The ability of a metal material to resist the indentation of a hard object is called hardness. According to the test method and the scope of application, the hardness can be divided into Brinell hardness, Rockwell hardness, Vickers hardness, Shore hardness, microhardness and high temperature hardness. Commonly used for pipes are Brinell, Rockwell and Vickers hardness.

 A, Brinell hardness (HB)

Press the steel ball or carbide ball of a certain diameter into the surface of the sample with the specified test force (F). After the specified holding time, remove the test force and measure the indentation diameter (L) on the surface of the sample. The Brinell hardness value is the quotient obtained by dividing the test force by the spherical surface area of ??the indentation. It is expressed by HBS (steel ball) and the unit is N/mm2 (MPa).

 The formula is:

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

D--diameter of test steel ball, mm;

d--the average diameter of the indentation, mm.

The Brinell hardness is more accurate and reliable, but the general HBS is only suitable for metal materials below 450N/mm2 (MPa). It is not suitable for hard steel or thinner plates. Among the steel pipe standards, Brinell hardness is the most widely used, and the hardness of the material is often expressed by the indentation diameter d, which is both intuitive and convenient.

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

 B, Rockwell hardness (HK)

The Rockwell hardness test is the same as the Brinell hardness test. The difference is that it measures the depth of the indentation. That is, under the action of the initial test force (Fo) and the total test force (F), the indenter (the steel mill cone or steel ball) is pressed into the surface of the sample, and after the specified holding time, the main body is removed. Test force, the hardness value was calculated using the measured residual indentation depth increment (e). Its value is an unnamed number, represented by the symbol HR. The scale used has 9 scales such as A, B, C, D, E, F, G, H, and K. The scales commonly used for steel hardness testing are generally A, B, C, ie HRA, HRB, HRC.

The hardness value is calculated by the following formula:

When testing with A and C scales, HR=100-e

When testing with the B scale, HR=130-e

In the formula, e--the residual indentation depth increment, which is expressed in the specified unit of 0.002 mm, that is, when the indenter is axially displaced by one unit (0.002 mm), it corresponds to a change in Rockwell hardness. The larger the e value, the lower the hardness of the metal, and vice versa.

The above three scales are applicable as follows:

HRA (diamond cone indenter) 20-88

HRC (diamond cone indenter) 20-70

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

The Rockwell hardness test is currently a widely used method in which HRC is used in steel pipe standards only after Brinell hardness HB. Rockwell hardness can be applied to the determination of metal materials from extremely soft to very hard. It compensates for the Brinell method. It is simpler than the Brinell method and can directly read the hardness value from the dial of the hardness machine. However, due to its small indentation, the hardness value is not as accurate as the Brinell method.

C, Vickers hardness (HV)

The Vickers hardness test is also an indentation test method in which a positive quadrilateral pyramid diamond indenter with an opposite face angle of 1360 is pressed into the test surface with a selected test force (F) after a specified hold time. Remove the test force and measure the length of the two diagonals of the indentation.

The Vickers hardness value is the quotient of the test force divided by the surface area of ??the indentation. The calculation formula is:

In the formula: HV--Vickers hardness symbol, N/mm2 (MPa);

F--test force, N;

d--the arithmetic mean of the two diagonals of the indentation, mm.

The test force F used for Vickers hardness is 6 (49.03), 10 (98.07), 20 (196.1), 30 (294.2), 50 (490.3), 100 (980.7) Kgf (N), etc. The hardness value ranges from 5 to 1000 HV.

Description of the method: 640HV30/20 indicates that the Vickers hardness value measured by holding the test force of 30Hgf (294.2N) for 20 seconds (second) is 640 N/mm 2 (MPa).

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