Which hardness scale is the most commonly used and why?

The Rockwell hardness scale is likely the most common hardness scale used today. A highly accurate and fast method of testing hardness, the Rockwell hardness test is used across a number of metrological settings and so you often see hardness represented using the Rockwell hardness scale. Information collected about hardness relates to other factors held by a material including strength, resistance, and ductility, making precision highly important. The differential depth measurement concept for hardness that is used by the Rockwell hardness scale was first developed by Pail Ludwik in . Then, Hugh Rockwell and Stanley Rockwell co-invented a machine called the Rockwell hardness tester, which was patented in , that implemented the differential depth measurement concept. The Rockwell hardness scale is defined by the standard American Society for Testing and Materials (ASTM) E18 and approved for measurement of commercial shipments, making it highly utilized. Getting a reliable Rockwell hardness scale value requires that the test materials be at least 10 times the depth of the indentation being made and the material must be measured on a flat and perpendicular surface.

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The Rockwell hardness test is based on an inverse relationship to the measurement of the additional depth to which an indenter is forced by a heavy total (major) load beyond the depth resulting from a previously applied preliminary (minor) load. Initially a minor load is applied, and a zero datum position is established. The major load is then applied for a specified period and removed, leaving the minor load applied. The resulting Rockwell number represents the difference in depth from the zero datum position as a result of the application of the major load. The entire procedure requires as little as a few seconds up to 15 for plastics. In the Rockwell test results are quickly and directly obtained without the need for a secondary, dimensional measurement requirement. The most common indenter type is a diamond cone ground at 120 degrees for testing hardened steels and carbides. Softer materials are typically tested using tungsten carbide balls ranging in diameters from 1/16&#; up to 1/2&#;. The combination of indenter and test force make up the Rockwell scale. These combinations make up 30 different scales and are expressed as the actual hardness number followed by the letters HR and then the respective scale. A recorded hardness number of HRC 63 signifies a hardness of 63 on the Rockwell C scale. Higher values indicate harder materials such as hardened steel or tungsten carbide. These can have HRC values in excess of 70 HRC. Rockwell test forces can be applied by either closed loop load cell or traditional deadweight systems.

Micro or Macro hardness testing, also commonly referred to as Knoop or Vickers testing, is also performed by pressing an indenter of specified geometry into the test surface. Unlike Rockwell testing, the Knoop or Vickers test applies only a single test force. The resultant impression or un-recovered area is then measured using a high powered microscope in combination with filar measuring eyepieces, or more recently, automatically with image analyzing software. The Knoop diamond produces an elongated rhombic based diamond shaped indent with a ration between long and short diagonals of about 7 to 1. Knoop tests are mainly done at test forces from 10g to g, Knoop tests are mainly known as microhardness or Microindentation tests and are best used in small test areas or on brittle materials as minimal material deformation occurs on the short diagonal area. The Vickers diamond produces a square based pyramidal shape with a depth of indentation of about 1%7th of the diagonal length. The Vickers test has two distinct force ranges, micro (10g to g) and macro (1kg to 100kg), to cover all testing requirements. The indenter is the same for both ranges therefore Vickers hardness values are continuous over the total range of hardness for metals (typically HV100 to HV). Vickers tests are mainly known as macro-indentation tests and are used on a wider variety of materials including case hardened, and steel components. Vickers indents are also less sensitive to surface conditions than the Knoop test. In both test types the measured area is used in a formula that includes applied force to determine a hardness value. Tables or automatic electronic or imaging measurements are a more common and convenient way to generate Knoop and Vickers hardness numbers.

Another common hardness test type, the Brinell test, consists of applying a constant load or force, usually between 500 and Kgf, for a specified time (from 10 to 30 seconds) using a 5 or 10 mm diameter tungsten carbide ball. The load time period is required to ensure that plastic flow of the metal has ceased. Lower forces and smaller diameter balls are sometimes used in specific applications. Similar to Knoop and Vickers testing, the Brinell test applies only a single test force. After removal of the load, the resultant recovered round impression is measured in millimeters using a low-power microscope or an automatic measuring device. Brinell testing is typically used in testing aluminum and copper alloys (at lower forces) and steels and cast irons at the higher force ranges. Highly hardened steel or other materials are usually not tested by the Brinell method, but the Brinell test is particularly useful in certain material finishes as it is more tolerant of surface conditions due to the indenter size and heavy applied force. Brinell testers are often manufactured to accommodate large parts such as engine castings and large diameter piping.

Hardness testing plays an important role in materials testing, quality control and acceptance of components. We depend on the data to verify the heat treatment, structural integrity, and quality of components to determine if a material has the properties necessary for its intended use. Establishing a correlation between the hardness result and the desired material property allows this, making hardness tests very useful in industrial and R&D applications and in insuring that the materials utilized in the things we use everyday contribute to a well engineered, efficient, and safe world.

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