|
Rockwell hardness reference test blocks are used primarily for the indirect
verification (see 5.2) and daily verification (see 6.2) of a Rockwell hardness
machine. Reference test blocks are also useful when comparing the relative
performance between two hardness machines by measuring the same blocks
on both machines or for comparing the performance of two indenters. Proper
care in the handling and use of reference test blocks is important to obtaining
accurate measurements. It is critical that the reference test blocks not
influence the hardness measurements due to improper use. The general
procedure for testing reference test blocks is the same as the hardness
measurement procedures discussed in 3.4. The following are additional
recommended practices for the proper use of reference test blocks.
Good Practice Recommendations
• Test temperature: As discussed previously, the test temperature can affect
Rockwell hardness measurement results due to the temperature dependency
of the material being tested. When using reference test blocks, it is desirable
to separate any effect due to the temperature dependency of the block
material. To the extent possible, prior to and during the verifications, the
reference test blocks should be maintained near to the temperature at
which they were calibrated.
• Anvil: When reference test blocks are being used for the verification of
a hardness machine, the same anvil must be used for the verification
(when possible) as will be used for normal testing following the verification.
In circumstances where the normally used anvil cannot be used for testing
test blocks, an initial verification of the machine should be made using an
anvil appropriate for testing reference test blocks. This anvil should then be
replaced with the anvil normally used for testing, and a second verification
should be performed. The second verification should be made on a typical
part of known hardness that is normally tested with the anvil or some other
appropriate test piece for which the correct hardness is known.
Secondary Reference Test Blocks
• Inspection: The bottom surface of reference test blocks should be visually
inspected prior to use. The slightest dent, scratch, or spot of corrosion can
significantly affect the measurement result. Attempts to repair mechanical
damage on the bottom surface of test blocks should be avoided.
• Cleaning: Prior to use, it is recommended that the reference test block be
cleaned. A recommended method for cleaning is to gently wipe the top and
bottom test block surfaces with clean cotton or a cloth, thoroughly wetted
with ethyl alcohol. The metal surfaces should immediately be dried using a
soft lint free cloth or paper towel before the alcohol evaporates in the air.
This cleaning must be performed in a manner that prevents a residue from
remaining on the top or bottom surfaces. The cleaning should be followed
by blowing the surfaces clean of dust using filtered air. The top and bottom
surfaces should not be touched after cleaning.
• Placement on the anvil: Immediately before placing the reference test
block on the hardness machine anvil, the top surface of the anvil and the
bottom surface of the test block should be blown free of dust as before.
The reference test block should be gently and carefully placed on the anvil
before dust can return. The top test surface of the reference block should
be blown free of dust prior to testing and occasionally during the period of
use. When a flat anvil is used, the reference test block should be slid several
times back and forth over the surface of the anvil to help seat the block on
the anvil. Anytime the reference test block is lifted from and replaced on the
anvil, the procedure described above in this paragraph should be repeated.
When a spot anvil is used, extreme care should be practiced to ensure that
the test block is supported parallel to the anvil surface until the indenter
contacts the block, and the preliminary force is applied.
• Preliminary indentation: When a flat anvil is used, it is recommended that
at least one preliminary Rockwell test be performed at any location on the
test surface of the reference test block. The preliminary test will help seat
the test block on the anvil. The measured hardness value of the preliminary
test should be ignored. The user is cautioned not to make the preliminary
indentation such that it contacts a previous indentation. Doing so may
damage the indenter. A preliminary indentation is not necessary when
using a spot anvil.
• Testing cycle: Reference test blocks are typically calibrated by performing
Rockwell tests using a specific testing cycle. When reference test blocks
are used for the verification of a hardness machine, a testing cycle should
be used that replicates, as closely as possible, the testing cycle used by
the standardizing agency when the block was calibrated. Deviations in the
testing cycle dwell times or force application rate may result in measured
hardness values that are shifted from measurements made using thestandar
dizing testing cycle. Frequently, the testing cycle is not reported
by the standardizing agency. In this case, a testing cycle should be chosen
that is within the stated tolerances of the test method standards.
• Measurement locations: The locations for making measurements on
reference test blocks should be as specified, or recommended by the test
method standards, keeping in mind proper indentation spacing. Indentations
should be randomly distributed over the surface of the test block when
determining the measurement performance of the testing machine with
respect to the certified average hardness value of the test block. Never fill
the test surface with indentations by starting at one side of the block and
progressively moving to the other side of the block.
• Storage: It is recommended that reference test blocks be stored in an
environment that protects the blocks from mechanical damage, excessive
oxidation and corrosion. Wrapping a test block in anti-corrosion paper is
a good method for protecting the test block surface from corrosion and
oxidation when not being used. Anti-corrosion paper for ferrous and
nonferrous metals is commercially available. Although a coating of oil
can protect a block surface, it is not recommended since the oil must be
completely removed prior to testing the block. Test blocks should not be
subjected to wide variations in temperature. Elevated temperatures should
be avoided; particularly in the case of brass test blocks, which in some
cases can age-harden the block changing its overall hardness.
Testing Precautions
• The certified hardness value provided with a reference test block is
applicable only to the top test surface of the block. It does not represent
the hardness of the bottom or edge surfaces of the test block, nor the
material inside the test block. As such, NEVER make indentations on
the bottom surface of a test block. Not only will the measurement values
obtained be invalid for comparing with the block’s certified hardness value
for verification purposes, but also the reference test block can no longer be
reliably tested on the top test surface. An indentation on the bottom surface
will significantly affect subsequent hardness measurements. Any reference
test block tested on the bottom surface must never be used for verification
purposes and should be discarded.
• Once the test surface of a reference test block is filled, it should not be
machined to remove the indentations for additional testing. As stated above,
the hardness of the sub-surface material may differ from the hardness of
the original test surface. Additionally, a Rockwell indentation deforms
material well below an indentation making it difficult to determine when
sufficient affected material has been removed from the block.
Back
| 
