All the essentials of the Brinell hardness tester explained
A Brinell hardness tester is a machine that is used for measuring the hardness of metal. Testers range in size from around 600mm high for the portable ones, to several metres high for the largest ones installed in steelworks.
Brinell hardness testers all work in the same way; they make an indentation in the metal being tested. The diameter of the indentation is then measured and the result of the measurement is put into a formula to calculate the hardness. More on the Brinell test can be read here.
The indentation is made by pressing a tungsten carbide ball into the metal surface for a specified period of time and with a precisely controlled force. Tungsten carbide is used as it is much harder than steel so it is not deformed by the process. The load ranges from 1 kilogram force (kgf) applied through a 1mm diameter ball (for testing very thin, sheet aluminium) to 3000 kgf applied through a 10mm ball, which is used on steel.
The material to be tested is placed on a level table – known as the anvil – which can usually move up and down to accommodate various sizes of sample. It moves up and down on a column that is rigidly mounted to the body of the hardness tester. The ‘test head,’ which holds the tungsten carbide ball, then descends onto the material to make the indentation.
Because of the force needed to make an indentation in steel, a Brinell hardness tester is a robustly constructed machine. In addition to the robust construction, testers usually have electronic circuitry and a computer to run the indentation process. There are exceptions to this: the smallest testers are hydraulic and hand operated and even some medium-sized machines are lever operated, with the operator using a stopwatch in place of software-driven electronic timing.
Most small and medium hardness testers have a characteristic shape somewhat like a square-ish ‘C’. The test sample is placed on an anvil that is fastened to the ‘bottom’ of the C and the indenting components descend from the ‘top’ of the C. An example is shown in this picture. The ‘open front’ of the ‘C’ allows long and unwieldy components like beams to be tested. Where testing of very large samples / components is required, the test head sits within a carriage that is mounted on a rail between two posts. An example can be seen here.
A major development in the evolution of the Brinell hardness tester
Brinell hardness testing takes its name from its inventor, the Swedish metallurgical engineer Johan August Brinell (1849-1925). He first demonstrated the system in 1900 and testing machines have become ever more sophisticated since then. In the 1980s the first automatic, optical Brinell indentation measurement system was launched, and a few years later it was incorporated into a testing machine (see pic).
Brinell hardness testers with automatic indentation measurement became the industry benchmark for accuracy and reliability and greatly reduced the chance of indentation mis-measurement. Because of this, the Brinell test became more widely accepted in industry, including among organisations where the hardness tolerances are narrow, usually because of safety considerations.