Classification of hardness of common steel and metal materials
The ability of a metallic material to resist indentation on its surface by a hard object is called hardness. Depending on the testing method and applicable scope, hardness can be classified into Brinell hardness, Rockwell hardness, Vickers hardness, Shore hardness, microhardness and high-temperature hardness, etc. For pipe materials, the commonly used ones are Brinell, Rockwell and Vickers hardness.
A. Brinell Hardness (HB)
Using a steel ball or a hard alloy ball of a certain diameter, press it onto the surface of the sample with a specified test force (F). After a specified holding time, remove the test force and measure the diameter (L) of the indentation on the sample surface. The Brinell hardness value is the quotient obtained by dividing the test force by the spherical surface area of the indentation. It is denoted as HBS (for steel ball) and is expressed in N/mm² (MPa).
B. Rockwell Hardness (HR)
The Rockwell hardness test is similar to the Brinell hardness test, both being indentation testing methods. The difference is that it measures the depth of the indentation. That is, under the successive application of the initial test force (Fo) and the total test force (F), the indenter (a steel cone or a steel ball) is pressed into the surface of the sample. After a specified holding time, the main test force is removed, and the hardness value is calculated using the measured increment of the residual indentation depth (e). The value is an unnamed number, represented by the symbol HR, and there are 9 scales used, 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 Rockwell hardness test is currently a widely used method. Among them, HRC is used in steel pipe standards after Brinell hardness HB, ranking second. Rockwell hardness can be used to measure metal materials ranging from extremely soft to extremely hard. It compensates for the inconvenience of the Brinell method and is simpler than the Brinell method, allowing the hardness value to be directly read from the dial of the hardness machine. However, due to its small indentation, the hardness value is not as accurate as that of the Brinell method.
C. Vickers Hardness (HV)
The Vickers hardness test is also a type of indentation test method. A square pyramid-shaped diamond indenter with a relative angle of 1360 degrees is pressed into the test surface with a selected test force (F). After a specified holding time, the test force is removed, and the lengths of the two diagonals of the indentation are measured.
With the advancements in hardness tester manufacturing technology, various portable instruments, especially high-precision portable instruments, have continuously emerged, enabling the hardness test to be carried out simply, quickly, and accurately. This has made it possible for on-site hardness testing, hardness control on production lines, and accurate hardness testing of large workpieces.
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