Rockwell scale

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The Rockwell scale characterizes the indentation hardness of materials through the depth of penetration of an indenter, loaded on a material sample and compared to the penetration in some reference material. [1] 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.


Contents

[edit] History

The Rockwell hardness test was devised by metallurgist Stanley Pickett Rockwell in Syracuse, NY, circa 1919, in order to quickly determine the effects of heat treatment on steel bearing races. [2] [3] Rockwell moved to West Hartford, CT, and improved on his initial invention in 1921. [4] 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. Rockwell collaborated with instrument manufacturer Charles H. Wilson of the Wilson-Mauelen Company in 1920 to commercialize his invention and develop standardized testing machines.[5] Rockwell started a heat-treating firm in the early 1930s, the Stanley P. Rockwell Co., which still exists in Hartford, CT. The Wilson company has changed ownership over the years, and was most recently acquired by Instron Corp. in 1993.

[edit] 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.

[edit] 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.

[edit] 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.

[edit] 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, 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 ceramics. 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.

[edit] 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-hardened specimens.

[edit] Standards

  • International (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)
  • ASTM E18 : Standard methods for rockwell hardness and rockwell superficial hardness of metallic materials

[edit] 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. ^ 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.
  3. ^ S.P. Rockwell, "Hardness-Testing Machine," US Patent 1 516 207, Nov 1924.
  4. ^ S.P. Rockwell, "Hardness-Testing Machine," US Patent 1 516 208, Nov 1924
  5. ^ V.E. Lysaght, Indentation Hardness Testing, Reinhold Publishing Corp., 1949, p 57-62.

[edit] See also

[edit] External links