Establishing a world-wide unified Rockwell hardness scale with metrological traceability

Abstract

Recently developed microform m easurem ent techniques have reduced the measurement uncertainties in the geom etry of Rockwell diamond indenters. It is now possible to establish standard-grade Rockwell diamond indenters characterized by high geometric uniform ity, high hardness performance uniformity, interchangeability and reproducibility. By using the standard indenters under differen t national standard machines and a standardized testin g cycle, a world-wide uni® ed Rockwell hardness scale could be established with m etrological traceability , stability and reproducibility. Geometric measurements and hardness tests in ® ve laboratories have shown that tightly controlled indenter geom etry can signi® cantly im prove the consistency of Rockwell C hardness (HRC) measurements. These results support the feasibility of establishing a world-wide uni® ed HRC scale with an expanded uncertainty of approxim ately 6 0,2 HRC and without signi® cant bias with respect to an ideal scale.

Effect of the Spherical Indenter Tip Assumption on the Initial Plastic Yield Stress

“…At the larger micrometer to millimeter scale of Rockwell hardness indenters, for example, NIST has played a leading role in the drive toward indenter standardization (Song et al, 1997). Direct metrology of indenter geometry, using well-calibrated stylus profilers, is the most effective method at these size scales, and it is able to provide uncertainties low enough to support the uncertainty goals in Rockwell hardness measurements themselves.…”

Length and dimensional measurements at NIST

“…The work is aimed at verifying the geometric correctness of the Rockwell indenters as an alternative to hardness performance comparisons. NIST standard indenters, combined with the use of the NIST standard testing machine and a standardized testing cycle, are being used to create, maintain, and reproduce the metrology-based Rockwell hardness scale in the United States and overcome any errors that might exist in the European Community’s performance-based HRC scale [ 56 ].…”

NIST recommended practice guide :

“…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 Use of NIST Rockwell C Scale SRM Test Blocks A complete certified value involves computation of three quantities: ( 1 ) the average hardness value itself, denoted by Jj \ (2) the standard uncertainty for test block uniformity and repeatability, <s…”