Bolt grades are classified based on the material properties and mechanical performance of bolts, primarily used to indicate the strength and quality level of bolts. The following is a detailed introduction to bolt grades:

I. Method for Representing Bolt Grades Bolt grades are typically indicated by a combination of numbers and letters, for example: 4.8, 8.8, 10.9, and so on. Among these, the digits represent the tensile strength of the bolt (in megapascals, MPa); the digit after the decimal point indicates the yield-to-tensile ratio, which is the ratio of the yield strength to the tensile strength. For example, an 8.8-grade bolt has a tensile strength of 800 MPa, a yield-to-tensile ratio of 0.8, and a yield strength of 640 MPa. Schematic diagram of the measurement method
II. Characteristics of Different Bolt Grades 1.   4.6 and 4.8 grade bolts - The tensile strength is relatively low, typically around 400 MPa. The yield-to-tensile ratio is 0.8, and the yield strength is 320 MPa. - A low yield-to-tensile ratio means that the bolt is prone to plastic deformation when subjected to significant loads. - This bolt belongs to a low-strength grade and is generally used in non-critical connections or applications involving relatively small loads. - It is relatively inexpensive and is commonly employed in general mechanical joints and building structures where high strength is not required. 2. 5.6 and 5.8 grade bolts - The tensile strength is slightly higher than that of grades 4.6 and 4.8, at around 500 MPa. - The yield-to-tensile ratio has also improved, resulting in a slight enhancement in the bolt’s strength and reliability. The yield-to-tensile ratio is 0.6, with a yield strength of 300 MPa. - The strength is slightly higher than that of grade 4.8 bolts, making it suitable for some medium-load connections. - It can be used for medium-load connections, such as small machinery and equipment, light steel structures, and the like. 3. 6.8 and 8.8 grade bolts - Belongs to the high-strength bolt grade. - The tensile strengths are as follows: 600 MPa and 800 MPa, with a yield strength ratio of 0.8. - It boasts high strength and toughness, capable of withstanding significant tensile and shear forces. - Widely used in fields such as mechanical manufacturing, the automotive industry, and construction engineering—for connections in critical areas like engine attachments and bridge structures. 4. 10.9-grade bolt - Ultra-high-strength bolt grade. - Tensile strength up to 1000 MPa, with a yield-to-tensile ratio of 0.9. - Suitable for applications requiring extremely high connection strength, such as large-scale steel structure bridges, steel connections in high-rise buildings, and heavy-duty machinery equipment. - Complex manufacturing process, resulting in higher costs. 5. 12.9 Grade Bolt - Top-grade bolt strength class. - Tensile strength can reach up to 1200 MPa, with a yield-to-tensile ratio of 0.9. - Yield strength is 1080 MPa. - Primarily used in specialized engineering fields such as aerospace and military equipment, where bolts are required to have extremely high strength, precision, and reliability. - Ultra-high-strength bolts, generally used in specialized engineering fields such as aerospace and military equipment. - Highly stringent requirements for materials and manufacturing processes, resulting in high costs. III. Factors Affecting Bolt Strength Grades 1. Materials - The grade of a bolt primarily depends on the chemical composition and mechanical properties of its material. - Generally speaking, using high-strength alloy steels allows for the manufacture of bolts with higher grades. For example, alloy steels such as 40Cr and 35CrMo are commonly used to produce high-strength bolts of Grade 8.8 and above. - The quality and purity of the material also affect the performance of bolts; the lower the impurity content, the better the bolt’s strength and toughness will be. 2. Processing technology - The manufacturing process of bolts significantly influences their grade. - Process steps such as cold heading, hot heading, and machining all affect the mechanical properties of bolts. For example, adopting advanced cold-heading technology can make the metal fibers in bolts more continuous, thereby enhancing their strength and toughness. - Heat treatment processes are also a key factor. Through heat treatments like quenching and tempering, the microstructure of bolt materials can be altered, improving their strength and hardness. 3. Thread accuracy - The thread accuracy of bolts also affects their grade. - High-precision threads can enhance the connection strength and reliability of bolts. Parameters such as thread tolerances, thread profile angles, and pitch must be strictly controlled to ensure proper engagement between the bolt and nut or other fastening components.
4.  Surface treatment - Surface treatments can enhance the bolt’s corrosion resistance, wear resistance, and aesthetic appeal, while also having a certain impact on the bolt’s grade. - Common surface treatment methods include galvanizing, chrome plating, and blackening. Different surface treatment methods alter the bolt’s surface properties, thereby affecting its performance in various environments. IV. Principles for Selecting Bolt Grades 1. Select according to load requirements. - First, determine the magnitude of the loads acting on the connection, including tensile force, shear force, bending moment, and others. - Based on the magnitude of the loads, select bolts of an appropriate grade. Generally speaking, the greater the load, the higher the bolt grade required. - At the same time, also consider the nature of the loads—such as static loads, dynamic loads, impact loads, and so forth—since different types of loads place varying demands on the bolts. 2. Consider the importance of the connection point. - For critical connection points, such as bridges, large-scale machinery, and high-rise buildings, bolts of higher strength grades should be selected to ensure the reliability and safety of the connections. - For some secondary connection points, bolts of lower strength grades can be chosen to reduce costs. 3. Consider the installation environment. - Factors such as the temperature, humidity, and corrosiveness of the installation environment can also affect bolt selection. - In harsh environments—such as those characterized by high temperatures, high humidity, or strong corrosivity—bolts made from materials with excellent corrosion resistance and of an appropriate grade should be chosen. - At the same time, it’s important to consider the bolt installation method and frequency of disassembly when selecting the appropriate bolt grade and surface treatment method. 4. Refer to relevant standards and specifications. - When selecting bolt grades, refer to relevant national standards, industry standards, and codes to ensure that the bolts meet the required quality and performance specifications. - Different countries and regions may have different standards and codes; therefore, selection should be made based on the actual situation. The classification of bolt grades is designed to meet the varying requirements for bolt strength and quality in different applications. When choosing a bolt grade, it is necessary to comprehensively consider factors such as load requirements, the importance of the connection point, installation environment, and other relevant standards and codes, in order to select an appropriate bolt grade and ensure the reliability and safety of the connection.