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Rockwell hardness testing is a general method for measuring the bulk hardness of metallic and polymer materials. Although hardness testing does not give a direct measurement of any performance properties, hardness correlates with strength, wear resistance, and other properties. Hardness testing is widely used for material evaluation due to its simplicity and low cost relative to direct measurement of many properties. Conversion charts from Rockwell hardness to tensile strength are available for some structural alloys, including steel and aluminum. Rockwell hardness testing is an indentation testing method. An indenter is impressed into the test sample at a prescribed load to measure the material's resistance to deformation. A Rockwell hardness number is calculated from the depth of permanent deformation of the sample after application and removal of the test load. Various indenter shapes and sizes combined with a range of test loads form a matrix of Rockwell hardness scales that are applicable to a wide variety of materials. |
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SAE Standard J429, Mechanical and Material Requirements for Externally Threaded Fasteners, has Rockwell hardness requirements for both core and surface. As hardness corresponds to material strength each grade of fastener has its own hardness requirements. ASTM A240 "Heat-Resisting Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip for Pressure Vessels" has a maximium Rockwell hardness requirement to assure that the materials is annealed for optimum corrosion resistance.
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Regular Rockwell hardness testing-Measures the bulk hardness of the material. There are separate scales for ferrous metals, nonferrous metals, and plastics. Common regular Rockwell hardness scales include B and C for metals and M and R for polymers.
Superficial Rockwell hardness testing-A more surface sensitive measurement of hardness than for the regular Rockwell scales. This technique is useful for samples with hardness gradients at the surface, to test small areas, and for thin samples. Superficial Rockwell hardness scales are N and T for metals and W, X, and Y for nonmetallic materials and coatings.
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Model: WILSON 3TT TWIN TESTER
Samples 6 in. (150 mm) thick or greater can be accommodated. The minimum sample size is dependent on the sample hardness and test scale. Cylindrical samples as small as 1/8 in. (3 mm) diameter and thin sheets 0.006 in. (150 µm) are the absolute minimum sample sizes.
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Testing is typically performed on flat or cylindrical samples. Cutting and/or machining are often required to obtain suitable test specimens from complex-shaped components. Smooth parallel surfaces are required for testing. The specific finish requirements are dependent on the material and test scale. Surfaces should be free from coatings, scale, and gross contamination.