and office equipment require high-function, high-value-added fasteners in many places. The following is a brief introduction to the demand situation and development trends of these fasteners and their materials.

6.1 TV

Among the fasteners for home appliances, the biggest change in recent years is the fasteners for televisions. The TV has changed from analog to digital TV, and due to advances in liquid crystal and plasma technology, the TV has been thinned. Therefore, many requirements for the cathode ray tube television era have emerged.

The internal components of a thin TV can be divided into three parts: the display, the main board, and the casing. The original thin TV case used aluminum alloy. This is because the advantage of a thin TV is that the display is large, and the display has the largest weight in the thin TV, so the chassis is required to be light and have a certain strength. The motherboard is mounted on the aluminum alloy casing. Because the heat dissipation of the motherboard is large, there are aluminum bumps between the motherboard and the casing to keep a certain distance between the two. Initially, aluminum bumps made by turning were used, and as the production of thin TVs increased, forging of bumps was required. The aluminum bumps should be able to be pressed into the aluminum alloy casing and not loosened when tightened with screws. Experiments were carried out on the manufacture of aluminum bumps for forging and rolling. The shape of the bumps and the heat treatment results were not satisfactory. Finally, "inner assembly" aluminum bumps were developed.

In order to reduce the fastening cost, a self-tapping method is adopted, and when the screw is tightened, a burr pattern is formed in the aluminum bump. The problem with this self-tapping method is the powder produced when the crease is formed. If the powder is scattered on the display or the motherboard, it will cause the TV function to fail. In addition, the frictional heat generated by the aluminum alloy self-tapping screws during the formation of the female ribs is also a problem caused by the sintering of the self-tapping screws and the aluminum bumps. The solution to these problems is to use an "aluminum seal" to improve the self-tapping properties of the aluminum alloy and prevent sintering. In addition, a powder adsorbent is applied to the front end of the aluminum seal to prevent the powder from scattering when the self-tapping screw is fastened. The combination of these measures improves the productivity and quality of thin TVs.

The biggest feature of thin TVs is "thin", because "thin" in other parts also produces problems that were not available in TV in the past. In the past, the screws inside the TV were exposed to the surface, and the thin TV was prone to heat. This heat caused the screws to be at a high temperature. If the screws were exposed, people who came into contact with the high temperature could cause burns. In order to prevent this from happening, a "heat-proof screw" in which a resin is embedded in the original steel screw head has been developed.

6.2 mobile phone

The requirement for the screw for the mobile phone is to ensure the realization of hardware requirements such as waterproof, thin and light. With the development of thin and light mobile phones, thin resin materials are increasingly used in mobile phones, and the requirements for screws for mobile phones are that the thin resin materials can be firmly fastened. To meet this requirement, the PS screw II was developed. This is a shallow-insertion type screw for thin resin, which ensures sufficient strength after fastening the thin resin. The waterproofness of the screw fastening portion of the waterproof type of mobile phone is very important, but it is difficult to ensure the waterproofness by the screw alone, and the screw with the O-shaped washer is mainly used at present.

6.3 digital camera

Digital cameras are now the mainstream of cameras, and the requirements for digital cameras are high functionality and miniaturization. In order to maintain the unique black appearance of the single lens reflex camera, the screw head exposed to the surface of the camera is painted, but when the painted screw is tightened, the coating near the screw traverse head is peeled off. In order to solve this problem, anti-paint peeling screws have been developed and are currently widely used for single-lens reflex cameras.

The miniaturization of digital cameras requires a compact structure. The use of the PS screw II with a head height of 0.2 mm can meet the requirements of fastening thin components and achieve space saving. By reducing the height of the screw head, the cross recessed head becomes shallower and the screw tightening operation rate is lowered. For this reason, a powerful driving cross wrench has been developed.

In order to achieve miniaturization, the casing material tends to be thinned, and stainless steel is used as the casing material in order to compensate for the strength reduction caused by the thin plate. Stainless steel is a material that is difficult to tap, and tapping before screw tightening is a cause of increased manufacturing costs. Therefore, through the research of screw materials and the improvement of heat treatment, the development of high-strength screws ensures the self-tapping of stainless steel materials and promotes the miniaturization of cameras.

7 Aircraft and spacecraft fasteners

7.1 Characteristics of aircraft bolt manufacturing

Aircraft bolt manufacturing is characterized by rigorous manufacturing and quality management in accordance with standards. For example, in Japan, aircraft manufacturing complies with the general standards such as MILNAS in procurement, and each airframe manufacturer and engine manufacturer has independently developed relevant standards, and has carefully defined the materials used, manufacturing methods, and test evaluation methods. Especially in the aspect of quality management, the technical requirements and operation methods for special heat treatment, surface treatment, non-destructive inspection and other special procedures are strictly regulated in the standards, and must be certified by airlines and NADCAP.

In terms of materials, it is required to comply with the material standards of AMS and MIL. In addition to the chemical composition and manufacturing methods of the materials, these standards also specify the mechanical properties after heat treatment related to the material characteristics. Some airlines have always specified the fuselage requirements to the aircraft manufacturer. No matter how good the material is developed, and after a long period of repeated experimentation and verification, it can not be used without the recognition of these standards. Especially for bolts, great importance is attached to their high reliability, so it is very important to use enough results. Since the aircraft bolts are strictly managed in accordance with the standards, the degree of freedom in the manufacture of the aircraft bolts is small compared to the bolts used in the general manufacturing industry, and the change in the bolts is small.

7.2 Aircraft bolt materials

1) Low alloy steel

Low-alloy steels such as Ni-Cr-Mo steel have long been used in parts of the fuselage where high strength is required. Representative steel grades are 8740 steel and 4340 steel. Ni-Cr-Mo steel has a wide tensile strength range from 125ksi to 180ksi and can be used in all parts of the fuselage. In order to meet the needs of lightweight body, with the development of materials and heat treatment technology, 4330M, H-11, 300M and other ultra-high strength bolts with tensile strengths from 220ksi to 260ksi are manufactured, but due to hydrogen embrittlement, multi-purpose Fastening in the state of shear stress such as pin. In order to prevent corrosion of such low alloy bolts, surface treatment must be carried out, the most common being cadmium plating.

2) Corrosion resistant and heat resistant steel

A part of the corrosion resistant bolt is made of 300 series or 400 series stainless steel, and more is made of precipitation hardening type stainless steel. The precipitation hardening type stainless steel used is 17-4PH, and the bolt for higher strength and toughness is used for PH13-8Mo.

The bolts with good corrosion resistance and heat resistance are made of austenitic heat-resistant steel A286 and have a wide range of uses. A286 can obtain two intensity levels of 130ksi and 140ksi depending on the solution temperature. When the bolt is machined, work hardening is produced by cold working and aging treatment, and the bolt strength can be further improved. With the work hardening property, the 160ksi and 200ksi high-strength steel can be drawn, and bolts with different strength requirements and different uses can be obtained. Work hardening increases the risk of hydrogen embrittlement of the bolt strength. Since austenitic heat-resistant steel is an iron-based alloy, the material cost is low, and the use of such materials for the manufacture of high-strength bolts will increase in the future.

Corrosion-resistant and heat-resistant bolt materials, in addition to austenitic heat-resistant steel, Co-based super alloy MP35N. MP35N has a normal temperature strength of 260 ksi and has good fatigue strength, toughness and corrosion resistance. Although the MP35N bolts are used in small amounts, they are used in harsh conditions.

3) Super heat resistant alloy

The bolts used in the high temperature environment of aircraft engines are superalloy bolts. Bolt materials are mainly Ni-based superalloys, mainly Waspaloy and Inconel718, of which Inconel718 is the most used. The usual strength of Inconel 718 is 185 ksi, which is achieved by solution and aging. Inconel 718, like the A286 described above, has a matrix of austenite and can be strengthened by cold working and aging treatment. In recent years, Inconel 718 with a strength of 220 ksi after wire drawing has been used to manufacture superalloy bolts. In the past, high-temperature resistant bolts with high temperature strength were mainly used in aircraft engines, and now they are used as high-strength bolts, and the amount used in aircraft bodies has also increased.

Co-based superalloy MP159 is a material that improves the heat resistance of MP35N. It has a normal temperature strength of 260ksi, high fatigue strength and toughness, and high resistance to high temperature relaxation. MP159 bolts are often used in engines with harsh working conditions. MP159 is also work hardenable by wire drawing and is used in bolts. Materials containing a large amount of Co such as MP159 and Waspaloy have very good properties, but the material cost is high and there is a problem of poor workability.

4) Ti alloy

Ti alloy has high specific strength, corrosion resistance and heat resistance, and is widely used in various parts of aircraft engines and fuselages. Although Ti alloy materials are expensive and have poor processability, the application of Ti alloys is expanding to achieve the flexibility of military aircraft requiring good maneuverability and civil aircraft requiring improved fuel efficiency. In particular, in recent years, the application of the composite material on the fuselage has been accelerated, and since the Ti alloy has a good property of preventing electrochemical corrosion, the application rate thereof is further improved.

The Ti alloy used for the manufacture of bolts is mainly Ti-6Al-4V of α-β alloy, which is treated by solution aging treatment and has a strength of up to 160 ksi. Other bolt materials include Ti-6Al-6V-2Sn which is slightly higher in strength than Ti-6Al-4V and α-β alloys such as SP-700 which are developed in Japan with good hot workability, but in a small amount. At present, aircraft fuselage manufacturers urgently need high-strength Ti alloy bolts with strength exceeding Ti-6Al-4V in order to achieve lightweight body. Since it is difficult to have good formability and fatigue strength, the material of such bolts has not yet been developed.

7.3 Spacecraft bolts

The aircraft bolt factory has won high praise from customers for its superb technology, stable product quality and high integrity. Therefore, the bolt products produced are also widely used by the aerospace industry. Spacecraft bolts vary in shape and shape, and the standard bolts of aircraft are rarely used, but most of the bolt materials are materials specified in the aircraft bolt standard. In order to improve the propulsion of the launch spacecraft, lightweight requirements were imposed on the spacecraft. In this case, a large number of Ti alloy bolts were used. In addition, due to the good application performance of the superalloy Waspaloy and Inconel718 aircraft bolts, the bolts made of these materials are also used in rocket engines under high temperature conditions. The satellite is in two completely different environments on the ground and in orbit, in a gravity-free state in the orbit, without a large external force. However, it is necessary to have corrosion resistance in the atmosphere of the launching process. In addition, the satellite itself requires miniaturization and weight reduction, so the Ti-6Al-4V bolt with higher specific strength is often used. In addition, because the satellite carries many sophisticated electronic instruments, non-magnetic austenitic stainless steel bolts and A286 bolts are also used in large quantities.