What IS the Rockwell Hardness Rating?

According to Wikipedia (the free online encyclopedia), The Rockwell scale is a hardness scale based on the indentation hardness of a material. The Rockwell test determines the hardness by measuring the depth of penetration of an indenter under a large load compared to the penetration made by a preload.^[1] There are different scales, which are denoted by a single letter, that use different loads or indenters. The result, which is a dimensionless number, is noted by HRX where X is the scale letter.

When testing metals, indentation hardness correlates linearly with tensile strength.^[2] This important relation permits economically important nondestructive testing of bulk metal deliveries with lightweight, even portable equipment, such as hand-held Rockwell hardness testers.

How this test came to be or the history behind the Rockwell Test.

The differential depth hardness measurement was conceived in 1908 by a Viennese professor Paul Ludwik in his book Die Kegelprobe (crudely, “the cone trial”).^[3] The differential-depth method subtracted out the errors associated with the mechanical imperfections of the system, such as backlash and surface imperfections.

The Rockwell hardness tester, a differential-depth machine, was co-invented by Connecticut natives Hugh M. Rockwell (1890-1957) and Stanley P. Rockwell (1886-1940). A patent was applied for on July 15, 1914.^[4] The requirement for this tester was to quickly determine the effects of heat treatment on steel bearing races.

The Brinell hardness test, invented in 1900 in Sweden, was slow, not useful on fully hardened steel, and left too large an impression to be considered nondestructive. The application was subsequently approved on Feb. 11, 1919, and holds patent number #1,294,171.

At the time of invention, both Hugh and Stanley Rockwell (not direct relations) worked for the New Departure Manufacturing Co. of Bristol, CT. New Departure was a major ball bearing manufacturer that, 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 Sept. 11, 1919, which was approved on Nov. 18, 1924. The new tester holds patent #1,516,207.^[5][6] Rockwell moved to West Hartford, CT, and made an additional improvement in 1921.^[6] Stanley collaborated with instrument manufacturer Charles H. Wilson of the Wilson-Mauelen Company in 1920 to commercialize his invention and develop standardized testing machines.^[7]

Stanley started a heat-treating firm circa 1923, the Stanley P. Rockwell Company, which still exists in Hartford, CT. The later-named Wilson Mechanical Instrument Company has changed ownership over the years, and was most recently acquired by Instron Corp. in 1993.^[8]

References

1. ^ E.L. Tobolski & A. Fee, “Macroindentation Hardness Testing,” ASM Handbook, Volume 8: Mechanical Testing and Evaluation, ASM International, 2000, p 203-211, ISBN 0-87170-389-0.
2. ^ Correlation of Yield Strength and Tensile Strength with Hardness for Steels , E.J. Pavlina and C.J. Van Tyne, Journal of Materials Engineering and Performance, Volume 17, Number 6 / December, 2008
3. ^ G.L. Kehl, The Principles of Metallographic Laboratory Practice, 3rd Ed., McGraw-Hill Book Co., 1949, p 229.
4. ^ H.M. Rockwell & S.P. Rockwell, “Hardness-Tester,” US Patent 1 294 171, Feb 1919.
5. ^ S.P. Rockwell, “The Testing of Metals for Hardness, Transactions of the American Society for Steel Treating, Vol. II, No. 11, Aug 1922, p 1013-1033.
6. ^ ^{a b} S.P. Rockwell, “Hardness-Testing Machine,” US Patent 1 516 207, Nov 1924.
7. ^ V.E. Lysaght, Indentation Hardness Testing, Reinhold Publishing Corp., 1949, p 57-62.
8. ^ R.E. Chinn, “Hardness, Bearings, and the Rockwells,” Advanced Materials & Processes, Vol 167 #10, Oct 2009, p 29-31.

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