Rockwell hardness test
Hardness scale
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Through Hardness, Engineering and Metallurgy
When testing metals, indentation hardness correlates linearly with tensile strength.
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, 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. The requirement for this tester was to quickly determine the effects of heat treatment on steel bearing races. The application was approved on February 11, 1919, for 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. 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 held U.S. 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 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.
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Models and operation
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 , 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:
- 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).
- 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.
- 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 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:
- 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.)
- Axes: 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 specimens.
Standards
- International (ISO)
- 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 E18: Standard Test Methods for Rockwell Hardness of Metallic Materials