In this article we discuss both the differences in the tests and the often overlooked difference between a Rockwell test block and a Brinell one.
The Brinell hardness test
In the Brinell hardness test a ball indenter is pressed, by a precisely controlled force, into the material being measured and the diameter of the indentation produced by this pressure is then fed into an equation to give a hardness rating. 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 here at Foundrax Engineering Products. You can read more about this here. Now, more than thirty years on, the Brinell system is highly regarded and still the hardness 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. Brinell hardness testing also has the advantage that test material does not have to be spotlessly clean.
The Rockwell hardness 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 Brinell hardness testing, 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 the Rockwell hardness test is block size. 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.