According to Chinese national standards, the core implementation standard for high-strength bolts is GB/T 3632-2018 "Specifications for Bolts, Screws, Studs and Nuts for High-Strength Fasteners". This standard clearly specifies the classification, dimensional parameters, material requirements, mechanical property indicators, surface treatment methods and inspection procedures of high-strength bolts.
Based on the standard, high-strength bolts are mainly divided into two performance grades: Grade 10.9 and Grade 12.9. Among them, the tensile strength of Grade 10.9 bolts is not less than 1000MPa, and the yield strength is not less than 900MPa; the tensile strength of Grade 12.9 bolts is not less than 1200MPa, and the yield strength is not less than 1080MPa. Such high-strength bolts are mainly used in engineering structures and equipment that require high connection reliability and bear large loads or dynamic loads, such as bridges, high-rise buildings, heavy machinery, automotive manufacturing and other fields.
The standard also defines the dimensional range of high-strength bolts (including thread diameter, thread length, total bolt length, etc.). The appropriate size should be selected according to the load requirements, connection thickness and other factors of specific application scenarios. In terms of materials, high-strength bolts are usually made of high-quality alloy steel (such as 40Cr, 42CrMo, etc.), and their high strength and good toughness are guaranteed through quenching and tempering heat treatment. For bolts used in some special environments, surface treatments (such as phosphating, black oxide, etc., to avoid hydrogen embrittlement risk) can also improve corrosion resistance. In terms of mechanical properties, in addition to tensile strength and yield strength, the standard also puts forward clear requirements for plastic indicators such as elongation after fracture and reduction of area of bolts, ensuring that bolts have a certain safety margin when bearing ultimate loads.
Following national standards for the design, manufacture, inspection and use of high-strength bolts is the key to ensuring the reliability and safety of bolted connections. In engineering practice, it is necessary to select high-strength bolts that meet the performance grade and dimensional requirements in combination with specific engineering requirements, load conditions and standard specifications to ensure the stability and safety of engineering structures or equipment.
In addition to the Chinese national standard GB/T 3632-2018, there are also widely recognized international standards, mainly including relevant specifications formulated by ISO (International Organization for Standardization) and ASTM (American Society for Testing and Materials).
In the ISO standards, the core standards related to high-strength bolts are ISO 898-1 "Carbon Steel and Alloy Steel Threaded Fasteners - Part 1: Mechanical Properties" and ISO 898-2 "Carbon Steel and Alloy Steel Threaded Fasteners - Part 2: Chemical Composition of Fasteners with Specified Performance Grades". ISO 898-1 specifies the mechanical performance requirements for high-strength bolts made of carbon steel and alloy steel, including tensile strength, yield strength, elongation after fracture, impact energy, etc.; while the dimensions and allowable tolerances of bolts shall comply with special dimensional standards such as ISO 4014 (hexagon head bolts) and ISO 8676 (hex socket cap bolts).
In the ASTM standards, the core standards for high-strength structural bolts are ASTM A325/A325M and ASTM A490/A490M. ASTM A325/A325M is applicable to medium and high-strength bolts for structural steel connections, and the material can be carbon steel or low-alloy steel; ASTM A490/A490M is a higher strength grade structural bolt made of alloy steel, suitable for connection scenarios bearing larger loads. Both standards detail the chemical composition, mechanical properties, dimensional accuracy, allowable tolerances and inspection methods of bolts.
It should be noted that high-strength bolt standards in different countries and regions may vary in performance grade definition, dimension series, application scenarios, etc. Therefore, in transnational projects or supporting export equipment, the corresponding high-strength bolt standards and products should be selected according to the standard requirements of the country or region where the project is located.
The formulation of relevant standards and specifications for high-strength bolts is mainly to ensure the quality and reliability of bolted connections and improve the safety factor of engineering structures and equipment. When using high-strength bolts, it is necessary to strictly follow the requirements of relevant standards for design and selection, manufacturing and processing, installation and construction, and later maintenance to ensure the firmness, durability and safety of bolted connections.
