Hardness index

Hardness is the ability of a metal material to resist hard objects pressing into a surface. According to the test method and scope of application, hardness can be divided into Brinell hardness, Rockwell hardness, Vickers hardness, Shore hardness, micro hardness and high temperature hardness. Commonly used to pipe material have brinell, rockwell, Vickers hardness 3 kinds.

A, Brinell hardness (HB)

A steel ball or carbide ball of a certain diameter is pressed into the sample surface with a specified test force (F), and the test force is removed after a specified holding time to measure the indentation diameter (L) on the sample surface. The Brinell hardness number is the quotient of the test force divided by the surface area of the indentation sphere. Expressed in HBS, the unit is N/mm2(MPa).

Its calculation formula is as follows:

Where: F-- the test force pressed into the surface of the metal sample, N;

D-- Test ball diameter, mm;

D -- Average indentation diameter, mm.

Brinell hardness is more accurate and reliable, but generally HBS is only suitable for metal materials below 450N/mm2(MPa), for hard steel or thin plate is not applicable. In steel pipe standard, brinell hardness is the most widely used, often to indentation diameter D to express the hardness of the material, both intuitive and convenient.

Example: 120HBS10/1000130: indicates that the Brinell hardness value measured by 10mm diameter steel ball under 1000Kgf(9.807kN) test force is 120N/ mm2(MPa) after 30 seconds.

B. Rockwell Hardness (HK)

Rockwell hardness test, like Brinell hardness test, is an indentation test. The difference is that it measures the depth of the indentation. That is, under the action of the initial Yong test force (Fo) and the total test force (F), the indenter (gold steel cone or ball) is pressed into the sample surface, after the specified holding time, the main test force is removed, and the measured residual indentation depth increment (E) is calculated hardness value. Its value is an unknown number, represented by the symbol HR, and the scale used is A, B, C, D, E, F, G, H, K and other nine scales. The scales commonly used for steel hardness test are generally A, B, C, that is, HRA, HRB, HRC.

The hardness value is calculated by the following formula:

When tested with A and C scales, HR= 100-E

When tested with B scale, HR= 130-E

In the formula, e-- residual indentation depth increment, its kind is expressed in the specified unit 0.002mm, that is, when the axial displacement of the indenter is 1 unit (0.002mm), it is equivalent to a number of rockwell hardness change. The higher the e value, the lower the hardness of the metal, vice versa, the higher the hardness.

The scope of application of the three scales is as follows:

HRA(Diamond cone Indenter)20-88

HRC(Diamond cone indenter)20-70

HRB(Diameter 1.588mm steel ball indenter)20-100

Rockwell hardness test is a widely used method, among which HRC is second only to Brinell hardness HB in steel pipe standard. Rockwell hardness can be applied to determine from very soft to very hard metal materials, it makes up for the Brinell method is not, is simpler than brinell method, can be directly from the hardness machine dial read hardness value. However, due to the small indentation, the hardness value is not as accurate as buchwald method.

C, Vickers Hardness (HV)

Vickers hardness test is also a kind of indentation test method, which is to press a square pyramid diamond indenter with a selected test force (F) into the test surface. After the specified holding time, the test force is removed, and the length of the two diagonals of the indentation is measured.

The Vickers hardness value is the quotient of the test force divided by the indentation surface area, and its calculation formula is:

Where: HV-- Vickers hardness symbol, N/mm2(MPa);

F-- Test force, N;

D -- The arithmetic mean value of the indentation diagonals, mm.

Vickers hardness test force F is 5(49.03), 10(98.07), 20(196.1), 30(294.2), 50(490.3), 100(980.7)Kgf(N), the range of hardness can be measured from 5 to 1000HV.

Presentation method example: 640HV30/20 indicates that the Vickers hardness value determined by 30Hgf(294.2N) test force holding 20S(seconds) is 640N/mm2(MPa).

Vickers hardness test can be used to determine the hardness of very thin metal materials and surface layers. It has the main advantages of Brucellosis and Rockwell method and overcomes their basic disadvantages, but it is not as simple as Rockwell method. Vickers method is rarely used in steel pipe standards.