Characterisation of the Sensitisation Behaviour of Thermo-mechanically Processed Type 304 Stainless Steel Using DL-EPR Testing and Image Analysis Methods

Standard test methods such as the Electrochemical Potentiokinetic Reactivation Test (EPR – ASTM G108) and the Double-Loop EPR test (DL-EPR – ISO12732) are commonly used to characterise sensitisation behaviour in austenitic stainless steels. These tests provide a quantitative assessment of microstructure susceptibility. Factors such as different grain size may be accounted for, but additional information on the network of sensitised boundaries is neglected. This paper reports a new approach to characterise the development of sensitisation, applied to a Type 304 austenitic stainless steel subjected to thermo-mechanical processing. DL-EPR testing is augmented by large area Image Analysis (IA) assessments of optical images to measure the dimensions and connectivity of the attacked grain boundary network. Comparison is made with the standard assessment methods, and a new method is proposed, based on normalisation by a cluster parameter to describe the network of susceptible grain boundaries. This parameter can be estimated by electron backscatter diffraction (EBSD) methods in the non-sensitised condition. The proposed method allows a simple quantitative assessment of the degree of sensitisation of different microstructures and heats of austenitic stainless steels.