Copper and its effects on microstructure and correlated tensile properties of super duplex stainless steels

In the last few years, Cu is handled as a promising element to improve the corrosion resistance of duplex steels. Although there is a very limited number of studies in the literature describing the influence of Cu on the microstructure of duplex steels in the aged condition, to the authors' knowledge, no comprehensive study has been presented so far that describes in detail its influence on the microstructure and correlated mechanical properties in the solution-annealed condition. Consequently, this work is intended to fill that gap to provide a fundamental base for material design of novel duplex steels. Microstructural investigations showed a preferred formation of austenite combined with an elemental redistribution of Cr and Mo. Especially at the highest Cu content investigations revealed precipitation of Cu particles causing - so far unknown - an intragranular austenite in ferrite. It is proposed that the enrichment of austenite forming elements, i.e. Ni, at their phase boundaries as well as a low misfit between Cu particles and the intragranular austenite nuclei play a significant role during its nucleation. Due to the identified microstructural changes triggered by Cu an increased imbalance of the flow stress between ferrite and austenite, and thus a decrease of the macroscopic yield strength can be proposed. In turn an increased work hardening with Cu addition causes an unaffected ultimate tensile strength.