Delineation of Prior Austenite Grain Boundaries in a Low-Alloy High-Performance Steel

Characterization of prior austenite grain size is important for understanding the microstructure–property relationships in steels. The prior austenite grain size plays an important role in defining the microstructural scale of low-temperature phases and the mechanical properties (e.g., strength, ductility, fracture toughness, etc.) of steels in the final product form. Moreover, in several failure analyses, the cracks are observed to propagate along the prior austenite grain boundaries (PAGBs). The delineation of PAGBs in steels of new composition can be quite challenging, as the response to a particular etching protocol is very sensitive to the chemical composition of steel. The objective of this study was to establish a methodology to delineate PAGBs in AF9628, a newly developed low-alloy high-performance steel. Several different etchants and etching techniques from the literature were evaluated. These methods were unsuccessful or had limited success in revealing PAGBs in AF9628. However, swab etching with a solution of 100 ml saturated aqueous picric acid and 0.5 g sodium dodecyl benzene sulfonate worked remarkably well for delineating the PAGBs in this steel. This etchant was found to have high selectivity, revealing PAGBs preferentially over packet, block, and sub-block boundaries.