Hardness Testing – A Brief History

Hardness, as applied to most materials, and in specific metals, is a valuable, revealing and extensively used mechanical test that has been in use in different forms for more than 250 years. Indeed, as a material property, its importance and value cannot be understated, and the information from a hardness test can complement and frequently be used in conjunction with other material verification techniques such as compression or tensile in order to provide critical performance information. How important and useful is material and hardness testing?

Consider the information offered and its significance in aerospace, automotive, quality control, structural, failure analysis, and several other forms of manufacturing and industry. Determining these material properties offers valuable insight into the strength, flexibility, durability, and capabilities of a wide range of component types from raw materials to prepared specimens, and finished goods. Over the years, different methods for determining the hardness of materials have been created and used at varying levels of success. From early forms of scratch testing to sophisticated automated imaging, hardness testing has advanced into an accurate, efficient and valued material test method.

While testing techniques and hardware have considerably improved, particularly in recent years and in step with rapidly advancing electronics, hardware, computer, and programming capabilities, earlier, basic forms of hardness testing, such as the simple scratch test, sufficed for the need of the relevant era. Some of the earliest forms of bar scratch testing date back to about 1722. These tests were carried out based on a bar that increased in hardness from end to end. The level at which the material being tested could develop a scratch on the bar was a determining factor in the specimens hardness. Later, in 1822, hardness testing forms were launched that included scratching material surfaces with a diamond and measuring the width of the resultant line, a test eventually known as the Mohs scale.

This method is still used today in some processes. The Mohs scale comprises of 10 minerals, ordered from hardest at 10 (diamond) to softest at 1 (talc). Each mineral is capable of scratching those that fall below it in the scale hierarchy. The Mohs scale is not linear; the difference in hardness between 9 and 10 is mainly more than that between 1 and 2. To put the Mohs scale into perspective, a tangible example is that of hardened tool steel which falls at about 7 or 8 on the scale. Over the next 75 years, other more advanced versions of the scratch test were launched including integrated stage, microscope, and diamond apparatuses that applied increasing loads up to 3 grams.

The material to be tested was scratched under load variants and then compared to a standard set of scratches of known value. A more advanced version of this system used a diamond mounted at the end of a tapered steel spring. The other end of the spring was connected to a balance arm with a 3-gram weight. The material being tested was shifted by a hand-actuated wheel and worm gear system, on top of which was placed a stage and holding fixture for the material. A fixed pressure was applied as the material was traversed leading to a "cut" in the material which was then measured under the microscope with the help of a filar micrometer eyepiece. A mathematical formula, essential to the process, was then used for deriving the hardness.

Indentation type hardness was later introduced, one early form developed about 1859, was based on the load needed to produce a 3.5 mm indent in the material. The depth was then measured with a vernier scale system and the total load required to reach the 3.5 mm was named the hardness. The penetrator comprised of a truncated cone that tapered from 5 mm at the top to 1.25 mm at the point. This method was mainly effective in soft materials. Another early form of indentation test involved pressing right angles geometries of the same test material into one another and then measuring the width of the resulting impression. Different formats developed from this technique during the early 1900s that likewise used "mutual" indentation of cylindrical test material with the longitudinal axis pressed at right angles to each other.

Brinell Hardness Testing

The first extensively accepted and standardized indentation-hardness test was projected by J. A. Brinell in 1900. Brinell's interest in materials science developed during his involvement in a number of Swedish iron companies and his wish to have a fast and consistent means of determining material hardness. The Brinell hardness test, still extensively used today, comprises of indenting the metal surface with a 1 to 10 mm diameter steel or, most recently, a tungsten carbide ball at heavy loads of up to 3,000 kg.

The resultant impression, the diameter of the indentation, is measured with the help of a low-power microscope after removal of the load. The average of two readings of the diameter of the impression at right angles are taken and mathematically calculated to a hardness value. The Brinell test basically introduced the production phase of indentation hardness testing and opened the way for further indentation tests that were more relevant to material types.

Scleroscope Hardness Tester

Around the same time as the Brinell was developing as a useful test, the Scleroscope hardness tester was launched as one of the first "non-marking" hardness-testing instruments. Albert F. Shore, who discovered the Shore Instrument Manufacturing Company in New York, and whose name is currently synonymous with durometer testing, engineered the Scleroscope as an alternative hardness test. The Scleroscope used a diamond tipped "hammer", held within a glass-fronted tube that fell, from a height of 10 inches, onto a test specimen.

The rebound of the hammer was measured on a graduated scale of "Shore" units, each separated into 100 parts that offer a comparison with the rebound that might be expected from hardened high-carbon steel. The hardness reading is technically a measure of the elasticity of the material. One key advantage of the Scleroscope was its "non-destructive" nature in that, unlike the other available methods of hardness testing at the time, a Scleroscope left only a slight mark on the material under test, apparently leaving it available for use after evaluation.

As the 20th century progressed and endured two world wars, with the simultaneous growth of the industrial revolution global industrialization and increased manufacturing requirements brought an urgent need for more efficient and refined test methods, and as a result, new techniques started to develop. Efficient, accurate forms of testing were required in response to heavy manufacturing demands, structural failures, and the need to design adequate material integrity into the growing global infrastructure.

Vickers Hardness Testing

The Vickers hardness test was developed in 1924 by two gentlemen, Smith and Sandland, at Vickers Ltd, a British Engineering conglomerate. The test, developed as an alternative to the Brinell, was developed in response to the need to have a more refined test over the material limitations that the Brinell was effective on. The Vickers test uses the same principle as the Brinell, that of a regulated impression on the material, but instead utilized a pyramid-shaped diamond rather than the Brinell ball indenter. This led to a more versatile and consistent hardness test. Later, in 1939, an alternative to the Vickers test was launched by Fredrick Knoop at the US National Bureau of Standards. The Knoop test made use of a shallower, elongated format of the diamond pyramid and was designed for use under lower test forces than the Vickers hardness test, allowing more accurate testing of thin or brittle materials. Both the Knoop and Vickers tests continue as popular hardness analysis methods today.

Rockwell Hardness Testing

Despite being conceived as an idea in 1908 by a Viennese professor, Paul Ludwik, the Rockwell indentation test did not become of commercial importance until around 1914 when brothers Stanley and Hugh Rockwell, working from a manufacturing company in Bristol Connecticut, succeeded in expanding the idea of using a conical diamond indention test based on displacement and applied for a patent for a Rockwell tester design. The main criterion for this tester was to offer a quick method for determining the effects of heat treatment on steel bearing races.

One of the key strengths of the Rockwell was the small area of indentation required. It is also much easier to use as readings are direct, without the requirement for calculations or secondary measurements. The patent application was approved on February 11th, 1919 and later, in 1924 a more enhanced design patent was granted. At the same time, Stanley Rockwell was starting commercial production of Rockwell testers in association with instrument manufacturer Charles H. Wilson in Hartford, Connecticut.

The company grew into the Wilson Mechanical Instrument Company and was called the premium producer of Rockwell testers. After some changes in ownership through the latter 1900s, Wilson was attained in 1993 by Instron, a global leader in the material testing industry and today it has become a vital part of Instron/Illinois Tool Works. Presently known as Wilson Hardness, the combined expertise of Instron/Wilson, along with the subsequent acquisitions of Wolpert Hardness and Reicherter Hardness, have resulted in the engineering and production of cutting-edge hardness systems. The Rockwell test continues to be one of the most efficient and extensively used hardness test types today.

Hardness Testing - Today and the Future

Presently, with significant enhancements in recent years in hardness testing instrumentation, computer hardware, imaging algorithms, electronics, and software capabilities, the door has opened to exceptionally precise and reliable testing processes that provide results more rapidly than ever before, frequently in automated fashion. These techniques and components have proven to be beneficial in increasing efficiency, accuracy, and speed to unparalleled levels. Over the past several years and no doubt increasingly in the future, more standard manual test processes have and will continue to rapidly make way for automation in every aspect of the testing process.

New techniques in material preparation and handling, stage movement, mount fixturing, results interpretation, and analysis, and even reporting have been introduced to the hardness testing industry. More and more automation technology is being incorporated into a number of hardness systems using stage traversing and image analysis of Knoop, Vickers, and Brinell indentations. An automatic hardness system usually comprises of a fully controllable tester, including an auto-rotating or revolving turret and also actuation in the Z axis either from the head/indenter housing or from a spindle driven system used for both applying the indent at a predetermined force and also for automatically focusing the specimen.

Add to this a standard computer with dedicated hardness software, a USB video camera, and an automatic XY traversing motorized stage, and the result is a powerful, completely automatic hardness testing system. These systems can be left alone to automatically produce, measure, and report on an almost unlimited number of indentation traverses. This newer technology prevents much of the hardware that in the past caused cluttered workspace and operational challenges.

Hardness testing plays a vital role in quality control, materials testing and acceptance of components. The data is needed to verify the structural integrity, heat treatment, and quality of components in order to determine if a material has the properties necessary for its intended use. Through the years, establishing means of increasingly more productive and effective testing through refining standard testing design has made way to new cutting-edge methods that execute and interpret hardness tests in a more effective manner than ever before.

The result is increased potential and dependence on "letting the instrument do the work," contributing to considerable increases in throughput and consistency and continuing to making hardness tests extremely useful in R&D and industrial applications and in assuring that the materials employed in the things people use every day contribute to an efficient, well-engineered, and safe world.

Buehler

This information has been sourced, reviewed and adapted from materials provided by Buehler.

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