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Test blocks, traceable to the National Institute for Standards and Technology (NIST), are now manufactured by a number of American hardness test block manufacturers. In fact, their use in calibration is mandated by an addendum to the ASTM Standard for Rockwell testing (currently only available in Rockwell C scale). These blocks derive their traceability to NIST because the standardizing hardness testers that these manufacturers use to make the blocks have themselves been calibrated using master "test blocks" from NIST, formally known as Standard Reference Materials (SRM®). NIST decided to manufacture these blocks in Rockwell scales after surveys by NIST and Newage Testing Instruments, Inc. revealed that the values from the test blocks of United States test block manufacturers were not consistent with other manufacturers nor equal with many other hardness testing standards internationally. (See article on new changes in ASTM E-18) This situation arose because the US had informally adopted a performance standard for the acceptability of diamond indenters which have a hemispherical diamond tip. If the indenter read well on a test block it was "good". This lead to a chicken-and-egg situation between the blocks and indenters that caused the de facto hardness values to be different from one US manufacturer to another and from other national standards. It was apparent that NIST needed to create test blocks as a part of the solution to this problem. NIST now produces hardness "test blocks", formally known as Standard Reference Materials (SRM®), in the Rockwell C scale, which is the most common hardness testing scale, and at certain loads in the Vickers and Knoop scales. Since these are the only scales available, hardness test block traceability is limited to these same scales. The test blocks available from NIST are:
When the test block manufacturer calibrates his standardizing hardness tester for Rockwell C, he uses all three Rockwell C blocks which ensures the linearity of the tester calibration. The linearity is important due to factors such as elastic recovery of the material during the depth measurement and variations in the curvature of the diamond tip. The manufacturer can then manufacture traceable blocks anywhere within that hardness range, e.g. HRC50, HRC25 etc. The case is somewhat different for Microhardness test blocks. Since the same factors do not apply as in the Rockwell method's depth measurement, the test block is essentially used to calibrate for the proper load. So one test block can be used to calibrate a micro tester for an entire range of hardness values within a given load. For more information on NIST and hardness SRMs please visit their website: National
Institute for Standards and Technology |