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The Rockwell hardness test is a hardness test based on indentation hardness of a material. The Rockwell test measures the depth of penetration of an indenter under a large load (major load) compared to the penetration made by a preload (minor load).E.L. Tobolski & A. Fee, "Macroindentation Hardness Testing," ASM Handbook, Volume 8: Mechanical Testing and Evaluation, ASM International, 2000, pp. 203–211, . There are different scales, denoted by a single letter, that use different loads or indenters. The result is a dimensionless number noted as HRA, HRB, HRC, etc., where the last letter is the respective Rockwell scale. Larger numbers correspond to harder materials.
When testing metals, indentation hardness correlates linearly with tensile strength.
Hugh M. Rockwell (1890–1957) and Stanley P. Rockwell (1886–1940) from Connecticut in the United States co-invented the "Rockwell hardness tester," a differential-depth machine. They applied for a patent on July 15, 1914.H.M. Rockwell & S.P. Rockwell, "Hardness-Tester," , Feb 1919. The requirement for this tester was to quickly determine the effects of heat treatment on steel bearing races. The application was subsequently approved on February 11, 1919, and holds . At the time of invention, both Hugh and Stanley Rockwell worked for the New Departure Manufacturing Co. of Bristol, CT.S.W. Kallee: Stanley Pickett Rockwell Stanley Pickett Rockwell - One of the Inventors of the Rockwell Hardness Testing Machine. Retrieved on 21 November 2018. New Departure was a major ball bearing manufacturer which in 1916 became part of United Motors and, shortly thereafter, General Motors Corp.
After leaving the Connecticut company, Stanley Rockwell, then in Syracuse, NY, applied for an improvement to the original invention on September 11, 1919, which was approved on November 18, 1924. The new tester holds .S.P. Rockwell, "The Testing of Metals for Hardness, Transactions of the American Society for Steel Treating, Vol. II, No. 11, August 1922, pp. 1013–1033.S. P. Rockwell, "Hardness-Testing Machine", , Nov 1924. Rockwell moved to West Hartford, CT, and made an additional improvement in 1921. Stanley collaborated with instrument manufacturer Charles H. Wilson of the Wilson-Mauelen Company in 1920 to commercialize his invention and develop standardized testing machines.V.E. Lysaght, Indentation Hardness Testing, Reinhold Publishing Corp., 1949, pp. 57–62. Stanley started a heat-treating firm circa 1923, the Stanley P. Rockwell Company, which operated until 2012.OpenCorporates, "STANLEY P. ROCKWELL COMPANY THE"
Retrieved 5/24/2023 The building, which still stands, was empty in 2016.CONNECTICUT STATE REGISTER OF HISTORIC PLACES
Retrieved 5/24/2023 The later-named Wilson Mechanical Instrument Company has changed ownership over the years, and was acquired by Instron Corp. in 1993.R.E. Chinn, " Hardness, Bearings, and the Rockwells," Advanced Materials & Processes, Vol 167 #10, October 2009, p 29-31.
The determination of the Rockwell hardness of a material involves the application of a minor load followed by a major load. The minor load establishes the zero position. The major load is applied, then removed while still maintaining the minor load. The depth of penetration from the zero datum is measured, on which a harder material gives a lower measure. That is, the penetration depth and hardness are inversely proportional. The Rockwell test does not use any optical equipment to measure the hardness indention, rather all calculations are done within the machine to measure the indention in the specimen.
The equation for Rockwell hardness is , where d is the depth in mm (from the zero load point), and N and h are scale factors that depend on the scale of the test being used (see following section).
It is typically used in engineering and metallurgy. Its commercial popularity arises from its speed, reliability, robustness, resolution and small area of indentation.
Legacy Rockwell hardness testers operation steps:
In order to get a reliable reading the thickness of the test-piece should be at least 10 times the depth of the indentation. Also, readings should be taken from a flat perpendicular surface, because convex surfaces give lower readings. A correction factor can be used if the hardness of a convex surface is to be measured..
| + Various Rockwell scales (2025). 9780080431529, Elsevier Ltd.. ISBN 9780080431529 | ||||||
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| * Except for the superficial scales where it is 3 kgf, the minor load is 10 kgf. | ||||||
| †Also called a Brale indenter, is made with a conical diamond of 120° ± 0.35° included angle and a tip radius of 0.200 ± 0.010 mm. | ||||||
| §The Rockwell number precedes the scale abbreviations (e.g., 60 HRC), except for the "Superficial scales" where they follow the abbreviations, separated by a ‘-’ (e.g., 30N-25). | ||||||
The superficial Rockwell scales use lower loads and shallower impressions on brittle and very thin materials. The 45N scale employs a 45-kgf load on a diamond cone-shaped Brale indenter, and can be used on dense . The 15T scale employs a 15-kgf load on a hardened steel ball, and can be used on sheet metal.
The B and C scales overlap, such that readings below HRC 20 and those above HRB 100, generally considered unreliable, need not be taken or specified.
Typical values include:
Several other scales, including the extensive A-scale, are used for specialized applications. There are special scales for measuring Case hardening specimens.
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