Microstructures and yield strength of nitrogen alloyed super duplex steels

Ferritic-austenitic duplex steels combine high mechanical strength with acceptable corrosion resistance and are attractive for applications in the chemical and petrochemical industries. Alloying with nitrogen increases further both strength and corrosion resistance. In this work attention is paid to the influence of nitrogen and the constituents' spatial arrangement in the microstructure on the yield strength σ^d_y of the compound. The matrix-inclusion character of ferrite and austenite, which can be quantified by the parameters contiguity C and fraction of clusters r, markedly influences σ^d_y. Besides the topology of the microstructure, the yield strengths of the single phases ferrite and austenite, σ^α_y and σ^γ_y, affect σ^d_y. σ^γ_y is calculated from the austenite's nitrogen content and the mean grain size of the austenite. σ^α_y is calculated in two ways: One method considers the ferrite mean grain size, the nitrogen content in the ferritic phase and the coherent α′-particles. In a second approach, σ^α_y is calculated from σ^γ_y and the microhardness ratio of ferrite to austenite. An extensive quantitative analysis of various duplex microstructures reveals that r is much more sensitive to variations in the topology than the contiguity. Therefore, it is sensible to establish a relationship for σ^d_y, which incorporates the yield strengths of the single phases and the parameter fraction of clusters of ferrite and austenite. For this purpose a non-linear rule of mixture-type equation for σ^d_y is proposed showing excellent agreement with experimentally obtained strength values.