The performance grades of bolts used for valve steel structure connections are divided into more than 10 grades, including 3.6, 4.6, 4.8, 5.6, 6.8, 8.8, 9.8, 10.9, 12.9, etc. Among them, bolts with grade 8.8 and above are made of low-carbon alloy steel or medium carbon steel and undergo heat treatment (quenching, tempering), commonly known as high-strength bolts, while others are commonly known as ordinary bolts. The bolt performance grade number consists of two parts of numbers, which respectively represent the nominal tensile strength value and yield strength ratio of the bolt material. For example:
The meaning of a bolt with a performance level of 4.6 is:
1. The nominal tensile strength of the bolt material reaches 400MPa level;
2. The yield strength ratio of bolt material is 0.6;
3. The nominal yield strength of the bolt material reaches 400 × 0.6=240MPa level
High strength bolts with a performance level of 10.9, made of materials that have undergone heat treatment, can achieve:
1. The nominal tensile strength of the bolt material reaches 1000MPa level;
2. The yield strength ratio of bolt material is 0.9;
3. The nominal yield strength of the bolt material reaches 1000 × 0.9=900MPa level
The meaning of bolt performance grade is an internationally recognized standard. For bolts with the same performance grade, regardless of the difference in material and origin, their performance is the same, and only the performance grade should be selected for design.