The history and application of the Rockwell Hardness Test and Rockwell Hardness Scale
Metal hardness testing is an essential part of numerous engineering and manufacturing processes and in most of these applications is essential to avoid catastrophic component failure and, indeed, consequential death and injury.
The Rockwell scale takes its name from Hugh and Stanley Rockwell, American engineers working in the ball bearing (and bearing race) industry prior to World War One. They built on the conceptual work (1908) of Viennese Professor Paul Ludwik, whose work Die Kegelprobe showed that a metal hardness test methodology involving a very small conical indenter could provide very accurate results, with minimal damage to the material being tested. The test would also overcome any problems due to surface imperfections by calculating the hardness using the difference in depth achieved by an indenter when pressed into the metal under a light, initial load then a much heavier full load.
Driven by the need to test the hardness of small ball-bearing races, Hugh and Stanley Rockwell invented a depth differential machine in 1914. Stanley Rockwell made significant improvements after World War One and the test, as used today, is essentially regarded as his. Rockwell collaborated with Charles Wilson of the Wilson-Maulen Company to produce a suitable testing machine and the test was in use from the mid 1920s.
The Rockwell test works as follows: a very small indenter is driven vertically downwards into the test material by a ‘minor’ load of 10 Newtons (N). The depth achieved by the indenter under this minor load establishes the datum. A much higher load (not less than 45N and as much as 150N is then applied for several seconds then removed, leaving the minor load applied. The difference in depth achieved by the indenter under the minor and major loads is then used to calculate the hardness. Obviously the harder the material being tested, the less the penetration depth but the harder the material, the higher the Rockwell hardness number (see below) so the Rockwell hardness number is inversely proportional to the penetration depth. The Rockwell test has two very significant advantages over the Brinell test, which was first demonstrated in 1900. First, the depth measurement is performed and displayed by the test machine itself. No additional optical measuring equipment is required (operator error in optical measurement by manual microscope is still the most problematic element in the use of Brinell testing by some organisations) and second, the indentations are much, much smaller than in Brinell testing, so much smaller components and samples can be tested in a non-destructive manner. You can read more on the Brinell / Rockwell distinctions here: https://foundrax.co.uk/all-about-brinell-hardness-testing/. For a comprehensive overview of hardness testing we have produced this article: https://foundrax.co.uk/hardness-testing-a-comprehensive-overview/
There are a number of Rockwell hardness scales, in order to cater for the widest range of materials, and they are named Hardness Rockwell A (HRA), Hardness Rockwell B (HRB) and so on. There are also a variety of indenters (tungsten carbide spheres of various diameters or a sphero-conical diamond). Most general testing requirements in mainstream manufacturing and engineering are covered by the HRB (for copper alloys, aluminium, soft steel) and HRC (for steel, cast iron and hardened steel) scales. Rockwell numbers in each scale fall between 0 and 100.
A second set of Rockwell scales – Superficial Rockwell – came about to permit the testing of very thin and / or brittle materials.
The employment of the Rockwell test (equipment and methodology) is governed by international standards (International Standards Organisation (ISO) and American Society for Testing and Materials (ASTM)).
Here’s a ‘household’ or domestic example to which a layman can relate: top quality tailoring scissors have a hardness around HRC 60. Chefs’ knives are usually around HRC 55. The chefs’ knife is thinner and the blade is likely to be subjected to a certain amount of bending force in use, so, although it has a cutting function, it must not be brittle. Scissor blades are thicker and not subject to repeated impact on bones, chopping boards etc and less likely to be inadvertently flexed, so can be of a harder and more brittle material.
For a comparison between the Brinell and commonest Rockwell scales see this page: https://foundrax.co.uk/resources/conversion-chart-brinell-rockwell/
For information on successful Rockwell testing, see here: https://foundrax.co.uk/rockwell-hardness-testing-tips/
