One of the internationally accepted methods of measuring metal hardness is by Brinell testing. Brinell testing expresses the hardness numerically and the “Brinell hardness number” is a colloquial term for what is more correctly called the HBW number; that is Hardness Brinell Wolfram (Wolfram being tungsten carbide, the material of which the indenter ball (see below) is made). The higher the HBW number of any material, the harder that material is. The HBW number is calculated from the diameter of an indentation that is made by a Brinell hardness testing machine forcing a tungsten carbide ball into the surface of the test material. The Brinell test, which is used worldwide, is a straightforward method of determining how well a metal resists deformation – there are several such tests and you can read more about them here. In this article we deal only with the Brinell method.
The Brinell test takes its name from Johan August Brinell, the Swede who invented it in 1900. It uses a large indenter (compared to the other methods) to create a depression, or indentation, in the surface of the test material and is thus least susceptible to providing an erroneous reading due to contamination of the test surface or indenter. This makes it especially suitable for environments where extreme cleanliness is difficult to achieve (eg steelworks and foundries) and for materials which have a surface finish too rough and/or a grain too coarse for the other testing systems.
As stated above, Brinell hardness testing is performed using a Brinell hardness tester (of course!); the machine that presses the indenter ball into the sample. The load is applied for several seconds to ensure maximum plasticity of the indentation (or, to put it another way, to prevent the metal, at a microscopic level, ‘pulling itself’ slightly back towards its original shape after the indenter has withdrawn). The smaller the diameter of the indentation, the harder the material and so the higher the HBW number.
Automated Brinell hardness testing
If the Brinell hardness tester is fully automated like our Helios and BRINscan machines, it will read the diameter of the indentation and calculate the HBW number from it. Yes – determining the HBW number requires no effort whatsoever with a fully automated machine.
If the hardness tester is not automated the operator will measure the indentation with a Brinell microscope and read the hardness value off a conversion chart like ours, here. Better than a conversion chart is a calculator, of course, which gives a more exact figure for the hardness than a conversion table.
Calculating the Brinell hardness number
The Brinell hardness number is a function of the force applied, the diameter of the indenter and the indentation diameter.
Working out the Brinell hardness number can be calculated “longhand” but by far the best way is to make use of one of the many online calculators. The Foundrax one is here. With one of these all you have to do is enter the value of the applied force, the diameter of the indenter, and the diameter of the indentation (either the mean of two measurements, on x and y axes, or the two measurements themselves, depending on how the calculator is set up) and the calculation will be performed for you. If you are doing several tests on similar materials you will almost certainly be using the same force and indenter so the values for these will stay the same and only the diameters of the indentations will need to be entered.
The Brinell formula
Of course, you may prefer to create a calculator yourself using an excel spreadsheet. The formula is:
where:
P is the applied force (kgf)
D is the diameter of the indenter ball in mm
d is the mean diameter of the indentation in mm (it’s measured twice, typically left-right and top-bottom)
Are you planning to purchase a Brinell hardness tester?
Hardness testing is a key element in many quality control procedures. If you are testing rough cast, bulky components in less than spotlessly clean conditions then the Brinell test is the obvious choice.
You can check out the extensive range of Foundrax Brinell hardness testers, and if you have any questions about which would be most suitable for your applications a member of our engineering staff will be delighted to assist. Please give us a call on 01458 274 888, email sales@foundrax.co.uk or fill in the online enquiry form.
