Rockwell hardness values obtained by performing a Rockwell hardness test give an indication about some of the mechanical properties of metals. The prevalence of this value and the widespread recognition it enjoys across the industry is due to the simplicity of its definition and the low cost of testing. However, for a unit to be trusted and internationally accepted, it must be well-defined, and its realization at the highest level of the calibration pyramid must be undertaken with the least amount of measurement uncertainty. Since the Rockwell scale is an empirical scale, the definition of the unit is mainly based on measurement procedures developed by national metrology institutes (NMI). Therefore, it is vital for low measurement uncertainties that a unified scale and well-defined standardized procedures are used by the NMIs.
A hurdle hampering the implementation of a standardized testing cycle is the adherence to the ISO 6508 without performing time-consuming pre-measurements on hardness reference blocks. This work builds on a method developed by the Physikalisch-Technische Bundesanstalt (PTB) that makes use of measurement data from the preload phase of a Rockwell hardness test to perform the calibration without pre-measurements. The definition of the estimating parameters is improved, and the method is extended to all Rockwell scales. Furthermore, this automated approach is developed to be independent of the hardness testing machine by identifying and measuring the key factors and validating the results through an interlaboratory comparison. This work also reviews the available methods of correction of the measured Rockwell hardness value on the basis of the measured indenter geometry. It has been shown that the determination of the accurate indenter radius and opening angle for Rockwell hardness diamond indenters is crucial for an effective correction of the measured hardness value. Traditionally, tactile methods are used to extract 2D profiles of the indenter topography and to determine the tip radius. In this work, a novel method to evaluate the entire 3D spherical tip measurement data is proposed. This method also assists in exploring the limits and capabilities of a confocal laser scanning microscope (CLSM). It has been shown that a CLSM can be used to perform calibrations of the Rockwell indenter tip radius, contingent upon a few caveats. Finally, a measurement uncertainty budget for calibrating the indenter tip radius with a CLSM and the improvement in the measurement uncertainty of the Rockwell hardness test resulting from using the measured radius are presented.