Is there a national standard for the hardness of screws? how much is it? How to detect?

There are national standards for the hardness of screws, and different grades of screws have different hardness standards.

If the grade is 4.8, the tensile strength of the bolt is 400MPa and the yield strength is 400*8/ 10=320MPa.

Bolt hardness mainly refers to the surface hardness of bolts. The hardness of bolts, screws and studs shall be measured at the head, end or rod.

Hardness detection method

Hardness testing is also stipulated. There is generally no requirement for the surface. The heat treatment manufacturer tests the hardness after heat treatment. Metric nuts are generally broken from the middle to test the hardness of the center. English nuts are generally broken from one face (that is, two knives are cut on one face) to test the hardness in the middle of the cross section of one face. Small nuts are generally tested after grinding off 20-30 wires in this section.

The performance grades of bolts for steel structure connection are 10 or above, such as 3.6, 4.6, 4.8, 5.6, 6.8, 8.8, 9.8, 10.9, 12.9, etc. Among them, the bolts of grade 8.8 and above are made of low-carbon alloy steel or medium-carbon steel and treated (quenched and tempered).

10.9 after heat treatment, the performance grade of high-strength bolts can reach:

1, the nominal tensile strength of bolt material reaches1000 MPa;

2. The yield ratio of bolt material is 0.9;

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

The meaning of bolt performance grade is an international standard. Bolts with the same performance grade have the same performance regardless of material and origin, so only performance grade can be selected in design.

The strength grades of 8.8 and 10.9 refer to the bolt shear stress grades of 8.8GPa and 10.9GPa.

8.8 Nominal tensile strength 800N/MM2 Nominal yield strength 640N/MM2

Generally, the strength of bolts is expressed by "X.Y".

X* 100= tensile strength of the bolt,

X* 100*(Y/ 10)= the yield strength of the bolt.

(Because according to the label: yield strength/tensile strength =Y/ 10)

Extended data

Routine inspection should be carried out after the hinge layer or other coating of the sample is removed and properly treated. The hardness of bolts, screws and studs shall be measured at the head, end or rod.

For all performance grades, if the maximum hardness is exceeded, the test shall be conducted again at the part with the thread diameter and radius of 1/2 from the end, and the hardness value shall not exceed the maximum hardness. If there is any dispute during acceptance, Vickers hardness shall be used as arbitration test.

Surface hardness should be measured on the end face or hexagonal plane. In order to ensure the accuracy of lateral positioning and maintain the original performance of the material surface, the tested parts should be polished or polished. The surface hardness should be determined by HV0.3 Vickers hardness test. Surface hardness value HV 0. 3 should be compared with the core hardness value of the same specimen to determine its actual control value.

The allowable surface hardness is higher than that in the center, and the maximum difference is 30 Vickers hardness values; Otherwise, it means that diagnostic carbon should be used to judge the surface carburization of bolts, screws or studs, and the difference between core hardness and surface hardness should be based on 8.8- 12.9.

There may be no direct conversion relationship between hardness and theoretical tensile strength. To determine the maximum hardness, besides the theoretical maximum tensile strength, other factors (such as brittle fracture) should be considered.

References:

Baidu encyclopedia-bolt hardness