Sensitisation behaviour of grain boundary engineered austenitic stainless steel.

Thermo-mechanical processes involving both single and multiple cycles of low level (5%) strain and annealing were applied to specimens of a type 304 austenitic stainless steel in order to encourage grain boundary engineering (GBE). As a result of the GBE processing the total length proportion of Σ3n coincidence site lattice (CSL) boundaries was increased from 43% up to a maximum of 75% in conjunction with moderate grain growth. The increases in Σ3 and Σ9 boundaries resulted in significant decreases in the degree of sensitisation following exposure at 650°C for up to 4 hours and assessment through Double Loop-Electrochemical Potentiokinetic Reactivation (DL-EPR) tests. Over 97% of Σ3 boundaries were immune to sensitisation and approximately 80% of Σ9 boundaries were either immune or partially resistant to sensitisation, whereas all other CSL boundaries and general boundaries did not resist sensitisation. Therefore, only Σ3 and Σ9 boundaries were ‘special’. Deformation applied by cold rolling was more effective than tensile deformation in bringing about GBE. In summary, the results presented here show that increasing the fraction of Σ3 and Σ9 boundaries through GBE processing, accompanied by only moderate grain growth, provides an effective route to protection from sensitisation and intergranular corrosion