This is based on an article that first appeared in the United States’ publication Heat Treat Today
In many steelworks producing large forgings and billets, in numerous heat treatment companies and near many factory lines producing components for safety-critical applications, you’ll find a Brinell hardness tester. These machines have been used all over the world for more than a century (the test was first demonstrated by its inventor, the Swedish metallurgist August Brinell, in 1900), determining metal hardness by means of a tungsten carbide indenter ball that leaves a dish-shaped indentation in the surface of the test material.
In the test the material sample is placed on a rigid anvil and the indenter descends onto it under loads ranging from 1 kg up to 3,000 kg, depending on the material. Indenters vary in diameter from 1mm to 10mm. Most tests use a 3000 kg load and a 10 mm ball, and the standards always refer to this as “HBW 10/3000”. HBW means “Hardness Brinell Wolfram”, Wolfram being another name for the Tungsten Carbide that the indenter ball is made of. After the (approximately) fifteen second indenting cycle, the indentation is measured across both its x and y axes, as a minimum, by a special calibrated microscope. The mean of the diameter readings is then fed into the Brinell equation (see image below).
Naturally, most technicians would rather not use that equation (!) so they look the indentation diameter up on a chart and ‘read across’ to the derived hardness.
The great advantage of the Brinell test, when considered alongside other metal hardness testing methods, is that the large indentation diameter (typically between 2.4mm and 6mm) means that the test result is generally unaffected by the grain structure of the metal. It also means that the surface of the test sample can be adequately prepared in just a few seconds with an angle grinder. For these reasons the test is regarded by many as the ‘default’ one for rough surfaced and/or coarse-grained samples.
That seems pretty simple but there are inherent weaknesses in the Brinell test. The test time, the load etc are straightforward enough to get accurate. Removing surface contaminants from the test material with a grinder is simple enough too. But then the real problem is encountered: measuring the indentation. In our previous article we used the black and white image below to illustrate just how difficult it could be to work out exactly where an indentation edge begins and ends.
You might look at that and think “I’m pretty confident about where that indentation edge is” but it’s trickier than it looks because the process of indenting doesn’t just push material downwards, it also spreads it sideways, and you get a ‘pile up’ around the rim of the indentation. The pile up may be difficult to see on hard material, or there may be a subtle ‘lip’ inside the pile up that represents the true edge, but considered in cross-section, indentations are not a simple ‘dish’ shape.
The image below with multiple indentations illustrates the fact that the indenting process moves material sideways, as well as downwards, very clearly. Now where, exactly, does the pile-up end and the true edge of the indentation begin? And bear in mind that 0.2mm can equal 20 hardness points. You could show an indentation to three experienced workshop technicians and receive three different answers to the diameter question, and this problem has been a challenge of the Brinell test from its inception. Special blocks are available for training technicians in measurement, but the problem of operator interpretation was such that, in some quarters, the Brinell test was regarded as a bit ‘rough and ready’. “Ok for the workshop but not for the lab” was perhaps how it was once seen.
So, the first question to consider when looking at the automation of the Brinell test is the measurement system because this is its major weakness. There are, of course, applications where only narrow tolerances are acceptable, and disagreements can arise between customers and suppliers. Over the years, certain suppliers to the oil tool industry, who had previously been using manual microscopes, have confided to us that they were regularly using expensive testing laboratories because of customers disputing the hardness figures of their products. Obviously, this has reputational, as well as financial consequences. If a manual microscope is employed on raw materials at the goods-in stage and there’s an error reading the hardness, you could find at final machining that you have put a lot of time and effort into a part that, in the end, is too hard or soft for the intended application. Taking every sample, manipulating the microscope into exactly the right position and making the best estimate of diameter is a time-consuming business in itself, even where results are adequate – and they might not be, despite the diligence of the operator, so there are potentially major cost savings from the employment of an automatic Brinell microscope.
The World’s first automatic measurement microscope hit the market 40 years ago. Sweating every last drop of performance out of the computer power of the day, it could measure the diameter of the indentation across over 100 axes, calculate the mean and determine the hardness in a split second. The system has been refined continuously over the years and remains the industry benchmark. It can handle almost any surface irregularity, works in poor light, warns operators of unacceptable surface preparation and does not require the user to get the indentation central to the field of view or to line it up with a graticule. Within a few years of launch, a major oil tool manufacturer’s quality chief recommended its use to his suppliers, and user uptake was rapid. Thanks to this invention, the status of the Brinell test was massively enhanced. There’s a photo below of one version of the microscope in use at a heat treatment plant. Smaller, lighter units are also available.
The next automation step would be to dispense with operator handling of the microscope entirely, by the acquisition of a tester with an integrated microscope. The microscope mentioned above, for example, is a feature on several hardness testing machines. The heavy-duty indenter holder pivots away from its normal line of thrust at the end of the indenting cycle, allowing a supra-mounted camera to view the indentation. This is hugely advantageous: No separate apparatus near the test machine, reduced handling time and hence much faster testing overall. Results from such machines are displayed next to the control panel and instantly uploadable to company quality systems.
Another automation option is to dispense with a hand-cranked anvil capstan and purchase a tester with a fixed anvil and movable test head The technician is not required to manually raise and lower the anvil to allow for variations in the size of sample. Instead, the test head automatically ‘takes up’ the space and also clamps the test piece very securely in place during the test cycle.
The fourth, and obviously most dramatic automation step to consider is incorporating a custom-designed hardness tester into the production line. In some industries this is essential. Large billets and forgings can’t be lifted into the jaws of a bench-top or floor-standing Brinell tester so, for highly accurate testing of such items, a larger machine is required. There’s an example shown in the photos – that machine is now in Texas.
The whole gantry moves on one axis of travel while the test head moves perpendicular to that and, of course, up and down, so you have full x, y, z movement. Large samples are manoeuvred on and off by crane. The test head assembly incorporates the automatic microscope and results are displayed on a screen beside the control panel. Test results, of course, can be instantly uploaded to factory quality systems. The head assembly can also incorporate a milling tool for surface preparation!
With any decision to purchase plant and machinery, some form of cost-benefit analysis is worthwhile. Clearly, if you’re doing a significant amount of business annually with a customer who is threatening to cease contracting with you because your hardness measurements are wrong too often, then the decision to buy an automatic microscope is not a difficult one. If staff are on overtime because mandatory hardness testing is adding too much time to production schedules, then a heavy-duty production machine with automatic microscope, movable test head and sample clamp will pay for itself easily. One such machine is running almost continuously, testing every link, for every track used by the tanks and other tracked vehicles of the British Army.
One thing is certain. Every automation option in Brinell testing increases accuracy and saves time.
