In this article we discuss the Brinell hardness test, we compare it to the Rockwell hardness test and discuss the advantages and disadvantages of both.
The Brinell hardness test
The need to understand the hardness of metal for various applications dates back to antiquity (in numerous engineering situations (and this includes hand-tool engineering) having the wrong hardness could cause catastrophic equipment failure and serious injury or death) and various methods had been employed (with varying degrees of accuracy). You can read more about this here. Indentation-based testing (measuring the depth or width of an indentation made in the metal under a particular load) was a 19th Century development and the Brinell test was ‘launched’ by its inventor, the Swedish engineer Johan August Brinell (1849-1925) at the 1900 Paris Exposition. The Brinell hardness test uses a spherical indenter which is pressed, by a precisely controlled force – most commonly 3,000 kgf – into the material being measured. The force builds between two and eight seconds then is sustained for several more to ensure that the indentation is a plastic deformation (see footnote). The diameter of the indentation produced by the sphere is then fed into an equation to give the Brinell hardness rating. Below is a photograph of a Brinell indenter in its housing, ready to descend onto the next test sample, and beside it is the Brinell equation.
For many years the test was regarded in certain quarters as somewhat rough and ready, more for the machinist than the professional engineer, due to the difficulty of measuring indentations accurately using a low-power microscope and ambient light. That perception began to change in the 1980s following a technical breakthrough at Foundrax Engineering Products. You can read more about this here. Now, more than thirty years on, Brinell hardness testing is a highly regarded method and still the test of choice in much of the transport, oil and gas and iron and steel industries because it can be used on large components, be they rough cast or machined, as the grain structure of the material does not influence the result. The Brinell hardness test also has the advantage that test material does not have to be spotlessly clean, though there must be no trace of lubricants. As with all industrial processes that have safety-critical consequences, the Brinell hardness test is governed by the International Standards Organisation and the tungsten carbide indenter balls, the force applied and the measurement of the indentations are subject to strict regulations and tight tolerances. In order to ensure that Brinell testing machines are working correctly they are regularly assessed by being put through a test cycle on calibration blocks of a precise, known hardness.
As mentioned above, here’s the Brinell hardness equation. HBW stands for Hardness Brinell Wolfram (Wolfram being tungsten carbide – the material from which the indenter ball is made), P is the applied force, D is the indenter diameter and d is the mean diameter of the indentation (it’s measured twice; usually top-bottom and left-right):
Comparing the Brinell hardness test to an alternative - the Rockwell test
Brinell hardness testing has its limitations, however, including the need to use a selection of different-sized indenters depending upon the material under test and, sometimes, it’s not suitable for testing very small components. It was the difficulty of testing very small components that led to the development of the Rockwell test shortly before the First World War. Stanley P Rockwell, a metallurgist, needed to test the hardness of small bearing races. He came up with a method that, like the Brinell test, involves indenting the material (with a ball for the ‘softer’ metals or a conical diamond with a tiny spherical tip for the ‘harder’ ones) but with the crucial difference that the indentations left behind are extremely shallow (under 0.3mm with the ball and at most 0.2mm with the diamond). There are two other critical differences: it is the depth of the deformation, not the width, that is measured and this measurement occurs as the test machine is going through the test cycle. Because there is no optical measurement system built into the machine (or required alongside it), the outlay for Rockwell hardness testing is relatively low and the fact that measurement occurs as part of the indenting cycle makes the Rockwell hardness test faster than the Brinell. This lower cost and swiftness led the Rockwell test to become the most widely employed hardness measurement system – and this remains the case in 2021.
Another advantage of Rockwell testing over Brinell hardness testing is the size of the calibration blocks which are used to regularly check that the test machines are giving accurate results. Because the forces involved are lower and the indenters are smaller (in general) one can make the indentations much closer to each other without one indentation affecting the result obtained from another. This means Rockwell test blocks can be much smaller than Brinell test blocks for the same number of tests (and you won’t be as dependent on safety boots for your wellbeing if you drop one!). Foundrax’s Rockwell test blocks are 65mm x 65mm x 14mm while our Brinell blocks are approximately 148mm x 115mm x 17mm and obviously Brinell test blocks are far heavier (2.3kg versus 450g for a Rockwell block). There are two disadvantages of Rockwell testing when considered against Brinell: The diamond indenters are very expensive and the test surface (and its underside) must be extremely clean.
Note for non-engineers: deformation is classed by engineers as either elastic or plastic. Elastic deformation is temporary and the material pulls itself back, as it were, to its previous shape. Plastic deformation is permanent, and the dwell time in Brinell hardness testing is to ensure the indentation is a plastic deformation. You can see elastic and plastic deformation in action with a rubber band and a spoon, respectively: If you apply ‘fingertip’ force to extend the length of a rubber band it will return to its original length when the force is no longer applied. If, however, you apply firm thumb and finger force to bend the handle of a teaspoon it will be permanently deformed. The elastic limit of a material is the maximum amount of stress it can withstand but still return to its original shape.