What are the standards for hardness testing?

The ability of a metal material to resist the indentation of its surface by hard objects is called hardness. Depending on the testing method and range of application, hardness can be further divided into Brinell hardness, Rockwell hardness, Vickers hardness, Shore hardness, microhardness, and high-temperature hardness, among others. For pipes, the three commonly used hardness tests are Brinell, Rockwell, and Vickers. A. Brinell hardness (HB): A steel ball or cemented carbide ball of a certain diameter is pressed into the surface of the specimen using a specified testing force (F); after a specified holding time, the testing force is removed, and the diameter of the indentation on the specimen’s surface (L) is measured. The Brinell hardness value is the quotient obtained by dividing the testing force by the surface area of the spherical indenter. It is denoted by HBS (steel ball), with the unit being N/mm2 (MPa). Its calculation formula is: Where: F -- the testing force applied to the surface of the metal specimen, N ； D--Diameter of the test steel ball, mm ； d--Average indentation diameter, mm. Determining Brinell hardness is relatively accurate and reliable, but HBS is generally only suitable for metal materials with a strength of 450 N/mm2 (MPa) or less; it is not applicable to harder steels or thinner sheets. In steel pipe standards, Brinell hardness is the most widely used; it is often used to indicate the hardness of a material through the indentation diameter d, which is both intuitive and convenient. Example: 120HBS10/1000130: This indicates that the Brinell hardness value, measured by using a steel ball with a diameter of 10 mm under a testing force of 1000 Kgf (9.807 KN) for 30 seconds, is 120 N/mm2 (MPa). B. Rockwell hardness (HK) The Rockwell hardness test, like the Brinell hardness test, is also a indentation testing method. The difference is that it measures the depth of the indentation. That is, under the sequential action of the initial testing force (Fo) and the total testing force (F), the indenter (a steel cone or steel ball from a steel mill) is pressed into the surface of the specimen; after a specified holding time, the main testing force is removed, and the hardness value is calculated using the measured increase in residual indentation depth (e). Its value is an unknown number, denoted by the symbol HR, and there are 9 scales used, namely A, B, C, D, E, F, G, H, and K. The scales commonly used for steel hardness testing are generally A, B, C, namely HRA, HRB, and HRC. The hardness value is calculated using the following formula: When testing with scales A and C, HR=100-e; when testing with scale B, HR=130-e. Here, e represents the increase in residual indentation depth, expressed in the specified unit of 0.002 mm. In other words, when the indenter moves axially by one unit (0.002 mm), it corresponds to a change in Rockwell hardness by that amount. The larger the e-value, the lower the hardness of the metal; conversely, the higher the e-value, the higher the hardness. The applicable ranges for the above three scales are as follows: HRA (diamond cone indenter): 20–88; HRC (diamond cone indenter): 20–70; HRB (steel ball indenter with a diameter of 1.588 mm): 20–100. The Rockwell hardness test is a widely used method, and among its variants, HRC is the most commonly used in steel pipe standards, second only to Brinell hardness HB. Rockwell hardness can be used to measure metal materials ranging from very soft to very hard. It overcomes the shortcomings of the Brinell method; it is simpler than the Brinell method, as the hardness value can be read directly from the dial of the hardness tester. However, due to its small indentation, its hardness value is less accurate than that of the Brinell method. C. Vickers hardness (HV): The Vickers hardness test is also a type of indentation test. In this test, a diamond indenter in the shape of a regular tetrahedron with an apex-angle of 136° is pressed into the test surface using a specified testing force (F). After a predetermined holding time, the testing force is removed, and the lengths of the two diagonals of the resulting indentation are measured. The Vickers hardness value is the quotient obtained by dividing the testing force by the surface area of the indentation. Its calculation formula is: Where: HV -- the symbol for Vickers hardness, N/mm2 (MPa) ； F--test force, N ； d--Arithmetic mean of the two diagonals of the indentation, in mm. The test force F used for Vickers hardness testing is available in six levels: 5 (49.03), 10 (98.07), 20 (196.1), 30 (294.2), 50 (490.3), and 100 (980.7) Kgf(N), with a hardness measurement range of 5 to 1000 HV. Example of notation: 640HV30/20 indicates that the Vickers hardness value, determined by applying a testing force of 30 Hgf (294.2 N) for 20 seconds, is 640 N/mm2 (MPa). The Vickers hardness method can be used to determine the hardness of very thin metal materials and surface layers. It possesses the main advantages of the Brinell and Rockwell methods while overcoming their fundamental disadvantages, but it is not as simple as the Rockwell method. The Vickers method is rarely used in steel pipe standards.

There are corresponding **standards** for hardness testing; I only have the standard for Rockwell hardness testing, GB/T230-2004. I can send it to you if you need it.

Like the original poster, I am also very curious about the hardness testing standards for pressure pipes. I hope someone can help.

Shouldn’t the hardness testing of pipes be the method we usually use to test materials?

Pipelines made of carbon steel or low-alloy steel in which the working medium contains humid H2S, or which may suffer from stress corrosion: for carbon steel, the hardness should not exceed 200HB; for low-alloy steel, it should not exceed 225HB, in order to prevent stress corrosion cracking caused by hardened microstructures

The standards for hardness testing should be applied in accordance with the standards adopted for the material and the standards referenced by it.

I’m troubled by this and hope professionals can provide some answers.