Shawn Byrd, the author of this article is the metal’s technical expert with Tinius Olsen.
The process of buying a new car can put us all through the paces. Some of us know precisely what we want and zero right in. But the rest have to weigh a myriad of factors. Is it a family carrier or a sports car? How much cargo room do I need? Should I get four-wheel drive for winter conditions? Which electronic dashboard options would make my life easier? Am I looking for a luxury vehicle or a lower price range?
The world of materials-testing machines, specifically hardness-testing machines, would seem to be very targeted and well defined. Even within this narrow realm, however, a user’s needs vary tremendously. As a result, so do the number of machine choices.
It is difficult to even agree on a common definition of metal hardness, from resistance-to-deformation to stiffness to scratch resistance. Most users buying test machinery are doing so to measure their metal sample’s ability to have its surface withstand being compromised – indented or abraded. Considering that such measurement can occur on the macro-, micro- and even nano-scale level only opens up the range of choices.
A look at various types of hardness-testing methods will help to identify the specific type of machine you might require, including whether a universal machine is in order to address multiple needs with a single piece of equipment.
Most materials engineers define hardness by one of three operational definitions: scratch hardness, which is resistance to fracture or permanent deformation due to friction from a sharp object; indentation hardness, which is resistance to permanent deformation due to a constant load from a sharp object; and rebound hardness, which measures height or speed of the bounce of an object dropped on the test material.
These scientists recognize a number of common tests, including Rockwell, Vickers, Knoop and Brinell, but even these are further segmented by Rockwell Superficial and micro-Vickers. Here is a rundown of hardness tests to help you determine which are best suited to your operation.
Whether you do Rockwell or Superficial Rockwell will depend on the material you are testing. Both use HR scales. Rockwell indents the test material with either a diamond core (HRC) or a hardened (tungsten) steel-ball indenter (HRB, etc.) by applying a preload of 10 kgf followed by a main test force of 60, 100 or 150 kgf. Depth of the impression is measured (Fig. 1).
If you are working with brittle or very thin materials (e.g., thin strip or lightly carburized surfaces, small parts or parts that might not hold up under a standard Rockwell test), then Superficial Rockwell scales should be used for lower force and shallower impressions, specifically a 3 kgf preload and main test force of 15, 30 or 45 kgf.
Here are Rockwell test samples broken down by HR scale: A (cemented carbides, thin steel, shallow case-hardened steel); B (copper alloys, soft steels, aluminum alloys, malleable iron); C (steel, hard cast irons, pearlitic malleable iron, titanium, deep case-hardened steel, plus other materials harder than B); D (thin steel and medium case-hardened steel, pearlitic malleable iron); E (cast iron, aluminum, magnesium alloys, bearing metals); F (annealed copper alloys, thin or soft sheet metals); G (phosphor bronze, beryllium copper, malleable irons); H (aluminum, zinc, lead); and K, L, M, P, R, S and V (bearing metals, plus other very soft or thin materials).
Vickers adds a microscope or USB camera to visualize and measure the indentation caused on a test sample, especially those with extremely hard surfaces, by a diamond indenter (Fig. 2). The indenter is an upside-down pyramid shape with a square base and an angle of 136 degrees between opposite faces. Test forces of 1-120 kgf are used. For testing where 1 kgf or less is needed, a micro-Vickers unit can be employed along with a precision microscope or high-resolution USB camera in order to visualize and measure indentations using magnifications up to 600x typically (up to 1,000x increasingly). Both tests carry the HV designation. Vickers hardness tests deliver extremely accurate readings and require only one type of indenter. The machine is typically a floor model and more expensive than Brinell and Rockwell units.
The Knoop indentation test is used to measure a material’s relative microhardness via the depth its indenter penetrates. Somewhat similar to Vickers, it employs an elongated diamond pyramid, test forces usually not exceeding 1 kgf, and a precision microscope or high-resolution USB camera. Typical magnifications are up to 600x. Knoop uses lower indentation pressures than Vickers, however, so it can be used beyond metals for brittle materials like glass and ceramics. Knoop hardness is HK derived by the formula, 14.229 (F/D2).
Brinell (HB) tests material using a 1-, 2.5-, 5- or 10-mm-diameter hardened-steel or carbide ball that is subjected to a load/force ranging from 1-3,000 kgf (Fig. 3). Like Vickers, a microscope or USB camera is part of the setup to visualize and measure the rather large indentations that result. The Brinell number is arrived at by dividing the load used (in kg) by the actual surface area of the indentation in square mm. It is a pressure measurement, but units are seldom given. Brinell machines can be either desktop or floor models.
Beyond these commonly used hardness tests, the industry has a number of others. Portable hardness-testing methods include Leeb (HL), also known as the rebound method. An impact body with a spherical tungsten-carbide tip is impelled onto the test surface by spring force – the softer the material, the greater the speed lost at rebound of the impact body.
Ultrasonic (UCI) uses a Vickers-shaped diamond indenter fixed on a vibrating rod that presses on the test surface with a specific force and measures hardness by applying ultrasonic vibrations and analyzing its damping effect. It ideally measures small, thin components that cannot be tested by rebound units.
Shore (HS scales) presses the indenter foot firmly onto the sample while a linear measuring device tracks the indent depth and converts it to the Shore value – the deeper the indent, the softer the material.
Webster places the sample between an anvil and a penetrator until a “bottom” is reached. Different indenters and force settings are used for different materials.
Still other less-common hardness-testing methods include: IRHD, which measures the indentation resistance of elastomeric or rubber materials; Martens (HM, but formerly HU for universal hardness); H for ball indentation hardness; Modified Vickers (HVT) for depth measurement; Modified Brinell (HBT) also for depth measurement; Mohs, which defines hardness by how well a substance will resist scratching by another substance; and Barcol, which, like Leeb, obtains a hardness value by measuring resistance of a sharp steel point under a spring load.
With such a wide diversity of hardness scales and applications, it is understandable why there is an equally diverse range of test machines to measure each. It is also understandable why universal hardness testers are becoming more popular, since the need for multiple testers for different hardness scales is eliminated.
We have found that our new model FH6 (Fig. 4) has the ability to test micro-Vickers, Vickers and micro-Brinell scales along with its capability of taking single or serial measurements. Powerful software allows file storing, test-program setting and storage, image zoom, auto-focus, limit settings, conversion to other hardness scales, system setup and (remote) control, and pattern testing. These features ensure high reproducibility of test results and limit operator error and interpretation.
Once you narrow down your options based on the types of materials you will need to test, the types of test methods best suited, the type of unit you require (benchtop, floor or portable) and what your budgetary limits are, you will have learned whether your needs point you in the direction of a machine performing a single type of hardness test or to a universal machine capable of multiple kinds of tests.
Some typical universal machines can combine Rockwell, Superficial Rockwell, Brinell, Vickers, and HVT or HBT. You will pay more for this added flexibility and capability. However, the range of information you’ll obtain is well worth the investment. A little bit of research and Internet comparison of manufacturers and available models will help you quickly narrow down what type of hardness tester(s) you need, steering you to a Honda, Jeep or Ferrari.