Evaluating microscopic hardness in ferritic steel based on crystallographic measurements via electron backscatter diffraction

Many researchers have investigated the relationship between hardness and crystallographic texture, including the Hall–Petch effect, crystal anisotropic-plasticity, dislocation strengthening, and strengthening by solutions. However, few studies have considered these effects simultaneously. This paper reports a method for estimating the impact of these multiple effects on the microscopic hardness of ferritic steel solely via electron backscatter diffraction (EBSD). Additionally, the correlation between the estimated hardness and fatigue damage is evaluated. The estimation method is based on the experimental correlation between the crystallographic texture obtained via EBSD and the hardness measured using nanoindenters. Three types of extra-low-carbon steel with different grain sizes were evaluated. First, a micro-Vickers test was conducted for each grain and the hardness distributions in the steel were evaluated. Second, EBSD measurements and nanoindentation tests were conducted at same locations to evaluate the relationship between the hardness obtained via nanoindentation and individual factors such as grain size, Taylor factor in the loading direction of indentation, kernel average misorientation (KAM), and indentation load. Each crystallographic factor had a rather low correlation with hardness, even though these low correlations were analogous to those of previous studies. Thus, we presume that the low correlations are due to the multiple effects of these factors, and a multiple-effect relational equation based on the strengthening mechanism is proposed with constants based on fitted data. The predicted and experimental hardness distributions were similar. Moreover, the grains with lower predicted hardness tended to be damaged after the fatigue test. These results suggest that the proposed equation can be used to predict the location of the lowest hardness in a material.