Hardness Indicators for Anti-corrosion Spiral Steel Pipes in Industrial Projects

Hardness is the ability of a metallic material to resist indentation by a hard object. Depending on the test method and applicable scope, it can be classified into Vickers hardness, Shore hardness, Brinell hardness, Rockwell hardness, microhardness, and high-temperature hardness etc. For anti-corrosion spiral steel pipes, Rockwell, Brinell, and Vickers hardness are commonly used.

A. Rockwell Hardness (HK) Like the Brinell hardness test, the Rockwell hardness test is an indentation test method. The difference is that it measures the depth of the indentation. That is, under the sequential application of the initial test force (Fo) and the total test force (F), an indenter (a steel cone or steel ball) is pressed into the sample surface. 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).

B. Brinell Hardness (HB) A steel ball or cemented carbide ball of a certain diameter is pressed into the surface of the sample with a specified test force (F). After a specified holding time, the test force is removed, and the diameter (L) of the indentation on the sample surface is measured. The Brinell hardness value is the quotient obtained by dividing the test force by the spherical surface area of the indentation.

C. Vickers Hardness (HV) The Vickers hardness test is also an indentation test method. A diamond indenter in the shape of a square pyramid with opposite faces at an angle of 136° 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. The Vickers hardness value is the quotient obtained by dividing the test force by the surface area of the indentation. The calculation formula is: HV = F / d, where: HV -- Vickers hardness symbol, N/mm² (MPa); F -- test force, N; d -- arithmetic mean of the two diagonals of the indentation, mm. The Vickers hardness test force F is 5 (49.03), 10 (98.07), 20 (196.1), 30 (294.2), 50 (490.3), and 100 (980.7) Kgf (N) in six grades, and the measurable hardness value range is 5 to 1000 HV.