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The Rockwell hardness test is a
hardness In materials science, hardness (antonym: softness) is a measure of the resistance to plastic deformation, such as an indentation (over an area) or a scratch (linear), induced mechanically either by Pressing (metalworking), pressing or abrasion ...
test based on
indentation hardness 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 o ...
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). 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. Larger numbers correspond to harder materials. When testing metals, indentation hardness correlates linearly with
tensile strength Ultimate tensile strength (also called UTS, tensile strength, TS, ultimate strength or F_\text in notation) is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials, the ultimate ...
.


History

The differential depth hardness measurement was conceived in 1908 by Viennese professor Paul Ludwik in his book ''Die Kegelprobe'' (crudely, "the cone test"). 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 The term hardened steel is often used for a plain-carbon steel, medium or high carbon steel that has been given heat treatment and then quenching followed by tempering (metallurgy), tempering. The quenching results in the formation of metastable ma ...
, and left too large an impression to be considered nondestructive. Hugh M. Rockwell (1890–1957) and Stanley P. Rockwell (1886–1940) from
Connecticut Connecticut ( ) is a U.S. state, state in the New England region of the Northeastern United States. It borders Rhode Island to the east, Massachusetts to the north, New York (state), New York to the west, and Long Island Sound to the south. ...
in the
United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...
co-invented the "Rockwell hardness tester," a differential-depth machine. They applied for a patent on July 15, 1914. 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 . At the time of invention, both Hugh and Stanley Rockwell worked for the New Departure Manufacturing Co. of Bristol, CT. 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 .S. P. Rockwell, "Hardness-Testing Machine", , Nov 1924. 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 operated until 2012. The building, which still stands, was empty in 2016. The later-named Wilson Mechanical Instrument Company has changed ownership over the years, and was acquired by Instron Corp. in 1993.


Models and operation

Force diagram of Rockwell test upA closeup of the indenter and anvil on a Rockwell-type hardness tester The Rockwell hardness test can be conducted on different types of hardness testers. Benchtop hardness testers can be found either in a digital or analog model. Digital models utilize a digital display whereas the analog models display results on a dial on the machine. Other testers are portable. 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, on which a harder material gives a lower measure. That is, the penetration depth and hardness are inversely proportional. The Rockwell test does not use any optical equipment to measure the hardness indention, rather all calculations are done within the machine to measure the indention in the specimen. The equation for Rockwell hardness is HR = N-h*d, 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 Engineering is the practice of using natural science, mathematics, and the engineering design process to Problem solving#Engineering, solve problems within technology, increase efficiency and productivity, and improve Systems engineering, s ...
and
metallurgy Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are known as alloys. Metallurgy encompasses both the ...
. Its commercial popularity arises from its speed, reliability, robustness, resolution and small area of indentation. Legacy Rockwell hardness testers operation steps: # Load an initial force: Rockwell hardness test initial test force is ; superficial Rockwell hardness test initial test force is . # Load main load: reference below form / table 'Scales and values'. # Leave the main load for a "dwell time" sufficient for indentation to come to a halt. # Release load; the Rockwell value will typically display on a dial or screen automatically. In order to get a reliable reading the thickness of the test-piece should be at least 10 times the depth of the indentation. 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.


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 Dimensionless quantities, or quantities of dimension one, are quantities implicitly defined in a manner that prevents their aggregation into unit of measurement, units of measurement. ISBN 978-92-822-2272-0. Typically expressed as ratios that a ...
. * Except for testing thin materials in accordance with A623, the steel indenter balls have been replaced by tungsten carbide balls of the varying diameters. When a ball indenter is used, the letter "W" is used to indicate a tungsten carbide ball was used, and the letter "S" indicates the use of a steel ball. E.g.: 70 HRBW indicates the reading was 70 in the Rockwell B scale using a tungsten carbide 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
ceramic A ceramic is any of the various hard, brittle, heat-resistant, and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcela ...
s. The 15T scale employs a 15-kgf load on a hardened steel ball, and can be used on
sheet metal Sheet metal is metal formed into thin, flat pieces, usually by an industrial process. Thicknesses can vary significantly; extremely thin sheets are considered foil (metal), foil or Metal leaf, leaf, and pieces thicker than 6 mm (0.25  ...
. 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: * Very hard steel (e.g. chisels, quality knife blades): HRC 55–66 (Hardened High Speed Carbon and Tool Steels such as M2, W2, O1, CPM-M4, and D2, as well as many of the newer powder metallurgy Stainless Steels such as CPM-S30V, CPM-154, ZDP-189. There are alloys that hold a HRC upwards 68-70, such as the Hitachi developed HAP72. These are extremely hard, but also somewhat brittle.) *
Axe An axe (; sometimes spelled ax in American English; American and British English spelling differences#Miscellaneous spelling differences, see spelling differences) is an implement that has been used for thousands of years to shape, split, a ...
s: about HRC 45–55 * Brass: HRB 55 (Low brass, UNS C24000, H01 Temper) to HRB 93 (Cartridge Brass, UNS C26000 (260 Brass), H10 Temper) Several other scales, including the extensive A-scale, are used for specialized applications. There are special scales for measuring
case-hardened Case-hardening or carburization is the process of introducing carbon to the surface of a low-carbon iron, or more commonly a low-carbon steel object, in order to Hardened steel, harden the surface. Iron which has a carbon content greater than ~ ...
specimens.


Standards

* International (
ISO The International Organization for Standardization (ISO ; ; ) is an independent, non-governmental, international standard development organization composed of representatives from the national standards organizations of member countries. Me ...
) ** ISO 6508-1: Metallic materials—Rockwell hardness test—Part 1: Test method ** ISO 6508-2: Metallic materials—Rockwell hardness test—Part 2: Verification and calibration of testing machines and indenters ** ISO 6508-3: Metallic materials—Rockwell hardness test—Part 3: Calibration of reference blocks ** ISO 2039-2: Plastics—Determination of hardness—Part 2: Rockwell hardness * US standard (
ASTM International ASTM International, formerly known as American Society for Testing and Materials, is a standards organization that develops and publishes voluntary consensus technical international standards for a wide range of materials, products, systems and s ...
) ** ASTM E18: Standard Test Methods for Rockwell Hardness of Metallic Materials


See also

*
Brinell hardness test The Brinell hardness test (pronounced /brəˈnɛl/) measures the indentation hardness of materials. It determines hardness through the scale of penetration of an indenter, loaded on a material test-piece. It is one of several definitions of hard ...
*
Hardness comparison 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 co ...
*
Knoop hardness test 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 ...
*
Leeb rebound hardness test 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 (mainly above 1 kg). It m ...
*
Meyer hardness test The Meyer hardness test is a hardness test based upon projected area of an impression. The hardness, H, is defined as the maximum load, P_\text divided by the projected area of the indent, A_\text. :H=\frac . This is a more fundamental measurem ...
*
Mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2011): Mi ...
* Mohs scale of hardness * Shore durometer *
Tensile strength Ultimate tensile strength (also called UTS, tensile strength, TS, ultimate strength or F_\text in notation) is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials, the ultimate ...
*
Vickers 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 scale, Brinell method to measure the hardness of materials. The Vickers test is often easier to use than ot ...


References


External links


Video on the Rockwell hardness test

Hardness Conversion Chart



Hardness Conversion Table

Rockwell Hardness Testing
{{Authority control Dimensionless numbers of physics Hardness tests