Brinell hardness test

The Brinell hardness test (pronounced /brəˈnɛl/) measures the indentation hardness of materials. It determines hardness through the scale of penetration of an indenter, loaded on a material test-piece. It is one of several definitions of hardness in materials science. The hardness scale is expressed in terms of a Brinell hardness value, sometimes referred to as the Brinell hardness number but formally expressed as HBW (Hardness Brinell Wolfram – Wolfram being an alternative name for the tungsten carbide ball indenter used during the test).

The test was named after Johan August Brinell (1849-1925) who developed the method at the end of the 19th century.

Premiered by Swedish engineer Johan August Brinell at the 1900 Paris Exposition, it was the first widely used and standardised hardness test in engineering and metallurgy. The large size of indentation and thus possible damage to test-pieces limits its usefulness. However, it also had the useful feature that the hardness value divided by two gave the approximate UTS in ksi for steels. This feature contributed to its early adoption over competing hardness tests.

The test uses a Tungsten Carbide ball indenter and a controlled force, the ratio of ball size to test force being a function of the material being tested. Most commonly the test is used for ferrous metals and uses a 10mm ball and a 3000 Kgf test force, although it can go as low as 1mm and 1 Kgf (HBW 1/1).

Although the maximum test force is 3000 kgf, there are hand-pumped, hydraulic, portable Brinell testers less than 58cm / 23 inches in height that can develop this force and hold it for sufficient time to perform a Brinell test. One such is pictured.

The advantage of the Brinell test over other measurement systems is that the indentation diameters usually range between 2.4mm and 6mm. This means that the indentation is unaffected by the grain structure of the metal under test, so Brinell testing is especially useful in testing materials such as rough castings with coarse grains. However, measurement of the indentation is normally carried out by a technician using a low-powered microscope, and it can be difficult to judge exactly where an indentation begins and ends. Three experienced technicians could obtain three slightly different readings using the same microscope - and an error of 0.2mm can equal 20 hardness points. The problem of operator interpretation errors was overcome in the 1980s in a collaboration between Birmingham University and the British company Foundrax Engineering Products. They developed a system which harnessed an optical microscope to a computer and which was able to measure indentations across multiple axes in under a second. Automatic measurement systems are now used in many production environments where accuracy is critical.

Brinell hardness is sometimes quoted in megapascals; the Brinell hardness value (expressed as HBW (see above)) is multiplied by the acceleration due to gravity, 9.80665 m/s², to convert it to megapascals.

The Brinell hardness value can be correlated with the ultimate tensile strength (UTS), although the relationship is dependent on the material, and therefore determined empirically. The relationship is based on Meyer's index (n) from Meyer's law. If Meyer's index is less than 2.2 then the ratio of UTS to HBW is 0.36. If Meyer's index is greater than 2.2, then the ratio increases.