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8.1 Traceability
Traceability is defined by the International Vocabulary of Basic and General
Terms in Metrology (VIM)(20)
as “Property of the result of a measurement or
the value of a standard whereby it can be related to stated references, usually
national or international standards, through an unbroken chain of comparisons
all having stated uncertainties." From this definition, it is clear that traceability
can only be obtained when uncertainties are determined. The traceability
hierarchy in Rockwell hardness from highest level to the hardness
measurement is the following:
(1) International definition of hardness: This is similar to a “fundamental
property value." This definition should precisely define all aspects of the
hardness test.
(2) National definition of hardness: At this time, there is no international
agreement on a well-defined definition for any Rockwell hardness scale.
National definitions are used instead, which are based on national and
international test method standards. These definitions vary from country to
country. The U.S. national definition of Rockwell hardness, as defined by
NIST, is based on the use of the NIST primary reference standardizing
machine with a specific indenter and following a specific testing cycle
(15).
(3) Primary reference standardizing machine: A primary reference
standardizing machine is usually maintained at a country’s National
Metrology Institute (NIST in the United States). The design and operation
of this machine is dependent on the hardness definition. The NIST primary
reference standardizing machine is shown in Figure 11.
(4) Primary reference test blocks: The primary reference test blocks
are calibrated using the primary reference standardizing machine in
accordance with the hardness definition.
(5) Secondary standardizing machine: The design and operation of this
machine is based on the hardness definition and calibrated using primary
reference test blocks.
(6) Secondary standardized test blocks: The secondary standardized test
blocks are calibrated using the secondary hardness machine in accordance
with the hardness definition.
(7) Laboratory hardness test machine: The design and operation of this
machine is based on the hardness definition and calibrated using
standardized test blocks.
errors with respect to the Rockwell hardness definition. In the first scenario,
a standardizing laboratory or a testing laboratory bases its measurement
uncertainty on measurement comparisons using standardized reference test
blocks. Traceability can then be linked to the highest reference level in the
traceability hierarchy through one or more reference levels. In the second
scenario, a standardizing laboratory’s measurement traceability is with respect
to the Rockwell hardness definition and is based on standardizing machine
errors as defined by the definition. This is the method that is used in
determining the measurement uncertainty of primary reference standardizing
machines.
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