The strength grade of the bolt is divided according to the hardness, force, and yield of bolt raw materials. For fastening bolts used in different products and applications, bolts of different grades are required to suit the uses. Bolt strength can be divided into ordinary grade and high strength grade. In this article, we’ll introduce the bolt grades system, the meaning of the bolt head markings grade, you can also check out the metric bolt grades chart.
The performance grades of bolts for steel structure connection are divided into more than 10 grades, such as 3.6, 8.8, and 12.9. Among them, bolt grade 8.8 and above are made of low-carbon alloy steel or medium carbon steel and are generally called high-strength bolts after heat treatment (quenching and tempering), and the rest are generally called ordinary bolts. The performance grade of the bolt is composed of two numbers, the first digit of bolt grade represents the one hundred part of nominal tensile strength or proof load of the bolt, the second digit of bolt grade is a factor that represents the yield strength of the bolt material. The grade of a bolt determines the maximum amount of stress that the bolt can handle. Tensile strength is the amount of pull the bolt can withstand before breaking.
Let’s take the bolt grade 4.6 as an example, it means the head marking of the bolt is 4.6, the nominal tensile strength of bolt material reaches 400MPa, and the yield ratio of bolt material is 0.6, so the nominal yield strength of the bolt material is 400 x 0.6 = 240MPa.
What are the different grades of bolts? Here are commonly used bolt grades:
The meaning of bolt performance grade is an international standard. Bolts with the same grade have the same performance regardless of their materials and origin. The difference in bolt grade is that its manufacturing materials are different, and the prices of different materials in the market are also different. Generally, the price of high-strength bolts will be much higher than that of ordinary bolts. Generally used bolt grades in the market are 4.8, 8.8, 10.9, and 12.9.
Below is a metric bolt grades chart that is used to identify the bolt head markings and their mechanical properties including strength and hardness.