9.1 United States
The past few years have seen significant changes to Rockwell testing in the United States with the introduction of NIST Rockwell standards, accreditation of Rockwell calibration laboratories, and the increased need to determine measurement uncertainty. Although the changes may not yet have impacted many users of Rockwell hardness, these and other proposed changes should soon affect every level of testing and will hopefully improve the accuracy and consistency of Rockwell measurements throughout the nation’s industries.
9.1.1 NIST Standards
In 1991, NIST began the development of a national Rockwell hardness standardization laboratory at the urging of the ASTM and U.S. industry.
The goals of this program are to standardize the Rockwell hardness scales for the United States and to provide industry with stable national transfer standards in the form of reference test blocks. In standardizing the Rockwell scales, NIST has employed instruments and procedures having the highest metrological accuracy as practicable.
In June 1998, NIST released the first Rockwell hardness reference test block standards for sale to industry. These blocks are for the HRC scale at three hardness levels, nominally 25 HRC, 45 HRC, and 63 HRC. The blocks are available to anyone wishing to purchase them; however, they are primarily intended for use by the secondary Rockwell hardness calibration laboratories.
A significant result of the NIST standardization of the HRC scale is that the hardness levels of the NIST HRC scale deviated from the HRC scale used by U.S. industry at that time. The magnitude of the deviation varies by hardness level and also depends on which calibration agency’s reference test blocks had been used previously. Figure12 demonstrates the general trend of the difference between NIST and U.S. industry HRC scales. The offset trend given in Figure 12 should not be used as an absolute offset. The relationship could possibly differ by as much as ± 0.5 Rockwell points; however, it is generally true that the greatest offset is at the high end of the scale. Also of interest is that the NIST HRC scale is in good agreement with other countries worldwide (see 9.2 below).
The next Rockwell scale for which NIST will release test blocks is the Rockwell B scale, likely followed by the HRA, HRN, and HRT scales. Eventually NIST hopes to provide a means for traceability to all Rockwell scales.
9.1.2 ASTM Test Method Standards
The ASTM Subcommittee E28.06 on Indentation Hardness Testing is always striving to improve the Rockwell hardness test. Subcommittee members are from industry and government, and they include manufacturers and users of Rockwell hardness equipment. Much of the effort to improve the Rockwell test has been through the requirements of the ASTM E18 Rockwell hardness test method(2). A significant revision was recently made to the standard requiring that performance verifications of Rockwell hardness indenters and hardness machines must be made using test blocks calibrated traceable to the Rockwell standards maintained by NIST. This can be accomplished through the use of commercial test blocks calibrated traceable to the NIST standard or by directly using the NIST SRMs. The new requirement will apply only to the Rockwell scale(s) for which NIST supplies primary reference test blocks. As NIST develops new SRMs for other Rockwell hardness scales, the same requirement will apply for those scales.
Currently, the ASTM hardness subcommittee also is developing a major revision of the E18 Rockwell hardness standard. The intention of the revision is to improve E18 by: clearly specifying when traceability is achieved; clarifying requirements and procedures; revising procedures to reflect current practice; and adding requirements and procedures to improve the Rockwell hardness test method.
The increasing need by industry to report uncertainties has led the ASTM hardness subcommittee to initiate the development of a general procedure for determining uncertainty in Rockwell hardness measurements. The procedure is being developed to assist hardness standardization, calibration, and verification laboratories by providing a basic approach to evaluating their uncertainty in order to simplify and unify the interpretation of uncertainty by users of Rockwell hardness.
9.1.3 Hardness Industry
Most U.S. secondary laboratories engaged in the manufacture and calibration of Rockwell hardness equipment are now producing Rockwell HRC test blocks and diamond indenters that are certified traceable to the NIST HRC reference test block standards. This has resulted in some users having to obtain a new Rockwell diamond indenter in order for their hardness machine to pass indirect verification of the HRC scale using NIST traceable HRC test blocks.
An increasing number of domestic and international customers of calibration and testing agencies are requiring that calibrations and measurements made by these agencies be traceable to national reference standards when possible, and, in many cases, that the laboratories be accredited to perform these measurements. This applies to the Rockwell hardness industry as well. Consequently, commercial and governmental programs have been developed for accrediting laboratories engaged in Rockwell hardness testing and calibrations.