Rockwell scale
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The '''Rockwell scale''' characterizes the [[indentation hardness]] of materials through the depth of penetration of an indenter under a major load on a material sample and compared to the penetration made by a minor load. <ref>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.</ref> It is one of several definitions of [[hardness]] in [[materials science]]. Its hardness values are noted by HR* where * is the letter for the scale used. Hardness' relation to strength is that both are measures of the pressure it takes to get plastic deformation to occur in materials.
== History ==
The differential depth hardness measurement was conceived in 1908 by a Viennese professor named Ludwig in his book ''Die Kegelprobe'' (crudely, "the cone method"). <ref>G.L. Kehl, ''The Principles of Metallographic Laboratory Practice'', 3rd Ed., McGraw-Hill Book Co., 1949, p 229.</ref> 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. <ref>H.M. Rockwell & S.P. Rockwell, "Hardness-Tester," US Patent 1 294 171, Feb 1919.</ref> The requirement for this tester was to quickly determine the effects of heat treatment on steel bearing races. The [[Brinell scale| 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 testing| 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 later, 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. <ref>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.</ref> <ref>S.P. Rockwell, "Hardness-Testing Machine," US Patent 1 516 207, Nov 1924.</ref> Rockwell moved to West Hartford, CT, and made an additional improvement in 1921. <ref>S.P. Rockwell, "Hardness-Testing Machine," US Patent 1 516 208, Nov 1924.</ref> Stanley collaborated with instrument manufacturer Charles H. Wilson of the Wilson-Mauelen Company in 1920 to commercialize his invention and develop standardized testing machines.<ref>V.E. Lysaght, ''Indentation Hardness Testing'', Reinhold Publishing Corp., 1949, p 57-62.</ref> 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.
==Operation==
The determination of the Rockwell hardness of a material involves the application of a minor load followed by a major load, and then noting the depth of penetration, vis a vis, hardness value directly from a dial, in which a harder material gives a higher number. The chief advantage of Rockwell hardness is its ability to display hardness values directly, thus obviating tedious calculations involved in other hardness measurement techniques. Also, the relatively simple and inexpensive set-up enables its installation in college laboratories.
It is typically used in [[engineering]] and [[metallurgy]]. Its commercial popularity arises from its speed, reliability, robustness, resolution and small area of indentation.
===Good practices===
*Cleaning indenter and test-piece to be clear of dirt, grease, rust or paint
*Measuring on a perpendicular, flat surface ("round work correction factors" are invoked to adjust for test-piece curvature)
*Ensuring that the thickness of the test-piece is at least 10 times the depth of the indentation
*Maintaining an adequate spacing between multiple indentations
*Controlling the speed of the indentation.
===How to use a Manual Rockwell Tester===
A manual tester will have a base plate where the part is located, directly below the indenter. It will also have a spoked wheel, designed to raise and lower the base plate, and a ridged wheel, to zero the scale on the display. Other important parts are the trigger (a flat area to be pushed to start the test) and a lever, which must be in the front position before you can start.
First, put the part on the base plate. Slowly turn the spoked wheel until it contacts the indenter. On the display, there will be a short indicator with a dot at the 12 o'clock position. Continue to turn the spoked wheel until the indicator points to the dot. If you overshoot you must start over in a new position- you can't just turn the wheel back. When the short indicator points to the dot, turn the ridged wheel until the long indicator points to the zero of the scale you're using (usually C or B). Press the trigger. Wait about 30 seconds. The lever will move backwards slowly as the indenter lowers. After the time is up, flip the lever to the front and read the measurement from the position of the long indicator. If you want to test again, raise the indenter, move it over, and repeat. If your reading not appropriate for the scale, change the indenter and weight and re-test.
==Scales and values==
There are several alternative scales, the most commonly used being the "B", and "C" scales. Both express hardness as an arbitrary [[dimensionless number]].
*The B-scale is used for softer materials (such as aluminum, brass, and softer steels). It employs a hardened [[steel]] ball as the indenter and a 100-kg weight to obtain a value expressed as "HRB".
*The C-scale, for harder materials, uses a [[diamond]] [[cone (geometry)|cone]], known as a ''Brale indenter'' and a 150-kg weight to obtain a value expressed as "HRC".
*The ''superficial'' Rockwell scales use lower loads and shallower impressions on brittle and very thin materials. The 45N scale employs a 45-kg load on a diamond cone-shaped Brale indenter, and can be used on dense [[ceramic]]s. The 15T scale employs a 15-kg load on a 1/16-inch diameter hardened steel ball, and can be used on [[sheet metal]].
Readings below HRC 20 are generally considered unreliable, as are readings much above HRB 100.
===Typical values===
* Very hard steel (e.g. a good knife blade): HRC 55 - HRC 62
* Axes, chisels, etc.: HRC 40 - 45
Several other scales, including the extensive A-scale, are used for specialized applications. There are special scales for measuring [[Case hardening|case-hardened]] specimens.
==[[International standard|Standards]]==
*International ([[International Organization for Standardization|ISO]])
:*ISO 6508-1 : Metallic materials -- Rockwell hardness test -- Part 1: Test method (scales A, B, C, D, E, F, G, H, K, N, T)
* US standard ([[ASTM International]])
:*[[ASTM]] E18 : Standard methods for rockwell hardness and rockwell superficial hardness of metallic materials
==References==
{{reflist}}
* [http://www.struers.com/default.asp?top_id=5&main_id=25&doc_id=344&target=_self&collapse=1 Struers hardness conversion table]
* "Metallographic and Materialographic Specimen Preparation, Light Microscopy, Image Analysis and Hardness Testing", Kay Geels in collaboration with Struers A/S, ASTM International 2006.
==See also==
* [[Vickers hardness test]]
* [[Brinell hardness test]]
* [[Knoop hardness test]]
* [[Shore durometer]]
* [[Hardness comparison]]
==External links==
*[http://www.indentec.com Indentec.com]
*[http://www.zwick.co.uk/productdisplay.php?type=hard Zwick.co.uk]
*[http://www.wilsoninstruments.com/resources/default.aspx?=MPID2538 WilsonInstruments.com]
* Rockwell to Brinell Conversion chart: http://www.engineersedge.com/hardness_conversion.htm
* [http://www.struers.com/default.asp?top_id=5&main_id=25&doc_id=344&target=_self&collapse=1 Struers hardness conversion table]
[[Category:Hardness tests]]
[[Category:Dimensionless numbers]]
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