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Primary reference test blocks are standardized using primary standardizing
machines in accordance with the hardness definition. Usually, the National
Metrology Institute (NMI) of a country standardizes the primary reference test
blocks and maintains the national hardness scales. The National Metrology
Institute in the United States for Rockwell hardness is NIST.
4.1.1 NIST SRMs for the Rockwell C Scale
Each Rockwell hardness scale covers a range of hardness levels. To transfer
the U.S. national HRC scale values to industry requires more than one transfer
standard for the entire scale. However, production of hardness blocks at all
levels of HRC hardness is not feasible for NIST. It was determined that
industry needs test blocks at the levels specified in test method standards for
the calibration and verification of Rockwell hardness testing machines. For
the HRC scale, ASTM and ISO specify three ranges of hardness. The NIST
reference test blocks for the HRC scale reflect these ranges and are certified
at three hardness levels: 25 HRC, 45 HRC, and 63 HRC, which are available
for purchase as a Standard Reference Material (SRM
®2810, 2811, and 2812,respectively)(18)Currently, NIST offers Rockwell
hardness test blocks for only the Rockwell Cscale. Because the NIST SRMs
are primary transfer standards, greater care in the usage of the test blocks is
recommended than for commercial test blocks that are standardized by
secondary calibration laboratories. Annex C provides recommended
procedures for the use of NIST Rockwell hardness test block
SRMs. These recommended procedures may be used as well when using
secondary standards to help improve measurement accuracy.
As a consequence of the variation in hardness across a test block, NIST
determines and provides the customer with two types of hardness certifications
with each reference test block: (1) the certified average HRC hardness across
the test surface of the block; and (2) certified HRC hardness values at specific
untested locations on the test surface (19).
These two types of certifications
characterize the hardness of the test block in distinctly different ways.
4.1.1.1 Certification of the Average Surface Hardness
As discussed previously, the certified average hardness value of reference
test blocks is usually determined by calculating the simple average of several
hardness measurement values taken across the surface of the block. The
certification of the NIST SRM test blocks was partly based on the calibration
measurements; however, it was also based on a NIST derived function that
models how the hardness varies across the surface of the test block material.
In the case of the NIST blocks, the certified average hardness value is the
average of the hardness values predicted by the hardness function for all test
surface locations, and not simply the arithmetical average of the seven NIST
measurements. However, because the locations chosen for the seven NIST
measurements provide a good representation of the range in surface hardness,
the two averages are nearly identical in value.
4.1.1.2 Certification of Hardness at Untested Locations
A hardness measurement is destructive in that a specific location on a
hardness block can be measured only once. For the second type of
certification, certified HRC hardness values and the associated uncertainties
are provided for specific untested locations on the test surface of the reference
block, as illustrated by the open circles in Figure 7. Because hardness blocks
are not uniform, NIST can only predict the hardness at these untested
locations. The HRC hardness values were calculated using the surface
hardness function. Using this formula, predicted hardness values may be
calculated for any single untested location or for the average of two or more
locations. For the NIST SRMs, however, certified values of only eleven
locations are provided with the SRM test block. Section C.2 of Annex C
provides the formulas used by NIST and gives examples of how to use these
formulas.
This second type of NIST hardness certification provides the customer with a
valuable tool for improving the hardness comparison measurements by reducing
the influence of the test block non-uniformity. For example, the user can
combine the seven NIST calibration measurements with the eleven predicted
values to produce a profile map approximating the block surface hardness, and
then, correct measurements depending on the test location on the block. The
customer can also calculate better corrections by calculating hardness values
using the same formula NIST used to determine the hardness at the eleven
untested locations, as described in Annex C.
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