What are the hardness testing standards for high-strength bolts

The hardness testing standards for high-strength bolts usually follow the national standard GB/T 3098.1-2010 Mechanical Properties of Fasteners – Bolts, Screws, and Studs.

According to this standard, the determination of bolt hardness should be carried out at the head, end, or rod. The commonly used hardness testing methods include Brinell hardness (HB), Rockwell hardness (HRC), Vickers hardness (HV), etc. For high-strength bolts, the HV0.3 Vickers hardness test is generally used as the arbitration test for surface hardness.

For example, the hardness test method for high-strength hexagonal bolts used in steel structures should follow the GB/T 230.1 or GB/T 4340.1 standards. When conducting hardness tests, four points should be selected on the surface of the nut, and the average of the last three points should be taken. If there is any dispute, the Vickers hardness test shall be used for arbitration. The hardness of the nut should comply with relevant regulations.

In addition, the hardness requirements vary for bolts of different performance levels. For example, after heat treatment, high-strength bolts of grade 10.9 can achieve a nominal tensile strength of 1000 MPa for bolt materials, a yield strength ratio of 0.9 for bolt materials, and a nominal yield strength of 1000 × 0.9=900 MPa for bolt materials.

In actual testing, to ensure the accuracy of the measurement and maintain the original performance of the material surface, the tested part should be ground or polished. Meanwhile, attention should also be paid to distinguishing whether the increase in hardness is caused by carburization, heat treatment, or surface cold work hardening. If the maximum hardness is exceeded, the test should be conducted again on a section with one thread diameter from the end, at a radius of 1/2, and the hardness value should not exceed the maximum hardness. If there is any dispute during acceptance, Vickers hardness shall be used as the arbitration test.

It should be noted that the specific hardness testing standards may vary due to factors such as the use, material, and specifications of bolts. When conducting tests, operations should be carried out according to relevant specific standards and requirements.