Optimization of Vickers Hardness Parameters for Micro- And Macro-Indentation of Grade 91 Steel

Hardness is being assessed as a potential life-assessment tool for tracking microstructural degradation and remaining life in creep strength enhanced ferritic (CSEF) steels. Such methodology is already being utilized for the CSEF steel Grade 91, which has been widely implemented in both replacement parts and new construction over the last two decades. Additionally, research into the complex microstructural features in welded joints often utilizes hardness surveys to characterize changes caused by thermal processing. New automated hardness testing equipment now affords the ability to develop statistically relevant datasets for these uses. However, proper application and understanding of the data requires knowledge of limitations and variations with the load and spacing of the hardness indents and the potential role of different material conditions in this complex steel. In this work, we have examined the effect of load in three Grade 91 material conditions, as well as the effect of spacing in as-received material to determine optimum parameters for both macrohardness (i.e., >1 kg) and microhardness mapping (i.e., <500 g). The results show that to minimize variations in microhardness and ensure consistency with macrohardness results, a 200 -g load is preferred for characterization. Laser scanning microscopy was utilized to clarify the potential for indentation size effects (ISE).