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.