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Rockwell hardness test blocks are reference standards for transferring
Rockwell hardness scale values from one standardizing level to a lower level;
for example, transferring national hardness scale values directly to secondary
standardizing laboratories, or transferring the national hardness scale values
to industry through the secondary standardizing level. Rockwell hardness
test blocks are also used for verifying or comparing the performance of
Rockwell hardness machines and indenters. The test method standards
specify requirements for the preparation, size, finish, uniformity, and
standardization of reference test blocks.
Historically, Rockwell test blocks are standardized (also referred to as
calibrated) to determine the average hardness of the test surface of the block.
Normally, the calibration laboratory accomplishes this by making a number of
measurements across the block surface and then calculating the average of the
measurements. This is the usual standardization process whether the
blocks are standardized by the primary national metrology institute level or
by secondary commercial laboratories.
Because no materials are perfectly uniform in hardness, all reference test
blocks will have some hardness variation across the test surface. In most
cases, the hardness varies smoothly across the surface, but the variation is
different from block to block. The hardness variation is primarily due to the
test block manufacturing process. Figure 6 illustrates examples of the hardness
variation in four 25 HRC level test blocks.
The certified hardness value provided with a test block is an estimation of
the average hardness of the entire test surface; however, the hardness at
individual test locations will vary within a range of values extending both
above and below the certified average hardness value. This variation in
hardness across the surface is referred to as the non-uniformity of the
test block. The test method standards specify tolerances on the degree of
acceptable non-uniformity, which varies depending on Rockwell scale and
hardness level.
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