Rockwell Hardness Test

2.1 Significance of the Test

The Rockwell hardness test is an empirical indentation hardness test. Its worldwide adoption has likely resulted from the many advantages provided by the test method. The test is fast, inexpensive, and relatively non-destructive, leaving only a small indentation in the material. The simplicity in the operation of a Rockwell hardness machine has provided the added advantage that Rockwell hardness testing usually does not require a highly skilled operator. By way of correlation with other material properties, the Rockwell hardness test can provide important information about metallic materials, such as the tensile strength, wear resistance, and ductility. The test is generally useful for material selection, for process and quality control, and for acceptance testing of commercial products. Consequently, in today’s manufacturing facilities, Rockwell hardness machines can be found in use in almost every testing environment, from the hot, oily surroundings of some manufacturing facilities, to environmentally controlled metallographic and calibration laboratories.

2.2 Rockwell Indentation

Test Principle The Rockwell hardness test is one of several common indentation hardness tests used today, other examples being the Brinell hardness test and Vickers hardness test. Most indentation hardness tests are a measure of the deformation that occurs when the material under test is penetrated with a specific type of indenter. In the case of the Rockwell hardness test, two levels of force are applied to the indenter at specified rates and with specified dwell times, as illustrated for the Rockwell C scale (HRC) test in Figure 1. Unlike the Brinell and Vickers tests, where the size of the indentation is measured following the indentation process, the Rockwell hardness of the material is based on the difference in the depth of the indenter at two specific times during the testing cycle, indicated by the X marks in Figure 1. The value of hardness is calculated using a formula that was derived to yield a number falling within an arbitrarily defined range of numbers known as a Rockwell hardness scale. Because the hardness value is dependent on the definition of the test method, there are no alternative measurement systems to directly or independently measure Rockwell hardness, nor are there intrinsic artifacts to reference. The general Rockwell test procedure is the same regardless of the Rockwell scale or indenter being used. The indenter is brought into contact with the material to be tested, and a preliminary force (formally referred to as the minor load) is applied to the indenter. The preliminary force is usually held constant for a set period of time (dwell time), after which the depth of indentation is measured. After the measurement is made, an additional amount of force is applied at a set rate to increase the applied force to the total force level (formally referred to as the major load). The total force is held constant for a set time period, after which the additional force is removed, returning to the preliminary force level. After holding the preliminary force constant for a set time period, the depth of indentation is measured for a second time, followed by removal of the indenter from the test material. The measured difference between the first and second indentation depth measurements, h, (see Figure 1) is then used to calculate the Rockwell hardness number. For many older models of Rockwell hardness machines, the operator must manually control most or all of the steps of the test procedure. Many of today's newer machines automatically perform the entire Rockwell test.

2.3 Rockwell Hardness Scales

Many manufactured products are made of different types of metals and alloys varying in hardness, size, and thickness. To accommodate the testing of these diverse products, several different indenter types (as discussed in 3.3.8) were developed for the Rockwell test to be used in conjunction with a range of standard force levels. Each combination of indenter type and applied force levels has been designated as a distinct Rockwell hardness scale. The ASTM(2) defines thirty different Rockwell scales, as shown in Table 1. Rockwell hardness scales are divided into two categories: regular Rockwell scales and superficial Rockwell scales. Both categories of tests use the same types of indenters. The regular Rockwell scales employ the heavier force levels. For
these scales, the preliminary force level is 98.07 N (10 kgf), and the standard total force levels may be 588.4 N (60 kgf), 980.7 N (100 kgf) or 1471 N (150 kgf). The superficial Rockwell scales employ lighter force levels, typically for use on thinner materials. For the superficial Rockwell scales, the preliminary force level is 29.42 N (3 kgf), and the standard total force levels may be 147.1 N (15 kgf), 294.2 N (30 kgf) or 441.3 N (45 kgf). Table 1 provides typical applications for the different Rockwell scales as recommended by ASTM(2) and it lists the appropriate type of indenter and force levels to be used with the particular scale.

2.4 Rockwell Hardness Number

A Rockwell hardness measurement is reported as a Rockwell hardness number, without units. The Rockwell hardness number is calculated from the difference in the indentation depths before and after application of the
total force, while maintaining the preliminary test force. The difference in indentation depths is measured as h as described above. The calculation of the Rockwell hardness number is dependent on the specific combination of
indenter type and the forces that are used.

For scales that use a ball indenter, the Rockwell hardness number is calculated
from h (in mm) as:

Regular Rockwell Hardness=-130 0 002 h/mm
Rockwell Superficial Hardness=-100 0 001 h/mm

2.5 Test Method Standards

The Rockwell hardness test method is specified by several national and international standards. In North America, most Rockwell hardness testing is performed in accordance with standards published by the ASTM
(2) . In other countries throughout the world, industry testing may be in accordance with a nationally published standard, but increasingly, countries are adopting the ISO Rockwell hardness standards (3,4,5) The International Organization of Legal Metrology (OIML) publishes Rockwell hardness documents referred to as International Recommendations (6,7,8,9) for countries desiring to regulate Rockwell hardness testing for legal purposes. Presently, use of the OIML documents is very meager. Listed below are the document standards specifying
requirements for Rockwell hardness testing, as well as other documents related to Rockwell hardness testing.

2.5.1 ASTM

ASTM E 18 – 2000, Standard Test Methods for Rockwell Hardness and
Rockwell Superficial Hardness of Metallic Materials

Related ASTM standards:
ASTM E 110 – 82 (Reapproved 1997), Standard Test Method for
Indentation Hardness of Metallic Materials by Portable Hardness Testers
ASTM E 140 – 97, Standard Hardness Conversion Tables for Metals

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ASTM standards can be purchased in the Store area of the ASTM web site.
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2.5.2 ISO

ISO 6508-1 Metallic Materials – Rockwell hardness test (scales A, B, C, D,
E, F, G, H, K, N, T) – Part 1: Test method, 1999-09-01
ISO 6508-2 Metallic Materials – Rockwell hardness test (scales A, B, C, D,
E, F, G, H, K, N, T) – Part 2: Verification of testing machines, 1999-09-01
ISO 6508-3 Metallic Materials – Rockwell hardness test (scales A, B, C, D,
E, F, G, H, K, N, T) – Part 3: Calibration of reference blocks, 1999-09-01

To contact or order documents:

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Electronic copies of ISO standards may be purchased from ANSI’s Electronic
Standards Store at the American National Standards Institute web site. Paper
copies of ISO standards may be purchased from Global Engineering
Documents as follows:
Global Engineering Documents
Phone: 800–854–7179 or 303–397–7956
Fax: 303–397–2740
Email: global@ihs.com

2.5.3 OIML

OIML International Recommendation No. 11 (1974), Verification and
calibration of “Rockwell B" hardness standardized blocks

OIML International Recommendation No. 12 (1974), Verification and
calibration of “Rockwell C" hardness standardized blocks

OIML International Recommendation No. 36 (1976), Verification of
indenters for hardness testing machines (Systems: Brinell – Rockwell B, F,
and T – Vickers – Rockwell C, A, and N)

OIML International Recommendation No. 39 (1981), Verification of
hardness testing machines (Rockwell B, F, T - C, A, N systems

To contact or order documents:

OIML Publications may be purchased (in French and in English in most cases)
from the Organization’s Secretariat (BIML).
Bureau International de Métrologie Légale
11, rue Turgot
F-75009 Paris
Tel.: +33 (0) 1 48 78 12 82 and 42 85 27 11 Fax: +33 (0) 1 42 82 17 27
E-mail: biml@oiml.org

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