Hardness Standardization

Our objective is to standardize and improve hardness measurement both in the U.S. and internationally. NIST is the U.S. National Metrology Institute (NMI) for hardness, and as such, is responsible for traceability in hardness measurements.

• Hardness is the most commonly used test in industry for quality control and acceptance testing of metals and metallic products.

• Of over 12 000 ASTM standards, hardness test method standards are among the most downloaded.

• NIST produces a variety of hardness Standard Reference Materials (SRMs) for industry.

• The NIST hardness standardization project impacts customers at all levels. Impact is evidenced through the interactions with the International Bureau of Weights and Measures (BIPM), during the development of international test method standards, by those performing hardness research, by those involved in laboratory accreditation, through the calibration of reference standards, by those involved in industrial hardness measurements, and through organized hardness conferences and workshops.

As the U.S. NMI for hardness, NIST’s role is to improve hardness measurement at all levels of traceability. The hardness traceability structure begins with the Consultative Committee for Mass and Related Quantities (CCM) Working Group on Hardness (WGH) under the umbrella of the International Committee for Weights and Measures (CIPM). NIST represents the U.S. in this committee by providing input in the development of the hardness definitions. The hardness definitions are the standard to which NMIs perform hardness measurements, although differences continue to exist between NMIs. NIST participates in worldwide comparisons between NMIs, and interacts bilaterally with the standardizing laboratories of other countries.

NIST transfers the U.S. national hardness scales to industry by way of hardness reference block SRMs sold primarily to secondary calibration laboratories, and takes additional steps to ensure that the hardness SRMs are used effectively, through participation in laboratory accreditations.  

Almost all industrial hardness measurements, whether made at calibration laboratories or industrial facilities, follow the test method procedures developed by Standard Development Organizations (SDOs) such as ASTM-International and the International Organization for Standardization (ISO). Through our participation and leadership in these hardness committees, NIST is able to communicate its expertise, research and experience.

The National Institute of Standards and Technology, formerly the National Bureau of Standards (NBS), has been involved with the measurement of hardness for decades. In 1937, for example, Frederick Knoop developed the Knoop hardness test at NBS, an important micro-indentation test still used today. However, an expansive NIST effort in hardness standardization didn't begin until the mid 1990s with the initiation of a Rockwell hardness standardization project, which had been requested by ASTM. The importance of this project increased with the creation of the WGH within, acknowledging hardness as one of the world’s important measurement quantities.

 In the past decade, the hardness standardization project has had a tremendous impact on hardness testing within the U.S. For example, in 1998, the U.S. Rockwell C scale (HRC), used for testing steel, was corrected to bring it in line with the rest of the world as a result of the release of SRMs for the HRC scale.

The development of HRC SRMs was made possible due to our collaboration with the NIST Precision Engineering Division’s facility to characterize indenters. In 2001, NIST published a Recommended Practice Guide on Rockwell Hardness Measurement. To date, about 7000 copies have been requested and distributed; many through hardness machine and test block manufacturers. Today, the NIST hardness standardization effort includes participation in the international effort within the WGH to better define the hardness scales and to reduce measurement differences between NMIs. In recent years, the WGH has been developing a revised definition for the Rockwell C scale, the most important industrial hardness scale. Previously, where test parameters have been defined with tolerance ranges, the new definition will define absolute values for these parameters.

NIST participates in the development of international hardness test method standards at ASTM and ISO. As Chair of the ASTM hardness committee, NIST led a 10 year effort to completely revise the Rockwell hardness (E18) and Brinell hardness (E10) standards to clarify and improve requirements. These proposed revisions were approved by the ASTM membership in 2007.

Currently, NIST produces hardness test blocks as SRMs for a variety of Vickers, Knoop and Rockwell hardness scales and hardness levels. The SRMs are used by industry to calibrate their hardness machines traceable to NIST. Two new instruments have been added that will support on-going development of a new Rockwell diamond indenter SRM and an SRM for the calibration of Brinell hardness indentation measuring systems.

NIST’s research efforts in recent years have included investigations of the influence of important parameters (e.g., force errors, indenter shape, loading rates, etc.) for each of the hardness scales. NIST’s modeling of the effect of different ball materials on indentation recently helped ASTM decide to specify tungsten-carbide as the standard Rockwell ball indenter.