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.
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.
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”). The
differential-depth method subtracted out the errors associated with the
mechanical imperfections of the system, such as backlash and surface
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. The requirement for this tester
was to quickly determine the effects of heat treatment on steel bearing races.
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. 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.
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.
E.L. Tobolski & A. Fee, “Macroindentation Hardness Testing,” ASM
Handbook, Volume 8: Mechanical Testing and Evaluation, ASM
International, 2000, p 203-211,
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
G.L. Kehl, The Principles of Metallographic Laboratory Practice, 3rd
Ed., McGraw-Hill Book Co., 1949, p 229.
H.M. Rockwell & S.P. Rockwell, “Hardness-Tester,” US Patent 1 294 171, Feb
S.P. Rockwell, “The Testing of Metals for Hardness, Transactions of the
American Society for Steel Treating, Vol. II, No. 11, Aug 1922, p
b S.P. Rockwell,
“Hardness-Testing Machine,” US Patent 1 516 207, Nov 1924.
V.E. Lysaght, Indentation Hardness Testing, Reinhold Publishing
Corp., 1949, p 57-62.
R.E. Chinn, “Hardness, Bearings, and the Rockwells,” Advanced Materials &
Processes, Vol 167 #10, Oct 2009, p 29-31.
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