The application of duplex steels is constantly increasing due to their excellent combination of high strength and fracture toughness complemented with superior resistance to localized chemical corrosion and stress corrosion. Since ultrafine-grained and nanocrystalline metals have shown improved mechanical and physical properties compared to their coarse grain counterparts, a further optimization of duplex steels could be realized by an additional nano-structuring process. Therefore, in the present study, a conventional duplex steel (X2CrNiMoN22-5-3) was deformed by high pressure torsion (HPT). The evolution of the microstructure and hardness upon deformation was examined. Special attention was devoted to the change of fracture toughness induced by HPT. In order to take grain shape changes during deformation into account, specimens with different orientations with respect to the principal shear deformation direction were tested. A pronounced anisotropy in the crack propagation behavior depending on the specimen orientation combined with an exceptional increase of strength was discovered.