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Three Hardness Standards of Metal The ability of a metal material to resist the indentation of the surface by hard objects is called hardness. 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. There are three commonly used hardnesses for pipes: Brinell, Rockwell, and Vickers hardness. A. Brinell hardness (HB) Use a steel ball or cemented carbide ball of a certain diameter to press into the sample surface with the specified test force (F). After the specified holding time, remove the test force and measure the indentation diameter (L) on the sample surface. Brinell hardness value is the quotient obtained by dividing the test force by the surface area of the indented sphere. Expressed in HBS (steel ball), the unit is N/mm2 (MPa). Its calculation formula is: in the formula: F--test force pressed into the surface of the metal sample, N ； D--Diameter of steel ball for test, mm ； d--average diameter of indentation, mm. The measurement of Brinell hardness is more accurate and reliable, but generally HBS is only suitable for metal materials below 450N/mm2 (MPa), and is not suitable for harder steel or thinner plates. Among steel pipe standards, Brinell hardness is the most widely used. The indentation diameter d is often used to express the hardness of the material, which is both intuitive and convenient. Example: 120HBS10/1000130: It means that the Brinell hardness value measured by using a 10mm diameter steel ball under the test force of 1000Kgf (9.807KN) and holding for 30s (seconds) is 120N/mm2 (MPa). B. Rockwell hardness (HK) 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 sequential action of the initial test force (Fo) and the total test force (F), the indenter (cone or steel ball of the gold steel mill) is pressed into the surface of the sample. After the specified holding time, the main test force is removed, and the measured residual indentation depth increment (e) is used to calculate the hardness value. Its value is an anonymous number, represented by the symbol HR, and the scales used include 9 scales, including A, B, C, D, E, F, G, H, and K. Among them, the scales commonly used for steel hardness testing are generally A, B, and C, namely HRA, HRB, and HRC. The hardness value is calculated using the following formula: When testing with A and C scales, HR=100-e When testing with B scale, HR=130-e where e--the residual indentation depth increment is expressed in the specified unit of 0.002mm, that is, when the axial displacement of the indenter is one unit (0.002mm), it is equivalent to a change in Rockwell hardness by one number. The larger the e value, the lower the hardness of the metal, and vice versa. The applicable scope of the above 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 currently a widely used method, among which HRC is used in steel pipe standards second only to Brinell hardness HB. Rockwell hardness can be used to measure metal materials from extremely soft to extremely hard. It makes up for the shortcomings of the Brinell method. It is simpler than the Brinell method and 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) The Vickers hardness test is also an indentation test method. It presses a square pyramidal diamond indenter with an angle of 1360 between opposite surfaces into the test surface with a selected test force (F). After the prescribed holding time, the test force is removed and the two diagonal lengths of the indentation are measured. Vickers hardness value is the quotient obtained by dividing the test force by the indentation surface area, and its 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 in Vickers hardness is six levels: 5 (49.03), 10 (98.07), 20 (196.1), 30 (294.2), 50 (490.3), 100 (980.7) Kgf (N), etc. The measurable hardness value range is 5 to 1000HV. Examples of expression methods: 640HV30/20 means that the Vickers hardness value measured with a test force of 30Hgf (294.2N) for 20S (seconds) is 640N/mm2 (MPa). The Vickers hardness method can be used to determine the hardness of very thin metallic materials and surface layers. It has the main advantages of Brinell and Rockwell methods and overcomes their basic shortcomings, but it is not as simple as Rockwell method. Vickers method is rarely used in steel pipe standards.