Grain boundary control for improved intergranular stress corrosion cracking resistance in austenitic stainless steels: new approach

Grain boundaries of special character have resistance to corrosion and intergranular stress corrosion. The character can be described using geometrical schemes such as the coincidence site lattice (CSL) model, in which boundaries with low CSL index (Σ) have lower energy and increased resistance. It has long been recognised that increasing the fraction of such boundaries will increase the resistance of a material to intergranular degradation.This paper describes work which has focussed on the behaviour of special grain boundaries in sensitised austenitic stainless steel. The aim of the work was to develop a general model for stress corrosion cracking, which would ultimately be capable of predicting the effects of the degree of sensitisation, the connectivity of special boundaries and the influence of stress gradients, such as those developed from surface preparation (machining or peening) or due to the stress concentration effect of pit formation.Experimental work using electron backscatter diffraction (EBSD) analysis and in-situ high-resolution computed X-ray tomography has correlated cracking with the microstructure in a Type 304 austenitic stainless steel. In-situ 3D observations demonstrated that annealing twins cause local crack arrest and diversion, leaving non-fractured ligaments in the wake of the cracking path. The mechanical effects of the deformation and failure of these bridges have been modelled, demonstrating that special grain boundaries cause crack tip shielding. Increasing the fraction of special boundaries, and decreasing grain size, are both predicted to increase stress corrosion cracking resistance. Experimental observations, using room temperature intergranular stress corrosion tests and high temperature autoclave tests confirm these predictions for thermo-mechanically processes microstructures. The effects of applied stress and stress gradients are also predicted by the model, which may be extended to include the kinetics of crack growth, clustering of grain boundary types and variation of the degree of sensitisation.