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.
