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International harmonization of the Rockwell hardness scales is yet to occur.
This is due to several factors, the most significant being differences in the
testing cycles used by the National Metrology Institutes (NMIs) throughout
the world and, in the case of the diamond indenter scales, differences in
the performance of the national indenters used by the NMIs
(21). The need for international harmonization is well recognized, and there are efforts to
achieve this goal currently being made under the auspices of the International
Committee for Weights and Measures (CIPM) and, to some degree, by the
ISO and OIML.
9.2.1 BIPM and CIPM
The Bureau International des Poids et Mesures (BIPM) [International
Bureau of Weights and Measures] was set up by the Convention of the
Metre, a diplomatic treaty that was signed in 1875. Under the terms of the
Metre Convention, the BIPM is financed jointly by the Member States of
the Convention and operates under the exclusive supervision of the Comité
International des Poids et Mesures (CIPM) [International Committee of
Weights and Measures]. The BIPM headquarters is located in Sèvres, France,
near Paris.
The CIPM is made up of eighteen individuals, each from a different Member
State. Its mandate is to provide the basis for a single, coherent system of
measurements throughout the world, traceable to the International System
of Units (SI). This task takes many forms, from direct dissemination of units
(as in the case of mass and time) to coordination through international
comparisons (key comparisons) of national measurement standards (as in
length, electricity, radiometry, and ionizing radiation). It operates through a
series of Consultative Committees, whose members are the national metrology
laboratories of the Member States of the Convention, and through its own
laboratory work. The CIPM meets annually at the BIPM and discusses reports
presented by its Consultative Committees. Reports of the meetings of the
CIPM, and all the Consultative Committees, are published by the BIPM.
In 1998, a new ad-hoc working group was formed under the CIPM to
investigate the present state and needs for international comparisons of
hardness standards and report to the CIPM on the most appropriate platform
for the comparison, if it is really necessary. The working group was given the
name Ad-Hoc Working Group on Hardness (AHWGH) and was comprised
of members representing ISO, OIML, the International Measurement
Confederation (IMEKO), and National Metrology Institutes having a strong
standardization program. Since its inception, the group determined that
international comparisons of hardness standards are important and necessary.
Consequently, in October 1999, the working group was officially approved as
the Working Group on Hardness (WGH) and has been placed under the
Consultative Committee on Mass (CCM).
Current efforts by the WGH include the adoption of a recent world-wide
intercomparison of Rockwell hardness scales using a diamond indenter
as an international key comparison, the initiation of a study on the shape
measurement of diamond Rockwell indenters, and the initiation of a key
comparison of Vickers hardness. An example of the results of the worldwide
intercomparison(22)is shown in Figure 13 for the Rockwell C scale. The
participants in the comparison were national metrology institutes throughout
the world, including NIST. As the figure illustrates, there continues to be
significant differences between the world’s national hardness scales.
9.2.2 ISO
The ISO technical committee on hardness testing, ISO TC164/SC3, is
comprised of hardness experts representing their nations’ standards
organizations. The schedule for review and revision of test method standards
is usually every five years; however, the committee meets each year to
discuss changes and improvements to the hardness tests, based on the latest
technical information presented by the delegations. The latest revisions of
the Rockwell hardness test method standards ISO 6508-1(3), ISO 6508-2(4), and
ISO 6508-3(5)were published in 1999.
9.2.3 OIML
The current OIML Recommendations related to hardness testing are under
revision at this time. The Recommendation R39 (1981), concerning the
verification of Rockwell hardness machines will be the initial document to
be revised.
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