Back to Basics: Hardness Testing
Hardness testing is an invaluable part of the quality process. There is no question that testing the hardness of a material is vital in industries ranging from automotive to aerospace. It is used for quality control, materials testing, research and development, and component testing. Think castings and forgings, for example. It is done in order to determine if a material is appropriate for a given situation or process, such as machining.
And it’s been around for a while. Hardness testing is older than the United States—the history of hardness testing goes back at least 250 years—and it continues to be used throughout industry today. If you are using hardness tests in your work, know that you are in good company. “The hardness test is, by far, the most valuable and most widely used mechanical test for evaluating the properties of metals as well as certain other materials,” according to “Hardness Testing,” 2nd Edition, 06671G Harry Chandler, editor, ASTM.
If you’re using this testing method, you may know some of the major players behind the technology, but perhaps not the details. For you quality history buffs, did you know the Rockwell test was named for Stanley and Hugh Rockwell? (The two men behind this patent worked together but were not related.) In the NIST guide “Rockwell Hardness Measurement of Metallic Materials,” Samuel R. Low writes, “Working in a ball-bearing manufacturing plant in 1919, Stanley P. Rockwell invented the Rockwell hardness test as a tool for obtaining a rapid and more accurate measure of the hardness of ball races.”
One hundred years after this pioneering work, hardness testing continues to be a key part of the quality technician toolbox. The NIST guide goes on to say that today it is “the most widely used method for acceptance testing and process control of metals and metal products.”
The NDT Resource Center explains several common hardness testing methods, from Mohs—one of the oldest methods—to Rockwell, Knoop, Brinell, and Vickers. Friedrich Mohs, a German mineralogist, devised this hardness scale during the early 1800s. Fredrick Knoop played a role in the history of hardness testing through his work at the U.S. National Bureau of Standards, and Swedish engineer Johann A. Brinell’s work in 1900 lead to the Brinell test. Although you might assume there was a person named Vickers behind the Vickers test, you would be incorrect—the name comes from the British company Vickers Ltd.
Other methods include the scleroscope and rebound hardness tests, durometer, and barcol. The NDT Resource Center site explains that the term hardness “can apply to deformation from indentation, scratching, cutting or bending. In metals, ceramics and most polymers, the deformation considered is plastic deformation of the surface.” The site goes on to say that “hardness is not be a basic property of a material, but rather a composite one with contributions from the yield strength, work hardening, true tensile strength, modulus, and others factors.”
In other words, it’s a complicated factor to consider. Thus, it makes sense that guidelines have been established to ensure it is measured correctly and consistently.
Standards abound for this testing method, and they have evolved along the way. Early versions of hardness testing standards were much different. According to an ASTM history, “The first Rockwell standards, ASTM E18.32T and BSI #891:1940 were really only directions on how to use the tester to get good results and did not contain a complete definition of the Rockwell hardness test.”
Meanwhile, today ASTM E384-17, Standard Test Method for Microindentation Hardness of Materials, ASTM International, explains the test this way: “Hardness tests have been found to be very useful for materials evaluation, quality control of manufacturing processes and research and development efforts. Hardness, although empirical in nature, can be correlated to tensile strength for many metals and alloys, and is also an indicator of machinability, wear resistance, toughness and ductility.” A sister standard, ASTM E92 – 17, is a Standard Test Methods for Vickers Hardness and Knoop Hardness of Metallic Materials.
With so many factors to consider, from the testing method to the material properties, hardness testing can seem difficult. But the process gets easier depending on what machine you choose to do the job.
Today’s hardness testers vary, from standalone models to fully automated testers with programmable movement and built-in cameras. The feature set of a hardness tester can help make the process as smooth as possible. Features may include automatic indentation and measurement of test patterns, specimen edge recognition, customizable test patterns, and programmable depth of hardening measurement.
When selecting a hardness tester, consider factors such as the skill level of the operator—automated systems may allow for less experienced users—as well as the size, volume and material that is to be tested. Other questions to consider are accuracy, issues with previous testing methods, and of course, budget.
The solution for your metallic materials hardness testing depends on a range of factors. But as the technology continues to develop, it seems certain that it will be around for hundreds of years to come.