High-Strength Bolt Materials: Current Situation and Trend

Abstract: With the increase of the working stress of high-strength bolts and the expansion of their application fields, higher requirements are put forward for the performance of high-strength bolt steel: not only to meet the high strength performance of bolts, but also to ensure the reliability of work.

This paper discusses the research status, strengthening mechanism and commonly used high-strength bolt materials of high-strength bolt materials, and puts forward the development trend of high-strength bolt materials.

Fasteners play the role of connection, positioning and sealing in mechanical components, among which the amount of bolts is the largest.

With the continuous enlargement of all kinds of machinery, equipment and construction projects, as well as the continuous improvement of power and speed, the working conditions of bolt parts are worse and the working stress is significantly increased.

Therefore, bolt steel is required to have higher strength. For example, high pressure feed water pump used in supercritical and ultra supercritical generating units is the key equipment of supercritical generating units.

The requirements for the water supply pressure of the pump are getting higher and higher, so that the strength requirements of the bolts for the sealing and pressure bearing functions of the pump are also increased;

Large building grid structures not only have large span, but also are mostly public buildings. High strength bolts are important parts used in spatial steel grid joints, which directly transfer the alternating internal forces caused by alternating loads, and their quality is directly related to the safety of people’s lives and property;

The original bolts for automobiles and motorcycles, especially the engine bolts, have been difficult to meet the requirements of high stress of the engine.

The high strength of the bolts can reduce the size of the bolts, reduce the mass of the bolts themselves, and help to reduce the vehicle mass and reduce energy consumption;

As a connecting and fastening component, the high strength of bolts is also conducive to the miniaturization and compactness of other automobile structures.

It can be seen that high-strength bolts have great use value and broad application prospects.

High-Strength Bolt Materials: Current Situation and Trend 1

1. Performance index of high-strength bolt

The strength level of high-strength bolts is divided into four levels: 8.8, 9.8, 10.9 and 12.9. See Table 1 for the mechanical properties of bolts at each level.

According to the quality of high-strength bolt steel, it can be generally divided into current quality, potential quality and final quality:

1) The current quality mainly refers to the most basic forging characteristics, such as low deformation resistance, not easy to crack, good steel quality, easy upsetting, and low tool and die loss.

2) On the basis of ensuring the current quality, the potential quality refers to the development of the selection of the best proportion of various alloy elements, the simplification or omission of the heat treatment process before and after upsetting, so as to obtain better functional properties than the original conventional steel.

3) The final quality refers to the high-strength bolt steel and its product bolts shall meet the high tensile strength to resist stretching, breaking, slipping and abrasion;

It has high plasticity and toughness to reduce the sensitivity to surface quality problems such as deflection and notch stress concentration;

Bolts working in humid or corrosive atmosphere shall have a sufficiently low delayed fracture sensitivity;

For bolts bearing alternating load and impact load, higher fatigue resistance and multiple impact tensile resistance are required to resist fatigue and multiple impact fracture;

For bolts working in severe cold areas, low ductile brittle transition temperature is also required for bolt materials.

Table.1 Indexes of high strength bolt’s mechanical properties

Mechanical properties

 

Bolt grade

8.8

9.8

1040-1180

10.9

12.9

≤M16

mm

>M16

mm

Tensile strength/MPa

800-980

830~980

32~39

1040-1180

1220~1380

Rockwell hardness/HRC

22~32

23~34

10.9

32~39

39~44

According to the service conditions of high-strength bolts, there are generally the following requirements for their mechanical properties:

1) High tensile strength and high yield ratio;

2) High enough plasticity, especially when tightening with plastic zone;

3) It can be tightened repeatedly, that is, it can withstand enough times of large stress amplitude loading, and has good low cycle fatigue performance;

4) When bearing alternating working load, it shall have good high cycle fatigue performance;

5) When bearing impact load, it shall have high impact toughness;

6) Good delayed fracture resistance;

7) Good low temperature resistance;

8) Good creep resistance and stress relaxation resistance;

9) Low notch sensitivity (bolts are multi notch parts);

10) Stable surface friction coefficient to obtain stable assembly preload.

High-Strength Bolt Materials: Current Situation and Trend 2

2. Research status of high-strength bolts

The use of high-strength bolts in China is not long.

It began to be used on some railway bridges in the 1960s, and began to be used on boiler steel structures in the 1980s.

In the 1990s, after introducing foreign cars and production technologies, China found bolts with strength grade of 12.9, tensile strength of 1200 MPa, and yield strength of 1080 MPa.

At that time, it belonged to the highest strength level among automotive bolts.

After FAW Group introduced the Chrysler 488 engine from the United States, the flywheel bolts have always depended on imports.

In order to achieve localization, FAW Group learned that the materials used for flywheel bolts in the United States are equivalent to ML35MnMo brand materials, and the materials used for high-strength bolts for German Audi cars are also equivalent to ML35CrMo by comparing the composition of foreign high-strength bolt materials with the existing materials in China.

Therefore, ML35CrMo is selected as the domestic trial production material of 12.9 grade flywheel bolt material

The decarburized layer on the surface of raw materials is removed by material peeling technology.

After cold upset forging and end quenching tests, annealing, quenching and tempering process tests, finished product performance tests, bench tests and loading tests, high-strength bolts with properties equivalent to those of CA488 engine flywheel bolts are finally successfully developed.

Wang Rongbin et al. used lath martensite structure to improve the performance of high-strength bolts, and they can also obtain high-performance bolts above grade 10.9, and can partially replace quenched and tempered high-quality structural steel.

Low carbon martensite (lath martensite) steel is widely used for its high strength, plasticity, toughness and low notch sensitivity.

Taiyuan Iron and Steel Co., Ltd. has developed a series of low carbon martensite fastener steels for automobile and standard parts industries.

For example, ML15MnVB, ML20MnVB, ML15MnB and ML15Mn are used to make grade 8.8, 9.8 and 10.9 high-strength bolts, which have achieved good results.

Leng Guangrong and others successfully controlled the properties of low carbon medium alloy steel (22Cr2Ni4MoV) to a tensile strength of 1560MPa, elongation of 12%, hardness of 45HRC and impact energy of 60J through appropriate heat treatment process.

The high-strength bolts made of this material can barely meet the requirements of the 2500 mm four high rolling mill for the mechanical properties of bolt materials, but after use, the average service life of the bolts is only 2 months, and the durability is not satisfactory.

On the basis of 22Cr2Ni4MoV material, Pan Zuyi et al. adopted the quenching+low temperature tempering or quenching+high temperature tempering heat treatment process by controlling the chemical composition, structure and properties, so that the strength, plasticity and toughness of the steel are well matched.

The 2500 mm universal joint bolt of four high reversing mill made of new high-strength bolt steel has a long service life.

With the improvement of bolt strength, especially when the tensile strength exceeds 1200 MPa, the delayed fracture becomes very prominent, which is a major problem encountered in the high strength of bolts.

High strength bolts are notched parts, which have high notch sensitivity and are easy to produce delayed fracture at the notch stress concentration position, so their application scope is limited.

On the basis of 42CrMo material composition, Hui Weijun et al. increased Mo content (mass fraction, the same below), added microalloying elements V and Nb, reduced Mn and impurity elements P and S content, and developed a high-strength bolt steel ADF1, which has good delayed fracture resistance at the strength level of 1300 MPa.

Further analysis shows that the grain size of the steel is refined from about 12 m to about 5 m, and the notch tensile I critical stress is significantly increased due to the secondary hardening effect of Mo and V carbides and the cyclic heat treatment.

It is concluded that the delayed fracture resistance of high-strength bolts can be improved by adjusting alloy content, adding corrosion resistant alloy elements, refining grains, reducing grain boundary segregation, increasing tempering temperature and making invading hydrogen harmless.

The ADS series of Sumitomo Metal, the KNDS series of Kobe Iron and the ADF series of China Iron and Steel Research Institute have successfully developed high strength bolt steel with good delayed fracture resistance through these measures.

Compared with developed countries, the research and development level of high-strength bolt steel in China is still relatively backward.

Only ML20MnVB, ML35CrMoV, 35CrMo and other materials can meet the requirements of 12.9 grade high-strength bolts.

In 2005, the 12.9 grade connecting rod bolts used in automobile engines in China were basically imported.

Although Hui Weijun and others have developed a 1300 MPa high-strength bolt material 42CrMoVNb based on 42CrMo material, its performance in practical application needs further investigation.

Different service environments of high-strength bolts have different requirements on material properties.

Yang Xinglin and others found that 35CrMnSiA material of high-strength bolts used in the marine environment is easy to fracture during service.

Through analysis, it is considered that the bolt fracture is not ordinary hydrogen embrittlement fracture, but stress corrosion fracture caused by the corrosion of the marine environment due to the serious corrosion of the marine atmosphere and seawater on the bolt materials.

At the same time, it is proposed to replace the coating and improve the detection level of finished products to improve the ability of bolt to resist stress corrosion cracking, but the problem of material performance defects has not been fundamentally solved.

After considering the service environment of materials, Fang Dong and others selected 16Co14Ni10Cr2Mo material to replace 35CrMnSiA material.

This steel has high strength, good plasticity and toughness, and excellent comprehensive performance.

Its application in aviation is basically mature, but it is the first time to use it to manufacture large section bolts and apply it to the marine environment.

Through the simulated marine environment test, it is found that the bolt will not break due to low temperature brittleness and notch brittleness.

In the pre tightening state, even if the coating is worn, stress corrosion cracking will not occur, and overload fracture will not occur during normal operation.

At the same time, the bolt product can be safely used for one year in practical application.

It is known that the 16Col4Nil0Cr2MoE steel M56 bolt is safe and reliable when used in the marine environment, thus solving the stress corrosion problem from the material.

In recent years, the research of Chinese scholars on high-strength bolts mainly focuses on the hydrogen embrittlement fracture mechanism of high-strength bolts, improvement of heat treatment process and failure analysis of high-strength bolts, which provides an important basis for the development of high-strength bolt materials in the future.

Alloy elements and trace elements in high-strength bolt materials play an important role in their performance.

The research shows that adding about 0.02% Ti and other microalloying elements into microalloyed non quenched and tempered steel can precipitate precipitation phase, which can prevent grain growth during heating and hot working, and precipitation phase can strengthen the matrix during cooling, thus improving the comprehensive properties of steel.

But not all precipitates can improve the comprehensive properties of steel.

The software Thermo scale and Dicta were used to calculate the precipitates in the microalloyed steel 40MnV.

The composition, morphology and distribution of the precipitates were studied by electrolytic analysis, X-ray diffraction and transmission electron microscopy.

The results show that a small amount of N and Ti in the steel can cause the precipitation of coarse TiN particles with a size of 50 nm in the solid-liquid two-phase zone.

According to Gladman’s theory, it can be concluded that (Ti, V) (C, N) particles precipitated in the solid-liquid two-phase zone can not prevent the grain growth during heating.

On the contrary, because these particles are coarse, they are harmful to the properties of steel.

The precipitation temperature and quantity of TiN in solid-liquid two-phase zone can be effectively reduced by reducing the content of N or Ti.

In order to ensure more VN precipitation, about 0.02% Ti in microalloyed steel should be reduced to the appropriate range, and the N content should also be controlled in the appropriate range.

By studying the influence of alloy elements on the properties of materials, we can have a basis for developing new high-strength bolt materials.

However, only proper alloy composition can not guarantee that the developed bolts can meet the actual performance requirements.

Only through reasonable heat treatment process and reasonable coordination of material hardness, strength, plasticity and toughness can the bolts with excellent performance be developed.

30NCD16 is a high-strength alloy steel with strong heat resistance and high strength and toughness after medium high temperature tempering.

Liu Xiangjiang and Liu Hua studied the influence of quenching and tempering temperature on the structure and properties of 30NCD16, and finally determined that the best heat treatment process of high strength steel 30NCD16 is 840~870 ℃.

After quenching and tempering at 560 ℃, fine and uniform sorbite structure can be obtained. The tensile strength of steel is>1200 MPa, and the impact energy Akus is>50 J

Wang Genji et al. studied the effect of different heat treatment processes on the microstructure and mechanical properties of Q390 low alloy high strength steel thick plate by means of microstructure observation and mechanical property measurement.

The results show that after normalizing at 920 ℃ for 36 min, the mixed crystal structure in the hot rolled Q39o low alloy high strength steel plate can be fully austenitized to achieve grain refinement, and then transformed into polygonal ferrite and pearlite in the subsequent cooling process.

Good comprehensive mechanical properties are obtained.

The elongation and impact toughness are much higher than those in the hot rolled state, and the tensile fracture delamination phenomenon is completely eliminated.

CrNiMoBNbl6-16 steel is an important high alloy steel for industrial production, which is mainly used as bolt material with high temperature strength requirements, such as steam turbine, gas turbine, engine, chemical reaction and high-pressure thermal equipment.

He Wei et al. analyzed the relationship between the structure and mechanical properties of CrNiMoBNb16-16 steel from two aspects: the influence of heat treatment process on the tensile properties at room temperature and high temperature and the influence of test temperature on the tensile properties.

The test results show that with the increase of test temperature (20 ~ 650 ℃), the strength and plasticity decrease obviously.

For this kind of test material, the comprehensive performance of warm forging is better than that of high temperature forging.

Finally, the thermomechanical treatment of this material is determined to be warm forging, so that its strength and toughness have the best match.

By adjusting the alloy composition of the material and through proper heat treatment, the tensile strength of most alloy structural steels will be increased to 1200MPa, which will lead to delayed fracture.

Therefore, further enhancing the strength will lose the use value and cause greater insecurity.

The results show that the delayed fracture resistance of high strength steel can be improved by reducing grain boundary segregation, refining grains, increasing tempering temperature, adjusting alloying elements, reducing the amount of hydrogen intrusion on the surface and making the hydrogen intrusion harmless.

High-Strength Bolt Materials: Current Situation and Trend 3

3. Common materials of high-strength bolts and their strengthening and toughening mechanism

3.1 Low alloy steel

Low alloy steels generally have medium carbon content. From the alloy composition, there are Cr, Cr Mo, Cr Ni, Ni Cr Mo, Mn and Mn Cr series.

It can be seen from Table 2 that low alloy bolt steel has a wide range of applications, and the strength grade can be selected from 700 to 100 MPa.

When the strength is 1200 MPa, the problem of delayed failure of bolts made of low alloy steel is prominent and needs to be solved.

At present, low alloy steel is still the main high-strength bolt steel.

The bolts made of low alloy steel need to be quenched and tempered, that is, quenched and then tempered.

In addition, due to the high content of carbon and alloy elements, the hardness and deformation resistance are large.

Therefore, spheroidizing annealing treatment is required before cold forging.

Low alloy steel contains various alloy elements. How to save the alloy in bolt steel and reduce the cost is a problem that needs to be considered.

Due to the relatively high content of carbon and alloy elements, the plasticity and toughness of steel are also poor.

To further improve the strength and ensure the necessary plasticity, it is also a problem that needs to be studied.

Tab.2 Some bolt steels’ strength grade MPa

Type of Steel400500~600700~800900~10001100
carbon steel  
non-quenched and tempered steel    
boron steel  
low-alloy steel  

3.2 Boron steel

With the development of cold forging technology, the demand for cold forging bolt steel is also greatly increased.

The materials used to manufacture high-strength bolts were medium carbon steel and medium carbon alloy steel.

However, this kind of steel has high hardness and large cold deformation resistance, so it needs spheroidizing annealing treatment before cold forging, which consumes a lot of energy.

Therefore, low-carbon boron steel has been developed.

The basic principle of low carbon boron steel composition design is to reduce the carbon content and improve the cold deformation ability of steel.

A small amount of boron is added to compensate for the loss of strength and hardenability caused by carbon reduction.

In addition, a small amount of Cr, Mn and other alloy elements can be added as required to further improve the hardenability.

The characteristics of low carbon boron steel are:

1) A small amount of boron is used to replace a large amount of alloy elements, and the economic effect is obvious;

2) With low content of carbon and alloy elements, the rolled products can be directly cold forged without pre spheroidizing treatment, saving a lot of energy;

3) The tendency of quenching deformation and cracking is small, which can be treated by water quenching, saving quenching oil, and improving the operating conditions and working environment;

(4) The properties are good, the toughness is obviously improved compared with medium carbon alloy steel at the same strength level, and the fatigue failure resistance is high, and the decarburization sensitivity is small.

Boron steel bolts have been used in automobile, construction, machinery and other departments, and the number is increasing.

It can be seen from Table 2 that bolts with strength ranging from 700 MPa to 1100 MPa can be made of boron steel.

High-Strength Bolt Materials: Current Situation and Trend 4

3.3 Non quenched and tempered steel

The non quenched and tempered steel does not contain a large amount of alloy elements, and does not need to be quenched and tempered.

Just control the hot working deformation and subsequent cooling rate, it can ensure the necessary mechanical properties, save energy consumption for heat treatment, shorten the production cycle, and reduce the cost of steel.

At present, non quenched and tempered steel bolts are mainly used in automobile manufacturing, but the total number is still small and the application scope is not wide.

Compared with quenched and tempered steel, its cost is reduced, but its toughness is low, the strength level is not stable enough, and the die life is low during cold forging.

These problems restrict the application scope of non quenched and tempered steel.

Non quenched and tempered steel is mainly used for bolts of 700~800 MPa grade, and sometimes it is also used for bolts above 900 MPa grade.

Generally, non quenched and tempered C-Mn system with carbon content of about 0.25% or C-Mo system with carbon content of about 0.10% is used for bolts of 700~800MPa grade, and trace Nb, V, Ti and other elements are added, and the structure is ferrite+pearlite.

When the strength level is above 900 MPa, Cr, Ti, B and other elements are usually added to the C-Mo Si system containing about 0.10% carbon to improve the hardenability and ensure satisfactory strength and toughness. The structure is ferrite+bainite.

In order to improve the toughness of non quenched and tempered steel and make the strength and toughness match well, in addition to controlling the chemical composition, the purpose can also be achieved by adjusting the processing technology (such as hot working temperature, rolling deformation and controlled cooling after rolling).

3.4 Low carbon martensitic steel

All unalloyed steel (carbon steel) or low carbon low alloy structural steel with carbon content less than 0.25% can obtain more than 80% or even 100% low carbon martensite structure after intensive quenching.

Such steel is collectively called low carbon martensite steel.

Its hardness is 45 ~ 50 HRC, yield strength is 1000 ~ 1300 MPa, and tensile strength is 1200 ~ 1600 MPa.

It has good plasticity (A ≥ 10%, Z ≥ 40%) and toughness (Axv ≥ 59 J).

Good cold workability, weldability, and small heat treatment distortion.

Therefore, the application of low carbon martensite is increasingly widespread, and it has become an important way to develop the strength and toughness potential of steel and prolong the life of machine parts.

Materials used in high-strength bolts include 15MnVB, 20MnSi, 20 steel, 20MnTiB, etc.

3.5 Strengthening and toughening mechanism

The strengthening and toughening mechanisms of high strength steel mainly include fine grain strengthening, solution strengthening, precipitation and dispersion strengthening and dislocation strengthening.

1) Fine grain strengthening.

By increasing grain boundaries to block dislocation movement and limiting plastic deformation to a certain range, the plasticity of steel can be improved.

It can not only effectively improve strength, but also obviously optimize plasticity and toughness.

Now, the technology of controlled rolling and controlled cooling (TMCP) is widely used in industry, which refines the final structure through austenite recrystallization, deformation induced ferrite transformation (DIFT), accelerated cooling and ferrite recrystallization.

2) Solution strengthening

The metal matrix (solvent metal) is strengthened by using the internal point defects of metal materials (interstitial atoms, replacement atoms).

As the difference in atomic diameters increases, the greater the distortion is

This results in greater strengthening effect.

Addition of Mn, Si, Ni, Mo and other elements in Fe can cause displacement type solid solution strengthening.

3) Precipitation and dispersion strengthening

When the particles of the second phase precipitate, the precipitated phase creates a stress field and a high energy region in the matrix, resulting in a sharp increase in strength and hardness and strengthening.

It is concluded that:

The larger the volume ratio of precipitation phase is, the more significant the strengthening effect is;

The greater the dispersion of the second phase, the better the strengthening effect;

The greater the resistance of the second phase particle to dislocation motion, the greater the strengthening effect.

4) Dislocation strengthening

Because of the existence of high density dislocations, it is difficult to move dislocations.

The mechanical property is that the metal strength is improved. For the actual metal with crystal defects, it can be strengthened by dislocation multiplication.

At the same time, the movement of dislocations is also the main reason for solution strengthening, fine grain strengthening, precipitation and dispersion strengthening.

The main reason that affects the strength and toughness of high-strength bolt materials is the micro defects of the matrix structure, including grain boundaries, precipitation particles, dislocation substructure and solution distortion.

The above micro defect structures will lead to the improvement of steel strength.

However, except that the increase of grain boundaries (i.e. fine grain strengthening) will increase the toughness, the other micro defect structures will reduce the toughness.

These strengthening mechanisms need to be fully utilized when strengthening high-strength bolt materials.

4. Research prospect of high-strength bolt materials

With the development of energy, automobile, machinery, construction, light industry and other industries, higher requirements are put forward for the materials used to manufacture various types of fastening bolts, and high-strength bolt materials are urgently needed.

In the past 10 years, both at home and abroad have attached great importance to the development of this technology.

China’s key basic research and development planning project (973) “Major basic research on new generation steel materials” has been launched, and the research and development of high-strength bolt steel is one of the important topics.

The development trend of high-strength bolt steel is summarized as follows:

1) High strength and high performance steel

With the increase of steel strength, its sensitivity to delayed fracture increases.

When the tensile strength>900 MPa and the hardness ≥ 31 HRC, the sensitivity of delayed fracture increases gradually.

The higher the service stress is, the greater the damage caused by the fracture is.

Therefore, the development of high-strength bolt steel with excellent resistance to delayed fracture is of great significance to protect people’s lives and property and expand the application field of high-strength bolts.

2) Reduce cost and energy consumption

Substitute cheap boron steel for high price alloy steel containing Ni, Cr, Mo, etc. to reduce the cost;

The extensive use of cold forging instead of hot forging, non quenched and tempered steel that reduces the heat treatment process, low carbon non annealed steel that does not need softening treatment before forging, and high-precision rolled products that do not need peeling and drawing can not only reduce energy consumption, but also reduce the decarburization tendency of thread tips and improve the yield of bolts.

3) Improving the quality and reliability of bolt steel

The reliability and service life of bolt parts are closely related to the metallurgical quality and surface condition of bolt steel, and even some processing properties.

By increasing the purity of steel and reducing the content of S and P, the deformation ability of steel can be improved, the grain boundary embrittlement can be reduced, the non-metallic inclusions in steel can be reduced, the toughness and plasticity of steel can be improved, and the delayed fracture resistance of steel can be improved.

In addition, the manufacturing accuracy, fastening technology and testing method of finished bolts are also important factors affecting the reliability of high-strength bolts.

Leave a Comment

Your email address will not be published. Required fields are marked *