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Hardness Testing

Vickers testing is performed by pressing an indenter of specified geometry into the test surface. Unlike Rockwell testing, the 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 a filar measuring eyepiece. 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 (10 -1000 g) and macro (1 -100 kg), 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.

Rockwell hardness values are expressed as a combination of a hardness number and a scale symbol representing the indenter and the minor and major loads. The hardness number is expressed by the symbol HR and the scale designation.

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 in up to 1/2 in. 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 are applied by traditional deadweight system.


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