【2026 Performance Grade】What does the number 48 indicated on the bolt mean?
Thread Content
The performance grades of bolts used for steel structure connections include over 10 grades such as 3.6, 4.6, 4.8, 5.6, 6.8, 8.8, 9.8, 10.9, and 12.9. Bolts of grade 8.8 and above are made of low-carbon alloy steel or medium-carbon steel and have undergone heat treatment (quenching and tempering); they are commonly referred to as high-strength bolts, while the rest are known as ordinary bolts.
The bolt performance grade designation consists of two digits, which represent the nominal tensile strength value of the bolt material and the yield strength ratio, respectively. 48 is 4.8.
For example, a bolt with a performance grade of 4.8 means that: 1. The nominal tensile strength of the bolt material is at the 400 MPa level； 2. The yield-to-tensile strength ratio of the bolt material is 0.8 ； 3. The nominal yield strength of the bolt material is 400×0.8=320 MPa
High-strength bolts of grade 10.9, whose material has been heat-treated, can achieve the following: 1. The nominal tensile strength of the bolt material reaches 1000 MPa； 2. The yield strength ratio of the bolt material is 0.9 ； 3. The nominal yield strength of the bolt material is 1000×0.9=900 MPa
The meaning of bolt performance grades is a standard recognized internationally; bolts with the same performance grade have identical properties, regardless of their material or place of origin. In design, only the performance grade needs to be considered.
The strength grades of 8.8 grade and 10.9 grade refer to the shear stress resistance levels of bolts being 8.8 GPa and 10.9 GPa respectively. For an 8.8 grade bolt, the nominal tensile strength is 800 N/mm2 and the nominal yield strength is 640 N/mm2. Generally, bolts have their strength indicated as “X.Y”; where X*100 represents the tensile strength of the bolt, and X*100*(Y/10) represents its yield strength (since according to the standard specification, yield strength/tensile strength = Y/10, meaning that Y is the yield-to-tensile strength ratio). For example, in the case of a 4.8 grade bolt, its tensile strength is 400 MPa； The yield strength is: 400*8/10=320MPa.
Additionally: Stainless steel bolts are usually labeled as A4-70, A2-70, etc.; the meaning of these designations is explained separately. Units of measurement: There are mainly two systems for measuring length in the world today. One is the metric system, whose units include meters (m), centimeters (cm), millimeters (mm), etc.; this system is widely used in Europe, China, Japan, and other Southeast Asian countries. The other is the imperial system, whose primary unit is the inch; this system is more commonly used in the United States, the United Kingdom, and other European and American countries. 1. Metric system: (decimal) 1 m = 100 cm = 1000 mm. 2. Imperial system: (octal) 1 inch = 8 inches; 1 inch = 25.4 mm. For products with a diameter of 3/8 inch or less, their diameter is indicated using numbers such as 4#, 5#, 6#, 7#, 8#, 10#, 12#.
Mechanical design tolerances: What is the difference between 6H, 6h, H6, and h6? 1. The schematic diagrams of the tolerance symbols for various types of threads are as follows: (1) Thread tolerance symbols generally consist of numbers and letters, and are used to indicate the tolerance grade and position of the thread. Among them, internal threads are represented by uppercase letters, while external threads are represented by lowercase letters. (2) For example, 7H, 6g, etc. represent thread tolerances, while H7, g6 represent thread fit grades.
2. Examples: (1) GB/T197-2003 specifies the basic deviations for external and internal threads, which are used to determine the position of the tolerance zones of these threads relative to the basic tooth profile. (2) For external threads, four basic deviations are specified and represented by the codes h, g, f, e; for external threads, two such deviations are specified and represented by H and G. (Refer to GB/T197-2003). (3) M10-6H is a metric internal thread; M10 represents the major diameter of the thread, the pitch is the standard value of 1.5, and 6H denotes the tolerance grade of the thread. For internal threads, the letter indicating this grade is usually written in uppercase as “H”, while for external threads it is written in lowercase.
2.1 M is the thread code: Generally, threads can be classified into coarse-thread and fine-thread types based on their pitch； (1) M10: Coarse-thread screw with a nominal diameter of 10 ； Coarse-threaded threads do not indicate the pitch; the pitch for M10 is 1.5 mm. (2) Fine-thread threads must be specified, for example: M10×1: the nominal diameter is 10, and the pitch is 1 (fine-thread thread) ； M10×1—LH means: nominal diameter of 10, pitch of 1, left-hand thread (left-hand threads are always denoted as LH).
2.2 The table below shows some thread sizes