YIELD STRENGTH OF NITROGEN ALLOYED DUPLEX STEELS - EXPERIMENTS AND MICROMECHANICAL PREDICTIONS

For the prediction of the yield strength of nitrogen alloyed ferritic- austenitic duplex steels, accurate knowledge on the single phases' yie ld strength and their geometrical arrangement within the duplex micros tructure is required. Since the matrix-inclusion character of the phas es markedly influences the duplex yield strength sigma(y)(d), linear m odels for sigma(y)(d) (Voigt-model) can not serve for an accurate pred iction of sigma(y)(d). A non-linear rule of mixture, however, is a mor e sophisticated approach to calculate sigma(y)(d). Micromechanical mod els combined with finite element computations are efficient tools to a ccurately predict the influence of the topology of the microstructure on sigma(y)(d) only if the yield strengths of the single phases are di stinctly different, i.e. the yield strength ratio of ferrite and auste nite, psi, is larger than 2. Experiments on duplex steels show, howeve r, a marked influence of the phase arrangement on sigma(y)(d) even for psi similar to 1. To explain this behavior a modified non-linear rule of mixture is proposed, which incorporates the in situ yield strength s of the phases as upper and lower bounds for sigma(y)(d). A Hall-Petc h analysis, which considers the interactions between adjacent grains g ives excellent agreement between experiments and computational predict ions. (C) 1997 Elsevier Science B.V.