The strength range of bolts of different grades is also not the same. Generally, the higher the grade of the bolt, the higher its strength range. When it comes to strength, let's first popularize the professional knowledge of strength. Strength represents a performance indicator value of bolts, which can be measured by a hardness tester.
There are three common ways to indicate strength:
1. Vickers hardness (HV) - The Vickers hardness value (HV) is obtained by dividing the load value by the surface area of the indentation pit of the raw material when a diamond square cone indenter is pressed into the surface of the workpiece with a load of less than 120kg and an angle of 136 °.
2. Brinell Hardness - The measurement principle is to use a test force F (N) of the corresponding shape (the unit of force is generally kilogram force kgf, note: 1kgf=9.8N, kgf means one kilogram of force), press a quenched steel ball or hard alloy ball with a diameter of D (mm) into the surface of the metal to be tested, maintain it for a limited time, remove the test force, measure the average diameter of the indentation d (mm) with a reading microscope, and then calculate the Brinell hardness HB value according to the formula, or find the HB value from the prepared Brinell hardness table based on d.
3. Rockwell hardness - an indicator that determines the hardness value based on the depth of plastic deformation of indentation, with 0.002 millimeters as a hardness unit. Choosing different indenters and types of test forces in the Rockwell hardness test will also result in different types of compositions, corresponding to different types of scales (HRA, HRB, HRC) for Rockwell hardness.
The strength range of bolts of different types and grades is shown in the following table:
Based on the above, for example, the Vickers hardness range for 6.8 grade bolts is 190-250HV, the Rockwell hardness range for 8.8 grade M12 bolts is 22-32HRC, and the Brinell hardness range for 12.9 grade bolts is 380-429HBW. These are the strength ranges represented by different types of bolt grades.
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