Introduction to Bolt Performance Grades

The performance grades of bolts for steel structure connections are divided into more than 10 levels, such as 3.6, 4.6, 4.8, 5.6, 6.8, 8.8, 9.8, 10.9, 12.9, etc. Among these, 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), commonly referred to as high-strength bolts, while the rest are referred to as ordinary bolts. The performance grade mark of bolts consists of two parts of numbers, which represent the nominal tensile strength value and the yield strength ratio of the bolt material, respectively.

For example: A bolt with a performance grade of 4.6 means:

The nominal tensile strength of the bolt material reaches 400 MPa;

The yield strength ratio of the bolt material is 0.6;

The nominal yield strength of the bolt material reaches 400 × 0.6 = 240 MPa.

For a high-strength bolt with a performance grade of 10.9, after heat treatment, it can achieve:

The nominal tensile strength of the bolt material reaches 1000 MPa;

The yield strength ratio of the bolt material is 0.9;

The nominal yield strength of the bolt material reaches 1000 × 0.9 = 900 MPa.

The meaning of bolt performance grades is an internationally recognized standard. Bolts with the same performance grade, regardless of their material and origin, have the same performance. In design, only the performance grade needs to be selected. The so-called strength grades of 8.8 and 10.9 refer to the bolt's shear stress grade being 8.8 GPa and 10.9 GPa, respectively.

8.8 Nominal tensile strength is 800 N/MM²; nominal yield strength is 640 N/MM².

Generally, bolts are represented by "X.Y" to indicate strength, where X100 = the bolt's tensile strength, and X100 * (Y/10) = the bolt's yield strength (because according to the identification regulation: yield strength / tensile strength = Y/10).

For example, for a grade 4.8 bolt, the tensile strength of this bolt is: 400 MPa; the yield strength is: 400 * 8/10 = 320 MPa.

Additionally, stainless steel bolts are usually marked as A4-70, A2-70, etc., with meanings that are interpreted differently.