This first appeared (with minor alterations) in the United States publication ‘Heat Treat Today’.
All heat treatment companies obviously test hardness and many do so with a Brinell tester. The Brinell test has been around since 1900 and (for those who don’t use it already) involves a tungsten carbide ball indenter being forced vertically into the surface of the test material, which is placed on a rigid anvil. The diameter of the indentation made by the ball is then measured, across both its x and y axes as a minimum, and the average of the measurements is taken as the working figure. The technician can then either feed that figure into an equation to arrive at the hardness, or read off a ‘diameter-to-hardness’ chart. There are various forces and indenter diameters available for Brinell testing, reflecting the very wide range of metals that need to be assessed, but most tests involve a 10mm ball under a 3,000kg load1. In large, floor-standing machines the indenter is usually motor-driven but some machines use levers and weights, while others are hydraulic or pneumatic. The Brinell test remains the default method for hardness measurement in many heat treatment facilities, for three reasons in particular:
Preparing the surface of a sample for Brinell testing takes just a few seconds with a grinder. Provided the sample is sitting steadily on the anvil and the top face of the sample is perpendicular to the direction of force of the indenter – as mandated by the standards – the surface does not need to be particularly smooth.
Minute surface contaminants under a Brinell indenter are unlikely to cause a ‘mis-test’ but in Rockwell testing (the most widely used method across all industries), where a tiny diamond indenter penetrates the surface by less than 10/1000 inch, any contaminants that could impede or assist the progress of the indenter are a problem, which means that Rockwell samples have to be carefully cleaned before testing. Something as simple as wiping the surface with a solvent cleaner can change the hardness result significantly.
Perhaps most significantly, rugged, hand-portable Brinell testers with hydraulic test heads enable large, heavy and awkwardly-shaped components of rough surface finish to be tested in situ. This feature is of such utility in industry that the international standards authorities give a dispensation – a special designation – to portable machines, though their performance cannot be directly verified like their floor-standing cousins.
Here’s a heavy-duty Brinell tester in situ:
With forces ranging from 3000kg down to 1kg, and indenter balls as small as 1mm, Brinell testing can be used on a huge range of metals, but forges, foundries and heat treatment plants are the places you are most likely to find a test machine, working at 10mm / 3000kg. We mentioned earlier that the surface of test samples doesn’t need to be particularly smooth, in fact roughly-ground surfaces on materials with a coarse grain structure can be measured quite safely because the diameter of the indentation is so large relative to any irregularities on the surface.
Here’s a close-up of a calibration-grade Brinell tester in action. You can see the tungsten carbide ball has been driven into the test sample. In this shot the ball is being held in position to stabilise plastic deformation (see later):
Standards for Brinell testing are laid out in great detail – ASTM E-10 and ISO 6506 are the authoritative documents – but the practical procedure for workshop technicians is very straightforward – training should not take longer than an hour. When testing forgings, billets and other samples, one indentation should suffice but obviously in certain critical applications more than one indentation may be used for assurance.
The question of whether to test every sample in a given batch will depend on how inconsistent those samples might be, it has nothing to do with any issues with Brinell testing itself. In certain industries every single product is tested because the risk of failure is too high. A good example of this is the production of links for the tracks used on tanks and some other armoured vehicles. Every link, in every tank track in use by the British Army has been Brinell-tested on a high-speed, fully automatic machine that features a powerful integral clamp to keep the component absolutely rigid during the test. Incidentally, that machine is the one in the first photo. Subject to reasonable care, a heavy-duty Brinell tester will perform many hundreds of thousands of tests. That one has performed several million.
Tests take approximately fifteen seconds because the indenter has to be driven smoothly into the material with no possibility of ‘rebound’, and ‘punching’ the indenter into the material at speed would also risk excess localised heating from friction. Also, the metal has to be loaded for a sufficient length of time to ensure that the indentation is as plasticly deformed as possible, that is, the risk of an indentation shrinking very, very slightly after the indenter is withdrawn is kept to a minimum.
This is the point, however, where things get tricky. You’ve carefully made your indentation and withdrawn the test sample from the ‘jaws’ of the test machine but now you have to measure the indentation across at least two diameters. Given that Brinell indentations are at most 6mm across and that 0.2mm difference in diameter might equal 20 hardness points, getting the measurement right is critical, and tricky. Most technicians will use an illuminated microscope to do this but even then it can be a challenge. Here’s an example:
Making an indentation leaves a ‘ridge’ at the indentation perimeter because metal is not just pushed downwards, but also sideways. This ridge can obscure where the real indentation begins, and three different technicians can easily make three different estimates of where that is. And this variation in operators’ interpretation of results is why, for over 80 years, the Brinell test was seen as a little ‘rough and ready’; ok for the workshop machinist, perhaps, but probably not for the laboratory scientist.
Manual measurement microscopes have improved over the years, and when you get a relatively ‘clean-edged’ indentation with a crisply illuminated graticule it can be less challenging for the experienced technician to make an accurate measurement. Here’s a less difficult scenario than the one above. Even so, how can we know if we have really judged the position of the edge precisely?
In 1982 the first automatic reader hit the markets. This was the culmination of years of research, and used proprietary software that pushed the computers of the day to their limits. The equipment could make hundreds of measurements across the indentation and calculate the mean diameter in a split second. Not long afterwards it was available as integral part of a Brinell test machine. Word of this equipment soon reached some critical users in the oil tool industry and they mandated its use to their suppliers. Within fifteen years the use of this technology was widespread and the perception of the Brinell test’s accuracy had been transformed. The Brinell test, in a sense, had come of age. We’ll discuss this further in a future article but here is the latest version of that automatic microscope in action:
Obviously, like any important measuring equipment, regular calibration and servicing is desirable, if not compulsory. Manufacturers typically stipulate a service schedule which must be considered alongside the calibration rules dictated by international agencies.
When considering your options for hardness testing of heat-treated samples you are ultimately looking at three test methods, Brinell, Rockwell and Vickers. Rockwell testing is the most widespread across manufacturing industry and it has the advantage that it is a depth measurement system and the depth measurement is performed by the machine during the test. Additional optical equipment for indentation measurement is not required, which makes the machines cheaper than their Brinell counterparts. Most Rockwell testing uses very small indenters, so is well suited to smaller and thinner components (it was actually developed to measure the hardness of small bearing races that could not reliably be tested by the Brinell method). However, a lot of Rockwell testing involves diamond-tipped indenters that can easily be damaged – and this damage can go unnoticed. They are also expensive and, as we mentioned earlier, the penetration depth is so small that Rockwell testing is vulnerable to contaminants and unsuitable for rough surfaced material.
Brinell testing isn’t suited to very small or very thin samples but it’s relatively ‘immune’ to small contaminants, the indenters are not expensive and the width of the indentation means that testing of coarse grained and roughly finished surfaces is not problematic. With the development of automatic indentation measurement 40 years ago, the one serious deficiency of the Brinell test was overcome, providing the assurance that was so important to critical component suppliers in oil and gas, aerospace, defence and transport.
Note 1. the 3,000kg figure is, strictly, 29,420N (3,000 x 9.80665)