Definitions

rockwell-test

Rockwell scale

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.

History

The differential depth hardness measurement was conceived in 1908 by a Viennese professor named Ludwig 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 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. 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 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. 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.

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.

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.

Various Rockwell scales
Scale Abbreviation Load Indenter Use
A RHA 60 kgf 120° diamond cone
B RHB 100 kgf 1/16 in diameter steel sphere Aluminium, brass, and soft steels
C RHC 150 kgf 120° diamond cone Harder steels
D RHD 100 kgf 120° diamond cone
E RHE 100 kgf 1/8 in diameter steel sphere
F RHF 60 kgf 1/16 in diameter steel sphere
G RHG 150 kgf 1/16 in diameter steel sphere
Also called a brale indenter

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 ceramics. The 15T scale employs a 15-kgf 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-hardened specimens.

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

See also

References

External links

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