Influence of Microstructure on the Mechanical Properties and Localized Corrosion of a Duplex Stainless Steel

The alloy studied, U50, is a ferritic-austenitic duplex stainless steel. In addition to the two major phases (that is, ferrite and austenite), several additional phases form during low and intermediate temperature aging, namely, M 7 C 3 , M 2 3 C 6 , σ, X, R, γ 2 , and a'. This study details the precipitation of these phases and their influence on the mechanical and localized corrosion properties. The sigma phase has the most deleterious influence on the impact properties. The α' produces the largest increase in the yield and ultimate tensile strength. The precipitation of M 2 3 C 6 and σ adversely affects the pitting resistance of U50 in 0.1NHCl because of the accompanying chromium and molybdenum depletion of the matrix. The formation of a' also severely reduces the chromium content of the ferrite phase and thereby inhibits its passivation. The large volume percent of ferrite in the alloy prevents intergranular corrosion that typically accompanies grain boundary precipitation of M 2 3 C 6 in single-phase stainless steels.