Cluster Properties of Sensitized Grain Boundaries in Type 304 Stainless Steel

Intergranular corrosion and stress corrosion crack propagation are influenced by the distribution and connectivity of susceptible and resistant grain boundaries. For example, in oxidising environments, the performance of austenitic stainless steels is strongly dependent on the propensity of grain boundaries to chromium depletion (i.e. sensitisation). This is linked to their structure, and therefore depending on the relative crystallographic and physical orientations of the grains and their boundaries (i.e. the so-called “5 parameter” description). In-situ observations of crack bridging by non-sensitised grain boundary segments have shown an effect on the intergranular crack propagation path, and 3D modeling of this mechanism has demonstrated significant effects on short crack growth rates of randomly distributed resistant grain boundaries. Grain boundary control may therefore be used to influence the distribution of beneficial grain boundaries for improved microstructure resistance to crack nucleation. This analysis, however, takes no account of the connectivity of grain boundaries, which may be non-random. Electron Backscatter Diffraction (EBSD) and Image Analysis (IA) techniques have been coupled with Electrochemical Potentiokinetic Reactivation (EPR) testing to characterize the development of grain boundary clusters in sensitised Type 304 stainless steel with thermo-mechanical processing. Clusters of potentially resistant and susceptible grain boundaries have been identified and were correlated to their sensitisation propensity and the intergranular stress corrosion resistances of investigated microstructure.