Summary: Increasing demands for high-strength fasteners, together with their expanding fields of application, have led to higher performance requirements for high-strength fastener steels. Steel must not only exhibit high strength, but also guarantee reliable performance.
This article explores the current status of research, strengthening mechanisms and commonly used materials for high-strength screws, and highlights future trends in their development.
Fasteners, such as screws, play a critical role in connecting, positioning, and sealing mechanical components. Screws are the most commonly used type of fastener.
As machinery, equipment and construction projects continue to expand and improve in terms of power and speed, working conditions and bolt tension levels have become increasingly demanding. This resulted in the need for higher strength bolt steel.
For example, high-pressure feedwater pumps used in supercritical and ultra-supercritical generating units require stronger bolts to ensure that their sealing and pressure support functions can meet the increasing pressure requirements of the water supply.
High-strength screws are also crucial components in the construction of large building grid structures, such as those found in public buildings. These bolts transfer alternating internal forces caused by alternating loads and are directly related to public safety.
Original bolts used in automobiles and motorcycles, especially engine bolts, are struggling to meet the high-stress requirements of engines. High-strength bolts can reduce the size and mass of bolts, which can help reduce vehicle weight and energy consumption.
Furthermore, the high strength of the screws contributes to the miniaturization and compactness of other automotive structures.
In conclusion, high-strength screws have significant practical value and a promising future for a wide range of applications.
1. Performance index of high strength screw
The strength levels of high-strength screws are divided into four categories: 8.8, 9.8, 10.9 and 12.9. See Table 1 for mechanical properties of screws at each level.
Based on the quality of high-strength screw steel, it can be divided into three categories: current quality, potential quality and final quality.
- Current quality mainly refers to the most fundamental characteristics of forging, such as low deformation resistance, good steel quality, easy upsetting and minimal loss of tools and dies, which are not easily prone to cracking.
- Potential quality refers to selecting the ideal proportion of various alloying elements and simplifying or omitting the heat treatment process before and after upsetting, while ensuring the current quality. This approach results in better functional properties than conventional steel.
- Final quality refers to the requirement that high-strength screw steel and screws in its products have high tensile strength to resist stretching, breaking, sliding and abrasion. The material must have high plasticity and toughness to reduce sensitivity to surface quality issues such as deflection and notch stress concentration.
Fasteners working in humid or corrosive atmospheres must have low sensitivity to delayed fracture. Bolts that support alternating and impact loads must have increased fatigue strength and multiple impact tensile strength to resist fatigue and multiple impact fractures. For fasteners operating in extremely cold areas, low ductile and brittle transition temperatures are required for the fastener materials.
Table.1 Mechanical property indices of high-strength screws
| Mechanical properties | Bolt Grade | ||||
| 8.8 | 9.81040-1180 | 10.9 | 12.9 | ||
| ≤M16mm | >M16mm | ||||
| Tensile strength/MPa | 800-980 | 830~980 | 32~39 | 1040-1180 | 1220~1380 |
| Rockwell/HRC hardness | 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:
- It must have high tensile strength and a high yield rate.
- It must present sufficient plasticity, especially in the plastic area during tightening.
- It must be able to withstand repeated tightening, which means it can withstand large stress amplitude loads multiple times and possess good low-cycle fatigue performance.
- Must demonstrate good high-cycle fatigue performance when subjected to alternating workloads.
- It must have high impact resistance when exposed to impact loads.
- It must have good resistance to delayed fracture.
- It must have good resistance to low temperatures.
- It should have good creep resistance and stress relaxation resistance.
- It should have low notch sensitivity, as screws are multi-notched parts.
- It must maintain a stable surface friction coefficient to obtain a stable assembly preload.
2. Search status of high strength screws
The use of high-strength screws in China is relatively recent. It was first used in some railway bridges in the 1960s and later in steel structures for boilers in the 1980s.
In the 1990s, China began to introduce foreign cars and production technologies and discovered bolts with a strength grade of 12.9, a tensile strength of 1,200 MPa, and a yield strength of 1,080 MPa. At that time, these screws had the highest strength level among automotive screws.
After the FAW Group imported the Chrysler 488 engine from the United States, flywheel bolts became dependent on imports. To achieve localization, FAW Group identified the materials used for steering wheel bolts in the United States and the high-strength bolts used for German Audi cars, both equivalent to ML35MnMo and ML35CrMo, respectively, by comparing the composition of foreign products High-strength screw materials with existing materials in China.
Therefore, ML35CrMo was selected as the material for the domestic experimental production of grade 12.9 flywheel screw material. The decarburized layer on the surface of raw materials has been removed by material peeling technology. After cold forging and final quenching tests, annealing, quenching and tempering process tests, finished product performance tests, bench tests and load tests, high-strength bolts with properties equivalent to those of CA488 engine flywheel bolts were successfully developed.
Wang Rongbin et al. used martensite slat structure to improve the performance of high-strength screws. They can also obtain high-performance bolts above grade 10.9 and partially replace quenched and tempered high-quality structural steel. Low carbon martensite steel (slat martensite) 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 the automobile and standard parts industries. For example, ML15MnVB, ML20MnVB, ML15MnB and ML15Mn are used to make high strength screws of grade 8.8, 9.8 and 10.9, which have achieved good results.
Leng Guangrong and his team successfully controlled the properties of low-carbon medium alloy steel (22Cr2Ni4MoV) to achieve a tensile strength of 1560 MPa, elongation of 12%, hardness of 45 HRC, and impact energy of 60 J through of appropriate heat. treatment process.
However, high-strength screws made of this material can barely meet the 2500mm four-height rolling mill's requirements for the mechanical properties of screw materials. Furthermore, the average lifespan of screws is only two months, which is not satisfactory in terms of durability.
To improve the strength of the material, Pan Zuyi et al. used the 22Cr2Ni4MoV material and controlled the chemical composition, structure and properties through the heat treatment process of quenching + low temperature quenching or quenching + high temperature quenching. This resulted in the strength, plasticity and toughness of the steel being well combined.
The newly developed high strength screw steel has a long service life for the four height reversible mill 2500mm universal joint screw.
However, when tensile strength exceeds 1200 MPa, delayed fracture becomes a significant problem. High-strength screws are slotted parts and have high notch sensitivity, making them susceptible to delayed fracture at the stress concentration position of the notch. As a result, its scope of application is limited.
To address this issue, Hui Weijun et al. increased the Mo content and added microligand elements V and Nb, while reducing the content of Mn and impure elements P and S, in the composition of the 42CrMo material. They developed a high-strength screw steel ADF1, which exhibits good delayed fracture resistance at the strength level of 1300 MPa.
Further analysis indicates that the grain size of the steel has been refined from about 12 μm to about 5 μm. This refinement, combined with the secondary hardening effect of Mo and V carbides and cyclic heat treatment, significantly increased the critical tensile stress of the notch.
Therefore, it can be concluded that the delayed fracture resistance of high-strength bolts can be improved by adjusting the alloy content, adding corrosion-resistant alloy elements, refining the grains, reducing grain boundary segregation, increasing the temperature of tempering and neutralizing the invading hydrogen.
Thanks to these measures, Sumitomo Metal's ADS series, Kobe Iron's KNDS series and China Iron and Steel Research Institute's ADF series have successfully developed high-strength bolt steel with good delayed fracture resistance.
However, compared with developed countries, the level of research and development of high-strength bolt steel in China is still relatively backward. Currently, only materials such as ML20MnVB, ML35CrMoV and 35CrMo can meet the requirements for grade 12.9 high strength screws.
In 2005, China was still importing grade 12.9 connecting rod bolts used in automobile engines due to a lack of domestically produced high-strength bolt steel.
Although Hui Weijun and others have developed a 1300 MPa high-strength screw material, 42CrMoVNb, based on 42CrMo, its performance in practical applications needs further investigation.
The material properties required for high-strength screws vary depending on service environments.
Yang Xinglin and colleagues found that the 35CrMnSiA material used for high-strength screws in the marine environment is prone to fracture during service.
Analysis revealed that the bolt's fracture was not due to ordinary hydrogen embrittlement, but to stress corrosion cracking caused by severe corrosion of the marine atmosphere and seawater on the bolt's materials.
It was suggested that replacing the coating and improving the detection level of finished products would increase the bolt's resistance to stress corrosion cracking, but the problem of material performance defects remained unresolved.
After considering the service environment, Fang Dong and his team chose 16Co14Ni10Cr2Mo material to replace 35CrMnSiA.
This steel has high strength, good plasticity, toughness and excellent overall performance.
Although it has been widely used in aviation, this is the first time it has been used in the manufacture of large section bolts and applied in the marine environment.
The simulated test in marine environment showed that the M56 bolt made of 16Col4Nil0Cr2MoE steel does not break due to low temperature brittleness or notch brittleness. Furthermore, stress corrosion cracking and overload fracture do not occur in the pre-tightening state even if the coating is worn, and normal operation does not cause overload fracture.
The screw product is safely applicable for one year in practical use. Chinese scholars' study of high-strength screws focused on the hydrogen embrittlement fracture mechanism, improving the heat treatment process, and failure analysis of high-strength screws. This research provides a crucial foundation for the development of high-strength screw materials in the future.
The performance of high-strength fastener materials strongly depends on the alloy and trace elements. Research has shown that the addition of microalloying elements, such as 0.02% Ti, to unquenched and tempered steel can precipitate a phase that prevents grain growth during heating and hot working, and strengthens the matrix during cooling, improving the general properties of the steel.
However, not all precipitates improve the comprehensive properties of steel. Using the Thermo scale and Dicta software, precipitation in 40MnV microalloyed steel was calculated.
The composition, morphology and distribution of the precipitates were studied through electrolytic analysis, X-ray diffraction and transmission electron microscopy. The results indicate that a small amount of N and Ti in steel can cause the precipitation of coarse TiN particles, with a size of 50 nm, in the solid-liquid biphasic zone.
Gladman's theory suggests that the (Ti,V)(C,N) particles precipitated in the solid-liquid two-phase zone cannot impede grain growth during heating. Instead, these coarse particles harm the properties of the steel.
By reducing the N or Ti content, the precipitation temperature and the amount of TiN in the solid-liquid two-phase zone can be effectively reduced, ensuring more VN precipitation. About 0.02% Ti in microalloyed steel should be reduced to an appropriate range, and the N content should also be controlled in the appropriate range.
By studying the influence of alloying elements on material properties, we can lay a foundation for the development of new materials for high-strength fasteners. However, a suitable alloy composition alone cannot guarantee that the developed fasteners meet actual performance requirements. Only through a reasonable heat treatment process and coordination of the hardness, strength, plasticity and toughness of the material can screws with excellent performance be developed.
30NCD16 is a high-strength alloy steel with robust 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.
They determined that the ideal heat treatment process for 30NCD16 high strength steel is between 840 and 870℃. After quenching and tempering at 560℃, a fine and uniform sorbite structure can be obtained. The tensile strength of steel is more than 1200 MPa and the Akus impact energy is more than 50 J.
Wang Genji et al. studied the effect of different heat treatment processes on the microstructure and mechanical properties of Q390 high-strength low-alloy steel thick sheet using microstructure observation and mechanical property measurement.
The results show that normalizing at 920℃ for 36 min can completely austenitize the mixed crystal structure in the hot-rolled Q390 high-strength low-alloy steel plate, achieving grain refinement. Subsequent cooling transforms it into polygonal ferrite and pearlite, resulting in excellent comprehensive mechanical properties.
The elongation and impact resistance of CrNiMoBNb16-16 steel are significantly higher than those in the hot-rolled state, and the phenomenon of tensile fracture delamination is completely eliminated. This high-alloy steel is an important material for industrial production, mainly used as a screw material in applications that require high temperature resistance, such as steam turbines, gas turbines, engines, chemical reactors and high-pressure thermal equipment.
He Wei et al. analyzed the relationship between the structure and mechanical properties of CrNiMoBNb16-16 steel from two perspectives: the influence of heat treatment processes on tensile properties at ambient and high temperatures and the influence of test temperature on tensile properties.
The test results indicate that with the increase of the test temperature (20～650℃), the strength and plasticity of the material decrease significantly. For this material, hot forging has better comprehensive performance than high-temperature forging. Consequently, hot forging is determined to be the ideal thermomechanical treatment for this material, resulting in the best combination between strength and toughness.
For most alloy structural steels, the tensile strength can be increased to 1200MPa by adjusting the alloy composition and carrying out appropriate heat treatment. However, further increasing strength may reduce the use value of the material and cause greater insecurity due to delayed fracture.
The results show that the delayed fracture resistance of high-strength steel can be improved by reducing grain boundary segregation, refining the grains, increasing the tempering temperature, adjusting the alloying elements, reducing the amount of hydrogen intrusion on the surface and rendering hydrogen intrusion harmless.
3. Common high-strength screw materials and their strengthening and toughness mechanism
3.1 Low alloy steel
Low alloy steels typically contain a medium amount of carbon. The alloy composition includes Cr, Cr Mo, Cr Ni, Ni Cr Mo, Mn and Mn Cr series.
Table 2 shows that low-alloy screw steel has a wide range of applications, and the strength grade can be selected from 700 to 1000 MPa.
However, when the strength exceeds 1200 MPa, the problem of delayed failure of bolts made of low-alloy steel becomes prominent and needs to be solved.
At present, low-alloy steel is still the main material for high-strength screws.
Bolts made from low-alloy steel require quenching and tempering, which means they must first be quenched and then tempered.
Furthermore, due to the high carbon content and alloying elements, steel has high hardness and resistance to deformation.
Therefore, spheroidizing annealing treatment is necessary before cold forging.
Because low-alloy steel contains multiple alloying elements, finding ways to save the alloy in bolt steel and reduce costs is a critical issue to consider.
In addition, due to the relatively high content of carbon and alloy elements, the plasticity and toughness of steel are poor.
To further increase strength and ensure the necessary plasticity, this issue requires further research.
Tab.2 MPa resistance degree of some steels for screws
| Steel Type | 400 | 500~600 | 700~800 | 900~1000 | 1100 |
| Carbon steel | √ | √ | √ | ||
| unquenched and tempered steel | √ | ||||
| boron steel | √ | √ | √ | ||
| light alloy steel | √ | √ | √ |
3.2 Boron steel
With the development of cold forging technology, there has been a significant increase in the demand for cold forged bolt steel.
High strength bolts were traditionally made from medium carbon steel and medium carbon alloy steel. However, these steels have high hardness and significant resistance to cold deformation, requiring spheroidizing annealing treatment before cold forging. This process consumes a lot of energy.
To solve this problem, steel with low carbon and boron content was developed. The basic design principle of low carbon and boron steel composition is to reduce the carbon content and improve the cold deformation ability of the steel. A small amount of boron is added to compensate for the loss of strength and hardenability caused by carbon reduction. Additionally, small amounts of Cr, Mn and other alloying elements can be added as needed to further improve hardenability.
The characteristics of low carbon and boron steel are as follows:
- A small amount of boron can replace a large amount of alloying elements, making it more economical.
- Rolled products can be directly cold forged without pre-spheroidization treatment due to the low content of carbon and alloy elements. This saves a lot of energy.
- The tendency for tempering, deformation and cracking is minimal, making it possible to treat the steel by water quenching, saving quenching oil and improving operating conditions and the working environment.
- Low carbon and boron steel has excellent properties, including improved toughness compared to medium carbon alloy steel at the same strength level, high resistance to fatigue failure, and low sensitivity to decarburization.
Boron steel screws have been increasingly used in the automobile, construction, machinery and other industries. As Table 2 illustrates, bolts with strengths ranging from 700 MPa to 1100 MPa can be made from boron steel.
3.3 Unquenched and tempered steel
Unquenched and tempered steel contains a small amount of alloying elements and does not require quenching and tempering. By controlling the hot working deformation and subsequent cooling rate, the required mechanical properties can be guaranteed, saving energy consumption for heat treatment, shortening the production cycle and reducing the cost of steel.
At present, unquenched and tempered steel screws are mainly used in automobile manufacturing, but their total number is still small and their scope of application is not wide. Although its cost is lower than quenched and tempered steel, its toughness is lower, the strength level is not stable enough, and the die life is shorter during cold forging. These limitations restrict the scope of application of unquenched and tempered steels.
Unquenched and tempered steel is mainly used for 700-800 MPa grade bolts, and sometimes for bolts above 900 MPa. Typically, unquenched and tempered C-Mn systems with a carbon content of about 0.25% or C-Mo systems with a carbon content of about 0.10% are used for grade 700-800 bolts MPa. Traces of 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 generally 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.
To improve the toughness of unquenched and tempered steel and obtain adequate strength and toughness, adjusting the processing technology (such as hot working temperature, rolling deformation and controlled cooling after rolling) in addition to controlling the chemical composition can also be an solution.
3.4 Low carbon martensitic steel
All non-alloy steel (carbon steel) or low-carbon and low-alloy structural steel with a carbon content of less than 0.25% can obtain more than 80% and sometimes even 100% low-carbon martensite structure after intensive quenching .
This type of steel is commonly referred to as low carbon martensite steel. It has a hardness of 45-50 HRC, a yield strength of 1000-1300 MPa and a tensile strength of 1200-1600 MPa.
It presents good plasticity (A ≥ 10%, Z ≥ 40%) and toughness (Axv ≥ 59 J), together with excellent cold workability, weldability and minimal distortion from heat treatment.
As a result, the use of low-carbon martensite steel is increasingly widespread and has become a crucial way to unlock the strength and toughness potential of steel and extend the life of machine parts.
The materials commonly used in the production of high-strength screws include 15MnVB, 20MnSi, 20 steel, 20MnTiB and so on.
3.5 Strengthening and hardening mechanism
The mechanisms that strengthen and harden high-strength steel mainly include fine-grain strengthening, solution strengthening, precipitation and dispersion strengthening, and dislocation strengthening.
1) Strengthening fine grains.
By increasing grain boundaries to obstruct the movement of dislocations and restrict plastic deformation within a certain range, it is possible to improve the plasticity of steel. This not only effectively increases strength, but also significantly optimizes plasticity and strength.
Currently, controlled rolling and controlled cooling (TMCP) technology is widely used in industry. It involves refinement of the final structure through austenite recrystallization, strain-induced ferrite transformation (DIFT), accelerated cooling, and ferrite recrystallization.
2) Strengthening the solution
The metallic matrix (solvent metal) can be strengthened using the internal point defects of metallic materials, such as interstitial atoms and replacement atoms.
As the difference in atomic diameters increases, the degree of distortion also increases, which leads to a greater strengthening effect.
Furthermore, the addition of elements such as Mn, Si, Ni, Mo to Fe can cause displacement-type solid solution strengthening.
3) Strengthening precipitation and dispersion
When second phase particles precipitate, they create a stress field and a region of high energy in the matrix, resulting in a marked increase in strength, hardness and overall strengthening.
It can be concluded that:
- The greater the volume ratio of the precipitated phase, the more significant the strengthening effect.
- The greater the dispersion of the second phase, the better the strengthening effect.
- The greater the resistance of the second phase particles to dislocation movement, the greater the strengthening effect.
4) Strengthening the dislocation
It is challenging to move displacements due to their high density.
A mechanical property of metals is improved strength. Multiplication of dislocations can strengthen real metals with crystalline defects.
The movement of dislocations is the main reason for solution strengthening, fine grain strengthening, precipitation and dispersion strengthening.
The microdefects of the matrix structure, including grain boundaries, precipitation particles, dislocation substructure and solution distortion, mainly affect the strength and toughness of high-strength screw materials.
The micro defect structures mentioned above can improve the strength of steel. However, although an increase in grain boundaries (i.e., strengthening of fine grains) can increase toughness, other microdefect structures can reduce toughness.
To strengthen high-strength screw materials, it is necessary to fully utilize these strengthening mechanisms.
4. Research perspective of materials for high-strength screws
With the advancement of the energy, automobile, machinery, construction, light industry and other sectors, there is an increasing demand for materials to produce various types of fastener screws, leading to an urgent need for high-strength screw materials.
Over the past decade, significant emphasis has been placed on advancing this technology, both nationally and internationally. In China, the “Main Basic Research on New Generation Steel Materials” project (973) was initiated, which includes research and development of high-strength bolt steel as one of its critical areas.
The development trend of high-strength screw steel can be summarized as follows:
1) High-strength, high-performance steel
As the strength of steel increases, its susceptibility to delayed fracture also increases. Specifically, when the tensile strength exceeds 900 MPa and the hardness is equal to or greater than 31 HRC, the delayed fracture sensitivity gradually increases. Furthermore, the higher the service stress, the more extensive the damage caused by fracture.
For this reason, it is crucial to develop high-strength screw steels with exceptional delayed fracture resistance. This will help protect people's lives and property while expanding the range of applications for high-strength screws.
2) Reduce costs and energy consumption
To reduce costs, consider replacing cheap boron steel with high-priced alloy steels containing Ni, Cr, Mo, etc.
Additionally, the following techniques can help reduce energy consumption, improve screw performance, and minimize the tendency for thread ends to decarburize:
- Use cold forging instead of hot forging
- Use unquenched and tempered steel to reduce the heat treatment process
- Use low-carbon unannealed steel that does not require softening treatment before forging
- Use high-precision laminated products that do not need to be peeled and stretched.
3) Improve the quality and reliability of screw steel
The reliability and service life of screw parts are closely related to the metallurgical quality and surface condition of screw steel, as well as some processing properties.
By increasing the purity of the steel and reducing the S and P content, the deformation capacity of the steel can be improved. This reduces the embrittlement of grain boundaries and the presence of non-metallic inclusions, thus increasing the toughness and plasticity of the steel. It also improves the delayed fracture resistance of the steel.
In addition, the manufacturing accuracy, fastening technology and testing methods of finished screws are crucial factors that affect the reliability of high-strength screws.
