This is part 5 of David Pye's series on hardness testing.
Rockwell Hardness Testing
Two brothers who owned and operated a commercial heat-treatment shop in the U.S. were faced with difficulty in accurately and consistently measuring hardness values. Hugh Rockwell and Stanley Rockwell came up with an invention for accurately measuring the hardness value of metals and materials. The brothers applied for a patent in July 1914, and it was approved in February 1919.
Stanley Rockwell then went into partnership with Charles Wilson. This company became the Wilson Mechanical Instrument Company. Wilson and the Rockwell are now well-known names in international hardness-testing equipment.
Rockwell is a method of hardness testing that simply relies on the metal’s resistance to indentation using a specific load application and a specific indenter. It has two load systems for measurement, which is light load and major load. The Rockwell hardness test is defined in ASTM E 18 and several other standards.
Rockwell hardness testing differs from Brinell testing in that the Rockwell hardness number is based on the difference of indenter depth from two load applications. Initially a minor load is applied, and a zero datum is established as seen on the front dial indicator. A major load is then applied for a period of time, causing a secondary penetration depth beyond the zero-datum point previously established by the minor load. After a specified dwell time for the major load, it is removed while still keeping 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 takes approximately 10 seconds.
In Rockwell testing, the minor load is 10 kg, and the major load is 60, 100 or 150 kg, depending on the material to be tested. With superficial Rockwell testing, the minor load is 3 kg, and major loads are 15, 30 or 45 kg. In both tests, the indenter may be either a diamond cone or a hardened ball made either from hardened steel or hardened carbide steel.
Diameters of the hardened-ball indenters are 1.588, 3.175, 6.35 and 12.7 mm. These indenter sizes are typically used to test softer materials such as annealed steels, softer grades of cast irons, plus a wide variety of nonferrous metals.
The next indenter option for hardness testing is the diamond indenter. Rockwell diamond indenters are used mainly for testing hard materials such as hardened steels and cemented carbides. The materials that are considered “hard materials” have a hardness greater than 100 HRB and greater than 83.1 HR30T.
While not out of tolerance, the old U.S. standard indenter is at the low end of the specification. This led to a change in tip radius closer to 200 μm used in the rest of the world.