Intergranular corrosion resistance of nanostructured austenitic stainless steel

Nanostructured metals and alloys possess very high strength but exhibit limited plasticity. Enhancement of the strength/ductility balance is of prime importance to achieve wide industrial applications. However, post-deformation heat treatment, which is usually used to improve plasticity, can lead to a decrease in other properties. In the case of austenitic stainless steels, heat treatment in the range from 480 to 815 °C can increase their susceptibility to intergranular corrosion. The aim of the work reported in this paper was to determine if nanostructured austenitic stainless steel is susceptible to intergranular corrosion if heat treated for 1 h at 700 °C. Samples of 316LVM austenitic stainless steel were hydrostatically extruded, in a multi-step process with the total true strain of 1.84 to produce a uniform microstructure consisting of nanotwins. These nanotwins averaged 21 nm in width and 197 nm in length. Subsequent annealing at 700 °C produced a recrystallised structure of 68-nm-diameter nanograins. The heat treatment improved the ductility from 7.8 to 9.2 % while maintaining the ultimate tensile strength at the high level of 1485 MPa. Corrosion tests were performed in an aqueous solution consisting of 450 ml concentrated HNO₃ and 9 g NaF/dm³ (according to ASTM A262-77a). The evaluation of the corrosion resistance was based on transmission and scanning electron microscopic observation of the microstructure and chemical analyses. The results revealed that both the as-received and HE-processed samples are slightly susceptible to the intergranular corrosion after annealing at 700 °C for 1 h.