National Standard GB/T 3098.23-2020, “Mechanical Properties of Fasteners—Bolts, Screws, and Studs from M42 to M72,” was released on March 31, 2020, and came into effect on October 1, 2020.

GB/T 3098.24-2020, "Mechanical Properties of Fasteners—Stainless Steel and Nickel Alloy Bolts, Screws, Studs, and Nuts for High-Temperature Applications," and GB/T 3098.25-2020, "Guidelines for Selecting Stainless Steel and Nickel Alloy Fasteners Based on Mechanical Properties," were published on November 19, 2020, and came into effect on June 1, 2021.

This article provides a brief analysis and introduction to the key design considerations for fasteners, material selection, and testing as outlined in the standard, helping readers better understand and apply these concepts in their work.

1. Introduction

Fasteners are a general term for a category of mechanical parts used to securely join two or more components (or structural elements) into a single, integral unit. The primary function of fasteners is to provide secure connections between mechanical parts. They are widely used and characterized by an extensive variety of types and specifications, diverse performance and applications, as well as a high degree of standardization and serialization.

To ensure that China’s demand for and technical standards of mechanical components—such as those used in machinery, equipment, and vehicular transportation—remain aligned with international standards, enhance the reliability and safety of machinery, marine equipment, wind power generation systems, and equipment used in construction and mining, and better meet market demands.

In recent years, the National Fastener Standardization Technical Committee has revised or developed a series of national standards related to fastener manufacturing and performance testing, with the aim of standardizing fastener design. These standards place great emphasis on the rational selection of materials; in addition to dedicating an entire chapter to "materials," many material-selection provisions have also been designated as mandatory. This means that designers must regard the proper and optimized selection of materials as a fundamental skill when carrying out fastener design work.

National Standard GB/T 3098.23-2020, “Mechanical Properties of Fasteners—Bolts, Screws, and Studs M42–M72,” was released on March 31, 2020, and came into effect on October 1, 2020. GB/T 3098.24-2020, “Mechanical Properties of Fasteners—Stainless Steel and Nickel Alloy Bolts, Screws, Studs, and Nuts for High-Temperature Applications,” was released on November 19, 2020, and came into effect on June 1, 2021.

GB/T3098.25-2020, "Mechanical Properties of Fasteners—Guidelines for Selecting Fasteners Made of Stainless Steel and Nickel Alloys," was published on November 19, 2020, and came into effect on June 1, 2021. The purpose of this article is to provide readers with an analysis and introduction to these standards, making it easier for everyone to understand and apply them in design. Should you find any inaccuracies in the text, we would greatly appreciate your constructive criticism and corrections.

2. GB/T3098.23-2020 "Mechanical Properties of Fasteners—Bolts, Screws, and Studs from M42 to M72"

GB/T3098.23-2020 "Mechanical Properties of Fasteners—Bolts, Screws, and Studs from M42 to M72" specifies the mechanical and physical properties of bolts, screws, and studs in the size range of M42 to M72 when tested under ambient temperatures ranging from 10°C to 35°C.

Fasteners that meet the requirements of this standard within the specified ambient temperature range may fail to achieve the prescribed mechanical and physical performance when used outside this temperature range. Fasteners manufactured according to GB/T3098.23 are suitable for use at temperatures ranging from -50℃ to +150℃.

For bolts, screws, and studs with coarse threads in the strength grades of 8.8 and 10.9, ranging from M42 to M64, and with fine threads ranging from M45×3 to M72×6, the fasteners must be made of alloy steel. The ordinary threads shall comply with the specifications stipulated in GB/T 192 and shall also conform to the diameter-pitch combinations specified in GB/T 192.

The GB/T3098.1-2010 standard, "Mechanical Properties of Fasteners—Bolts, Screws, and Studs," emphasizes that the materials used for products of Grade 10.9 and higher must exhibit good through-hardening properties. For fasteners of Grade 8.8 with thread diameters exceeding 20 mm, in order to ensure good through-hardening performance, materials specified for Grade 10.9 must be employed.

The new standard GB/T3098.23-2020, "Mechanical Properties of Fasteners—Bolts, Screws, and Studs from M42 to M72," stipulates that fasteners with performance grades 8.8 and 10.9 must be made from alloy steel to ensure that the core of the fastener’s threaded section achieves approximately 90% martensitic structure in the quenched state, prior to tempering.

For this reason, metallographic examination has received considerable attention in the fastener industry. Metallographic examination not only involves studying the internal microstructure of metallic materials using a metallographic microscope, but also includes macroscopic inspection conducted with the naked eye or under low-magnification magnifiers.

In actual production environments, during tensile testing, it is essential to follow standard procedures and pay close attention to key considerations. Mechanical property testing of large-diameter bolts ranging from M42 to M72 is a top priority in quality control. Currently, the most common method for measuring strength in China is to use a universal material testing machine, which can determine the tensile strength and yield strength of materials, as well as perform wedge-load tests on bolts and measure ductility indicators such as the post-fracture elongation A and the reduction of area Z.

The preparation of the specimens complies with the standard specifications outlined in GB/T 3098.1. For tensile specimens of bolts with a specification ≤ M39, the machining method involves direct turning. According to the standards specified in GB/T 3098.23, tensile specimens of bolts with a specification ≥ M42 are prepared by first taking an eccentric sample and then proceeding with turning.

Take a cross-section of the hardness specimen at a distance of 1d from the thread end. While ensuring parallelism, sequentially polish the cross-section using fine sandpaper to ensure that the surface roughness meets the requirements of the test standard.

After the tensile specimens are machined, their dimensions are verified. Only specimens that meet the dimensional requirements can be used to start the tensile test on the testing machine. The testing machine is equipped with an automatic centering device, which ensures that during the test, the clamped specimen is subjected only to axial tensile force, thereby preventing oblique tension. Oblique tension could lead to uneven stress distribution in the specimen being tested, thus compromising the accuracy of the test results.

a. The influence of specimen preparation: Different cross-sectional shapes and specimen diameters can affect the test data. According to the new standard, for diameters ranging from M42 to M72—and particularly when mechanical machining tensile specimens are prepared—the specimens should be machined at a location 1/4 of the bolt or stud diameter (eccentric specimens), with the specimen bar diameter being 3/8 of the original diameter. When the same bolt is used to prepare specimens of different diameters, the resulting test outcomes will show certain variations.

b. The influence of specimen clamping: During tensile testing, eccentric forces are not allowed. Particularly when conducting actual tensile tests on large-specification bolts, if the specimen is improperly clamped and an eccentric force is present, the resulting test values will be lower than the true values. When performing tensile tests on fasteners, it’s crucial to ensure that the middle portion of the fixture has a threaded section longer than 1d, which should remain unclamped. If too much thread is clamped, it may lead to lower plasticity indices and inaccurate tensile strength readings.

c. The effect of tensile speed: The national standard specifies a tensile speed (generally no more than 10 mm/min). At room temperature, the tensile test conducted at different speeds has little impact on the tensile strength of fastener products. However, it significantly affects the proof stress (yield strength), which is defined as the non-proportional extension strength. Typically, it is recommended to perform the tensile test at a slower rate and maintain a uniform speed throughout the test—avoiding a pattern of initially fast and then slowing down.

d. Environmental factors: During tensile testing, the temperature should be maintained between 10°C and 35°C, and vibrations and electromagnetic interference should be minimized as much as possible.

3. GB/T3098.24-2020 "Mechanical Properties of Fasteners—Stainless Steel and Nickel Alloy Bolts, Screws, Studs and Nuts for High-Temperature Applications"

The newly released standard GB/T3098.24-2020, “Mechanical Properties of Fasteners—Stainless Steel and Nickel Alloy Bolts, Screws, Studs, and Nuts for High-Temperature Applications,” specifies the chemical composition, mechanical properties, and physical performance requirements for bolts, screws, studs, and nuts made from corrosion-resistant stainless steel and nickel alloys, which are intended for use under high-temperature conditions up to 800℃. The standard also stipulates that these fasteners are suitable for use at low temperatures down to -50℃.

Fastener types include bolts, screws, studs, and nuts, with coarse-thread specifications ranging from M3 to M39; fine-thread specifications ranging from M8×1 to M39×3; and fully load-bearing fasteners of any shape.

This standard specifies the performance characteristics of fasteners made from martensitic steels, precipitation-hardened austenitic steels, and nickel alloys. The category codes for these three material types are as follows: martensitic stainless steels: CH0 (20Cr13), CH1 (30Cr13), CH2 (17CrNi16, 14Cr17Ni2), V/VH (22CrMoV12, ML21CrMoV), VW (19CrMoNbVN11);

Precipitation-hardened austenitic steel: SD (ML06Cr15Ni25Ti2MoAlBV); nickel alloys: SB (GH4080A, NiCr20TiAl) and 718 (NiCr19NbMo). For detailed information on material grades, refer to GB/T3098.25-2020, which specifies the technical requirements for selecting appropriate stainless steels and nickel alloys as well as their performance characteristics.

When mechanical property tests are conducted according to the prescribed methods, bolts, screws, and studs identified by their specified fastener designations shall meet all the required mechanical property specifications under room-temperature conditions and shall be comparable to those stipulated in GB/T 3098.1 and GB/T 3098.23. This applies whether the tests are performed during the manufacturing process or on the finished products. Fasteners manufactured in accordance with this standard shall undergo heat treatment; their physical and mechanical properties as well as the heat-treatment process shall comply with the provisions of this standard, taking into account both the maximum service temperature and the actual operating temperature of the fasteners.

The heat treatment process flow is as follows: Martensitic stainless steels CH0, CH1, CH2, V, VH, and VW shall be quenched and tempered (QT). Precipitation-hardened austenitic steels and nickel alloys SD, SB, and 718 shall undergo solution annealing (AT) followed by age hardening (P); it is strongly recommended that this process be carried out after fastener forming. When the tensile strength (Rm) of external-threaded fasteners is equal to or greater than 1100 MPa, with mutual agreement between supplier and customer, solution annealing (AT) may be applied directly to the raw materials (prior to fastener forming). For cold-forged and hot-forged fasteners, heat treatment should be performed after the fastener has been formed.

For fasteners formed by machining from bar stock, heat treatment can be applied either to the raw material or after the fastener has been machined and formed. In the case of externally threaded fasteners, threads can be rolled either before or after heat treatment, or between solution annealing (AT) and age hardening (P).

During use, nuts should be matched with bolts, screws, and studs (and washers) of the same designation—for example, CH0 bolts should be paired with CH0 nuts. While it is permissible—or even possible—to combine fasteners made of different materials, such combinations must meet the minimum corrosion resistance requirements for the joint assembly and take into account the risk of wear.

4. GB/T3098.25-2020 "Mechanical Properties of Fasteners—Guidance for Selecting Fasteners Made of Stainless Steel and Nickel Alloys"

Internationally, innovation in fastener technologies for stainless steel and nickel-alloy materials has remained highly active, with a continuous stream of innovative products emerging and the industry scale steadily expanding. In recent years, new technological trends have gradually emerged, including diversification of material types and stabilization of performance.

The United States and several European countries have long held a leading position in technologies and industries related to stainless steel and nickel-alloy fasteners. In recent years, China has seen growing demand for stainless steel and nickel-alloy fastener products in terms of both variety and specifications. However, we still rely on imports for certain materials, the forming processes for large-sized components remain unstable, and the functional characteristics of our fastener products are yet to effectively meet the requirements of equipment applications. Moreover, there is still a gap in the stability and reliability of these products.

The demand for stainless steel and nickel-alloy fasteners is steadily increasing in fields such as clean energy, aerospace, rail transit, marine engineering, and gas turbines, as well as in major equipment sectors.

Currently, there is no ISO standard yet. This project is being carried out in parallel with ISO/NP 3506-5 and ISO/NP 3506-6. Based on actual domestic production conditions and both domestic and international trade demands, we will develop a national standard for the mechanical properties of stainless steel and nickel-alloy fasteners in China. This will provide strong technical support for addressing the aforementioned issues and play a positive role in perfecting and optimizing the fastener standards system, guiding specialized fastener production, enhancing product quality, and improving market satisfaction levels.

The new standard GB/T3098.25-2020, "Mechanical Properties of Fasteners—Guidelines for the Selection of Stainless Steel and Nickel Alloy Fasteners," takes into account China’s specific conditions and draws on ISO/DIS 3506-6. Guided by the principles of scientific rigor, rationality, applicability, and promoting the fulfillment of Chinese stainless steel and nickel alloy fastener products for various applications, this standard has undergone in-depth research and comparative analysis. Based on stainless steel grades and classes, the chemical compositions of stainless steel and nickel alloys, resistance to stress corrosion cracking, resistance to pitting and crevice corrosion, intergranular corrosion, sensitivity to the formation of intermetallic compounds, and magnetic permeability characteristics of stainless steels, this standard provides guidelines suitable for the selection of coarse-thread and fine-thread stainless steel and nickel alloy fasteners in China.

This standard provides a guide for the selection of stainless steels and technical information on their performance characteristics. It includes technical specifications for corrosion-resistant stainless steels and nickel alloys suitable for fastener manufacturing, covering austenitic, martensitic, ferritic, and duplex (austenitic-ferritic) stainless steels as well as nickel-alloy fasteners.

According to the GB/T 3098 stainless steel series standards, the following groups and grades are covered: austenitic steels A1–A5 and A8; martensitic steels C1, C3, and C4; ferritic steel F1;

Duplex steels (austenite-ferrite) D2, D4, D6, and D8. Stainless steels encompass a wide variety of materials, offering diverse corrosion resistance and functional characteristics. The specific fasteners made from stainless steel should be carefully selected based on all the conditions expected in the environment where the bolted connections will be used. The surface condition of the fasteners (such as passivation and surface roughness) can influence their corrosion resistance.

In special cases, it is recommended to consult experts in fastener manufacturing and/or stainless steel materials to ensure the right choice is made for the specific application conditions. Corrosion is related to several aspects of fasteners, including bolted joint design, service environment, material and surface treatment, stress state, temperature, and corrosion caused by contact between dissimilar metals (electrochemical corrosion or galvanic corrosion).

The specified operating temperature ranges for different categories of stainless steel fasteners are as follows: Austenitic stainless steel fasteners: –196℃ to +300℃; Martensitic stainless steel fasteners: –40℃ to +230℃; Ferritic stainless steel fasteners: –20℃ to +250℃.

Duplex steel (austenitic-ferritic) fasteners: -40℃ to +280℃. The standards covered include GB/T3098.6, GB/T3098.15, GB/T3098.16, GB/T3098.2, and GB/T3098.24—these are the most widely used standardized materials, also covering ISO15510, EN10269, and/or DIN 267-13.

Appendix A lists stainless steel materials suitable for fasteners (but not included in the GB/T 3098 series standards). The use of these stainless steels requires prior agreement between the supplier and the buyer. The purpose of specifying these standards is to provide a standardized and guiding framework for the selection of stainless steel and nickel alloy fasteners.

5. Closing Remarks

The main manufacturing process flow for fasteners includes the following steps: raw material procurement → re-inspection → material cutting → cold heading or hot forging (for bolts and nuts) → heat treatment → performance testing → machining → thread rolling → surface treatment → surface inspection → non-destructive testing → dimensional inspection → packaging and transportation.

To meet the evolving needs of the fastener industry, the National Technical Committee for Standardization of Fasteners has revised or developed a series of new national standards for fastener manufacturing and performance testing. As a result, China has established the GB/T 3098.23, 24, and 25 series of standards on mechanical properties of fasteners, enriching China’s range of fastener products and standard specifications. These standards provide data support for research and development of domestic fastener products and material processing technologies, thereby breaking the monopoly of foreign technologies and standards.

More importantly, the establishment of the standard “Mechanical Properties of Fasteners—M42 to M72 Bolts, Screws, and Studs; Mechanical Properties of Stainless Steel and Nickel Alloy Bolts, Screws, Studs, and Nuts for High-Temperature Applications; and Guidelines for Selecting Stainless Steel and Nickel Alloy Fasteners Based on Mechanical Properties” addresses the domestic shortage of comprehensive standards covering the entire fastener industry chain. This standard will provide methodologies and data references for the subsequent development and revision of related standards, thereby promoting the growth of the industry.

A large number of fatigue failure analyses of high-strength bolts indicate that more than 70% of fatigue failures originate from surface damage, decarburization at the head-shank junction, obvious fine cracks or discontinuities in the thread machining marks and surface corrosion, as well as uneven quenching microstructure—precisely because these areas experience high stress concentrations.

To this end, vigorous efforts are needed to promote and implement the new standards GB/T 3098.23, 24, and 25. It is recommended that fastener quality management be strengthened and optimized across all stages—including design, procurement, manufacturing, installation, nonconformity management, inspection, and testing.