The ability of a metal material to resist the indentation of a hard object is called hardness. Hardness is not a simple physical quantity, but a comprehensive performance index reflecting the elasticity, plasticity, strength and toughness of materials.
According to different test methods and scope of application, hardness can be divided into Brinell hardness, Rockwell hardness, Vickers hardness, Shore hardness, microhardness and high temperature hardness. For
carbon seamless steel pipes, three commonly used hardnesses are Brinell, Rockwell and Vickers.
A. Brinell hardness (HB)
Use a steel ball or cemented carbide ball with a certain diameter to press into the surface of the sample with the specified test force (F), remove the test force after the specified holding time, and measure the indentation diameter (L) on the surface of the sample. The Brinell hardness value is the quotient of the test force divided by the surface area of the indentation spherical. Expressed in HBS (steel ball), the unit is N/mm2 (MPa).
B. Rockwell hardness (HR)
The Rockwell hardness test, like the Brinell hardness test, is an indentation test method. The difference is that it measures the depth of the indentation. That is, under the successive action of the initial test force (Fo) and the total test force (F), the indenter (cone or steel ball) is pressed into the surface of the sample, and after the specified holding time, the main force is removed. For the test force, use the measured residual indentation depth increment (e) to calculate the hardness value. Its value is an unnamed number, represented by the symbol HR, and the scales used are A, B, C, D, E, F, G, H, K, etc. 9 scales. Among them, the scales commonly used in steel hardness tests are generally A, B, and C, namely HRA, HRB, and HRC.
Rockwell hardness test is a widely used method, among which HRC is second only to Brinell hardness HB in steel pipe standards. Rockwell hardness can be used to measure metal materials from extremely soft to extremely hard. It makes up for the inconvenience of the Brinell method and is simpler than the Brinell method. The hardness value can be read directly 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)
Vickers hardness test is also an indentation test method, which is to press a regular quadrangular pyramid diamond indenter with an angle of 1360 opposite sides into the test surface with a selected test force (F), and remove the test after a specified holding time. Force, measure the length of the two diagonals of the indentation.
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