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.
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.
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.