Primary reference test blocks are standardized using primary standardizing machines in accordance with the hardness definition. Usually, the National Metrology Institute (NMI) of a country standardizes the primary reference test blocks and maintains the national hardness scales. The National Metrology Institute in the United States for Rockwell hardness is NIST.
4.1.1 NIST SRMs for the Rockwell C Scale
Each Rockwell hardness scale covers a range of hardness levels. To transfer the U.S. national HRC scale values to industry requires more than one transfer standard for the entire scale. However, production of hardness blocks at all levels of HRC hardness is not feasible for NIST. It was determined that industry needs test blocks at the levels specified in test method standards for the calibration and verification of Rockwell hardness testing machines. For the HRC scale, ASTM and ISO specify three ranges of hardness. The NIST reference test blocks for the HRC scale reflect these ranges and are certified at three hardness levels: 25 HRC, 45 HRC, and 63 HRC, which are available for purchase as a Standard Reference Material (SRM ®2810, 2811, and 2812,respectively)(18)Currently, NIST offers Rockwell hardness test blocks for only the Rockwell Cscale. Because the NIST SRMs are primary transfer standards, greater care in the usage of the test blocks is recommended than for commercial test blocks that are standardized by secondary calibration laboratories. Annex C provides recommended procedures for the use of NIST Rockwell hardness test block SRMs. These recommended procedures may be used as well when using secondary standards to help improve measurement accuracy. As a consequence of the variation in hardness across a test block, NIST determines and provides the customer with two types of hardness certifications with each reference test block: (1) the certified average HRC hardness across the test surface of the block; and (2) certified HRC hardness values at specific untested locations on the test surface (19). These two types of certifications characterize the hardness of the test block in distinctly different ways.
18.104.22.168 Certification of the Average Surface Hardness
As discussed previously, the certified average hardness value of reference test blocks is usually determined by calculating the simple average of several hardness measurement values taken across the surface of the block. The certification of the NIST SRM test blocks was partly based on the calibration measurements; however, it was also based on a NIST derived function that models how the hardness varies across the surface of the test block material. In the case of the NIST blocks, the certified average hardness value is the average of the hardness values predicted by the hardness function for all test surface locations, and not simply the arithmetical average of the seven NIST measurements. However, because the locations chosen for the seven NIST measurements provide a good representation of the range in surface hardness, the two averages are nearly identical in value.
22.214.171.124 Certification of Hardness at Untested Locations
A hardness measurement is destructive in that a specific location on a hardness block can be measured only once. For the second type of certification, certified HRC hardness values and the associated uncertainties are provided for specific untested locations on the test surface of the reference block, as illustrated by the open circles in Figure 7. Because hardness blocks are not uniform, NIST can only predict the hardness at these untested locations. The HRC hardness values were calculated using the surface hardness function. Using this formula, predicted hardness values may be calculated for any single untested location or for the average of two or more locations. For the NIST SRMs, however, certified values of only eleven locations are provided with the SRM test block. Section C.2 of Annex C provides the formulas used by NIST and gives examples of how to use these formulas.
This second type of NIST hardness certification provides the customer with a valuable tool for improving the hardness comparison measurements by reducing the influence of the test block non-uniformity. For example, the user can combine the seven NIST calibration measurements with the eleven predicted values to produce a profile map approximating the block surface hardness, and then, correct measurements depending on the test location on the block. The customer can also calculate better corrections by calculating hardness values using the same formula NIST used to determine the hardness at the eleven untested locations, as described in Annex C.