Rockwell scale

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A Rockwell hardness tester Rockwell hardness tester 001.jpg
A Rockwell hardness tester

The Rockwell scale is a hardness scale based on indentation hardness of a material. The Rockwell test measures the depth of penetration of an indenter under a large load (major load) compared to the penetration made by a preload (minor load). [1] There are different scales, denoted by a single letter, that use different loads or indenters. The result is a dimensionless number noted as HRA, HRB, HRC, etc., where the last letter is the respective Rockwell scale. Higher numbers correspond to harder materials.

Contents

When testing metals, indentation hardness correlates linearly with tensile strength. [2]

History

The differential depth hardness measurement was conceived in 1908 by Viennese professor Paul Ludwik in his book Die Kegelprobe (crudely, "the cone test"). [3] The differential-depth method subtracted out the errors associated with the mechanical imperfections of the system, such as backlash and surface imperfections. The Brinell hardness test, invented in Sweden, was developed earlier – in 1900 – but it was slow, not useful on fully hardened steel, and left too large an impression to be considered nondestructive.

Hugh M. Rockwell (1890–1957) and Stanley P. Rockwell (1886–1940) from Connecticut in the United States co-invented the "Rockwell hardness tester," a differential-depth machine. They applied for a patent on July 15, 1914. [4] The requirement for this tester was to quickly determine the effects of heat treatment on steel bearing races. The application was subsequently approved on February 11, 1919, and holds U.S. Patent 1,294,171 . At the time of invention, both Hugh and Stanley Rockwell worked for the New Departure Manufacturing Co. of Bristol, CT. [5] New Departure was a major ball bearing manufacturer which 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 September 11, 1919, which was approved on November 18, 1924. The new tester holds U.S. Patent 1,516,207 . [6] [7] Rockwell moved to West Hartford, CT, and made an additional improvement in 1921. [7] Stanley collaborated with instrument manufacturer Charles H. Wilson of the Wilson-Mauelen Company in 1920 to commercialize his invention and develop standardized testing machines. [8] 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 acquired by Instron Corp. in 1993. [9]

Models and operation

Force diagram of Rockwell test Durete rockwell.svg
Force diagram of Rockwell test
A closeup of the indenter and anvil on a Rockwell-type hardness tester Rockwell hardness tester closeup 001.jpg
A closeup of the indenter and anvil on a Rockwell-type hardness tester

The Rockwell hardness test can be conducted on several various hardness testers. [10] [11] All testers, however, fall under one of three categories. Bench model hardness testers can be found either in a digital or analog model. Digital bench models utilize a digital display and typically take more technical training to be able to operate, whereas the analog models are simpler to operate as well as very accurate and display results on a dial on the front of the machine. All bench model testers are usually found within a workshop or laboratory setting. Other testers are portable, and all portable testers will come in a digital model including a digital results screen similar to that of the bench digital model. Portable testers are practical and easy to use.[ citation needed ]

The determination of the Rockwell hardness of a material involves the application of a minor load followed by a major load. The minor load establishes the zero position. The major load is applied, then removed while still maintaining the minor load. The depth of penetration from the zero datum is measured from a dial, on which a harder material gives a lower measure. That is, the penetration depth and hardness are inversely proportional. The chief advantage of Rockwell hardness is its ability to display hardness values directly, thus obviating tedious calculations involved in other hardness measurement techniques.

The Rockwell test is very cost-effective as it does not use any optical equipment to measure the hardness based on the small indention made, rather all calculations are done within the machine to measure the indention in the specimen, providing a clear result in a manner in which is easy to read and understand once given. This also prevents any reworking or finishing needing to be done to the specimen both before and after testing. However, it is critical to double check specimens as the smallest indentions made from testing could potentially result in incorrect measurements in hardness, leading to catastrophe. After time, the indenter on a Rockwell scale can become inaccurate as well and need replacing to ensure accurate and precise hardness measurements. [12]

The equation for Rockwell Hardness is , where d is the depth in mm (from the zero load point), and N and h are scale factors that depend on the scale of the test being used (see following section).

It is typically used in engineering and metallurgy. Its commercial popularity arises from its speed, reliability, robustness, resolution and small area of indentation.

Legacy Rockwell hardness testers operation steps:

  1. Load an initial force: Rockwell hardness test initial test force is 10 kgf (98 N; 22 lbf); superficial Rockwell hardness test initial test force is 3 kgf (29 N; 6.6 lbf).
  2. Load main load: reference below form / table 'Scales and values'.
  3. Leave the main load for a "dwell time" sufficient for indentation to come to a halt.
  4. Release load; the Rockwell value will typically display on a dial or screen automatically. [13]

In order to get a reliable reading the thickness of the test-piece should be at least 10 times the depth of the indentation. [14] Also, readings should be taken from a flat perpendicular surface, because convex surfaces give lower readings. A correction factor can be used if the hardness of a convex surface is to be measured. [15]

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 [16] [17] [18]
ScaleAbbreviation§Major Load* (kgf)IndenterUseNh
AHRA60spheroconical diamond† Cemented carbides, thin steel, shallow case-hardened steel100500
BHRB1001⁄16 in (1.59 mm) ballCopper alloys, soft steels, aluminum alloys, malleable iron130500
CHRC150spheroconical diamond†Steel, hard cast irons, pearlitic malleable iron, titanium, deep case-hardened steel, other materials harder than 100 HRB100500
DHRD100spheroconical diamond†Thin steel and medium case-hardened steel and pearlitic malleable iron100500
EHRE1001⁄8 in (3.18 mm) ballCast iron, aluminum and magnesium alloys, bearing metals, thermoset plastics130500
FHRF601⁄16 in (1.59 mm) ballAnnealed copper alloy, thin soft sheet metals130500
GHRG1501⁄16 in (1.59 mm) ballPhosphor bronze, beryllium copper, malleable irons.130500
HHRH601⁄8 in (3.18 mm) ballAluminum, Zinc, Lead [19] 130500
KHRK1501⁄8 in (3.18 mm) ballBearing alloy, tin, hard plastic materials [19] 130500
LHRL601⁄4 in (6.35 mm) ballBearing metals and other very soft or thin materials.130500
MHRM1001⁄4 in (6.35 mm) ballThermoplastics, bearing metals and other very soft or thin materials130500
PHRP1501⁄4 in (6.35 mm) ballBearing metals and other very soft or thin materials130500
RHRR601⁄2 in (12.70 mm) ballThermoplastics, bearing metals, and other very soft or thin materials130500
SHRS1001⁄2 in (12.70 mm) ballBearing metals and other very soft or thin materials130500
VHRV1501⁄2 in (12.70 mm) ballBearing metals and other very soft or thin materials130500
15T, 30T, 45T15, 30, 451⁄16 in (1.59 mm) ballSuperficial: for soft coatings1001000
15N, 30N, 45N15, 30, 45spheroconical diamond†Superficial: for case-hardened materials1001000
* Except for the superficial scales where it is 3 kgf, the minor load is 10 kgf.
†Also called a Brale indenter, is made with a conical diamond of 120° ± 0.35° included angle and a tip radius of 0.200 ± 0.010 mm.
§The Rockwell number precedes the scale abbreviations (e.g., 60 HRC), except for the "Superficial scales" where they follow the abbreviations, separated by a ‘-’ (e.g., 30N-25).

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 (1.588 mm) hardened steel ball, and can be used on sheet metal.

The B and C scales overlap, such that readings below HRC 20 and those above HRB 100, generally considered unreliable, need not be taken or specified.

Typical values include:

Several other scales, including the extensive A-scale, are used for specialized applications. There are special scales for measuring case-hardened specimens.

Standards

See also

Related Research Articles

An operational definition specifies concrete, replicable procedures designed to represent a construct. In the words of American psychologist S.S. Stevens (1935), "An operation is the performance which we execute in order to make known a concept." For example, an operational definition of "fear" often includes measurable physiologic responses that occur in response to a perceived threat. Thus, "fear" might be operationally defined as specified changes in heart rate, galvanic skin response, pupil dilation, and blood pressure.

<span class="mw-page-title-main">Brinell scale</span> Brinell scale of hardness

The Brinell scale characterizes the indentation hardness of materials through the scale of penetration of an indenter, loaded on a material test-piece. It is one of several definitions of hardness in materials science.

<span class="mw-page-title-main">Vickers hardness test</span> Hardness test

The Vickers hardness test was developed in 1921 by Robert L. Smith and George E. Sandland at Vickers Ltd as an alternative to the Brinell method to measure the hardness of materials. The Vickers test is often easier to use than other hardness tests since the required calculations are independent of the size of the indenter, and the indenter can be used for all materials irrespective of hardness. The basic principle, as with all common measures of hardness, is to observe a material's ability to resist plastic deformation from a standard source. The Vickers test can be used for all metals and has one of the widest scales among hardness tests. The unit of hardness given by the test is known as the Vickers Pyramid Number (HV) or Diamond Pyramid Hardness (DPH). The hardness number can be converted into units of pascals, but should not be confused with pressure, which uses the same units. The hardness number is determined by the load over the surface area of the indentation and not the area normal to the force, and is therefore not pressure.

Indentation hardness tests are used in mechanical engineering to determine the hardness of a material to deformation. Several such tests exist, wherein the examined material is indented until an impression is formed; these tests can be performed on a macroscopic or microscopic scale.

<span class="mw-page-title-main">Knoop hardness test</span> OnePlus OS

The Knoop hardness test is a microhardness test – a test for mechanical hardness used particularly for very brittle materials or thin sheets, where only a small indentation may be made for testing purposes. A pyramidal diamond point is pressed into the polished surface of the test material with a known load, for a specified dwell time, and the resulting indentation is measured using a microscope. The geometry of this indenter is an extended pyramid with the length to width ratio being 7:1 and respective face angles are 172 degrees for the long edge and 130 degrees for the short edge. The depth of the indentation can be approximated as 1/30 of the long dimension. The Knoop hardness HK or KHN is then given by the formula:

In materials science, hardness is a measure of the resistance to localized plastic deformation induced by either mechanical indentation or abrasion. In general, different materials differ in their hardness; for example hard metals such as titanium and beryllium are harder than soft metals such as sodium and metallic tin, or wood and common plastics. Macroscopic hardness is generally characterized by strong intermolecular bonds, but the behavior of solid materials under force is complex; therefore, hardness can be measured in different ways, such as scratch hardness, indentation hardness, and rebound hardness. Hardness is dependent on ductility, elastic stiffness, plasticity, strain, strength, toughness, viscoelasticity, and viscosity. Common examples of hard matter are ceramics, concrete, certain metals, and superhard materials, which can be contrasted with soft matter.

<span class="mw-page-title-main">Johan August Brinell</span> Swedish metallurgist (1849–1925)

August Brinell was a Swedish metallurgical engineer.

Nanoindentation, also called instrumented indentation testing, is a variety of indentation hardness tests applied to small volumes. Indentation is perhaps the most commonly applied means of testing the mechanical properties of materials. The nanoindentation technique was developed in the mid-1970s to measure the hardness of small volumes of material.

<span class="mw-page-title-main">Shore durometer</span>

The Shore durometer is a device for measuring the hardness of a material, typically of polymers.

A variety of hardness-testing methods are available, including the Vickers, Brinell, Rockwell, Meyer and Leeb tests. Although it is impossible in many cases to give an exact conversion, it is possible to give an approximate material-specific comparison table for steels.

<span class="mw-page-title-main">Ceramography</span> Preparation and study of ceramics with optical instruments

Ceramography is the art and science of preparation, examination and evaluation of ceramic microstructures. Ceramography can be thought of as the metallography of ceramics. The microstructure is the structure level of approximately 0.1 to 100 µm, between the minimum wavelength of visible light and the resolution limit of the naked eye. The microstructure includes most grains, secondary phases, grain boundaries, pores, micro-cracks and hardness microindentions. Most bulk mechanical, optical, thermal, electrical and magnetic properties are significantly affected by the microstructure. The fabrication method and process conditions are generally indicated by the microstructure. The root cause of many ceramic failures is evident in the microstructure. Ceramography is part of the broader field of materialography, which includes all the microscopic techniques of material analysis, such as metallography, petrography and plastography. Ceramography is usually reserved for high-performance ceramics for industrial applications, such as 85–99.9% alumina (Al2O3) in Fig. 1, zirconia (ZrO2), silicon carbide (SiC), silicon nitride (Si3N4), and ceramic-matrix composites. It is seldom used on whiteware ceramics such as sanitaryware, wall tiles and dishware.

The Barcol hardness test characterizes the indentation hardness of materials through the depth of penetration of an indentor, loaded on a material sample and compared to the penetration in a reference material. The method is most often used for composite materials such as reinforced thermosetting resins or to determine how much a resin or plastic has cured. The test complements the measurement of glass transition temperature, as an indirect measure of the degree of cure of a composite. It is inexpensive and quick, and provides information on the cure throughout a part.

Brinelling is the permanent indentation of a hard surface. It is named after the Brinell scale of hardness, in which a small ball is pushed against a hard surface at a preset level of force, and the depth and diameter of the mark indicates the Brinell hardness of the surface. Brinelling is permanent plastic deformation of a surface, and usually occurs while two surfaces in contact are stationary and the material yield strength has been exceeded.

<span class="mw-page-title-main">Meyer hardness test</span>

The Meyer hardness test is a hardness test based upon projected area of an impression. The hardness, , is defined as the maximum load, divided by the projected area of the indent, .

Meyer's law is an empirical relation between the size of a hardness test indentation and the load required to leave the indentation. The formula was devised by Eugene Meyer of the Materials Testing Laboratory at the Imperial School of Technology, Charlottenburg, Germany, circa 1908.

<span class="mw-page-title-main">Nanoindenter</span>

A nanoindenter is the main component for indentation hardness tests used in nanoindentation. Since the mid-1970s nanoindentation has become the primary method for measuring and testing very small volumes of mechanical properties. Nanoindentation, also called depth sensing indentation or instrumented indentation, gained popularity with the development of machines that could record small load and displacement with high accuracy and precision. The load displacement data can be used to determine modulus of elasticity, hardness, yield strength, fracture toughness, scratch hardness and wear properties.

The Leeb Rebound Hardness Test (LRHT) invented by Swiss company Proceq SA is one of the four most used methods for testing metal hardness. This portable method is mainly used for testing sufficiently large workpieces.

The Korsunsky work-of-indentation approach is a method of extracting values of hardness and stiffness for a small volume of material from indentation test data.

Indentation plastometry is the idea of using an indentation-based procedure to obtain (bulk) mechanical properties in the form of stress-strain relationships in the plastic regime. Since indentation is a much easier and more convenient procedure than conventional tensile testing, with far greater potential for mapping of spatial variations, this is an attractive concept.

References

  1. ↑ E.L. Tobolski & A. Fee, "Macroindentation Hardness Testing," ASM Handbook, Volume 8: Mechanical Testing and Evaluation, ASM International, 2000, pp. 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," U.S. Patent 1,294,171 , Feb 1919.
  5. ↑ S.W. Kallee: Stanley Pickett Rockwell Stanley Pickett Rockwell - One of the Inventors of the Rockwell Hardness Testing Machine]. Retrieved on 21 November 2018.
  6. ↑ S.P. Rockwell, "The Testing of Metals for Hardness, Transactions of the American Society for Steel Treating, Vol. II, No. 11, August 1922, pp. 1013–1033.
  7. 1 2 S. P. Rockwell, "Hardness-Testing Machine", U.S. Patent 1,516,207 , Nov 1924.
  8. ↑ V.E. Lysaght, Indentation Hardness Testing, Reinhold Publishing Corp., 1949, pp. 57–62.
  9. ↑ R.E. Chinn, "Hardness, Bearings, and the Rockwells," Advanced Materials & Processes, Vol 167 #10, October 2009, p 29-31.
  10. ↑ "Rockwell Hardness - an overview | ScienceDirect Topics".
  11. ↑ "Rockwell Test - an overview | ScienceDirect Topics".
  12. ↑ Hardness Tester, JM (17 April 2019). "Rockwell Hardness Testing: The Ultimate Guide". JM Hardness Tester. Retrieved 21 September 2021.
  13. ↑ "Hardness tester, metallographic microscope, surface roughness tester – EBPU". Hardnesstesting-machine.com. Retrieved 18 February 2022.
  14. ↑ Fundamentals of Rockwell Hardness Testing, archived from the original on 2010-01-29, retrieved 2010-09-10
  15. ↑ PMPA's Designer's Guide: Heat treatment, archived from the original on 2009-07-14, retrieved 2009-06-19.
  16. ↑ Smith, William F.; Hashemi, Javad (2001), Foundations of Material Science and Engineering (4th ed.), McGraw-Hill, p. 229, ISBN   0-07-295358-6
  17. ↑ Sundararajan, G.; Roy, M. (2001). Encyclopedia of Materials: Science and Technology. Hardness Testing: Elsevier Ltd. pp. 3728–3736. ISBN   978-0-08-043152-9.
  18. ↑ Broitman, Esteban (2017). "Indentation Hardness Measurements at Macro-, Micro-, and Nanoscale: A Critical Overview". Tribology Letters. 65 (23): 4–5. doi: 10.1007/s11249-016-0805-5 . S2CID   20603457.
  19. 1 2 EBP company R-150T Rockwell hardness tester manual book.
  20. ↑ E18-08b Section 5.1.2.1 & 5.2.3
  21. ↑ "Knife blade materials". 31 May 2008. Archived from the original on 2008-05-31. Retrieved 18 February 2022.
  22. ↑ "MatWeb, Your Source for Materials Information". Matweb.com. Retrieved 2010-06-23.
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