Quantification of hardening contribution of G-Phase precipitation and spinodal decomposition in aged duplex stainless steel: APT analysis and micro-hardness measurements

Ageing of cast duplex stainless steels (DSS) is attributed to the decomposition of the ferrite: spinodal decomposition and precipitation of G-phase particles. This leads to an increase in hardness and a decrease in Charpy toughness. According to the literature, spinodal decomposition is accepted to play a major role on the hardening even if the role of G-phase precipitation on mechanical properties is still not clear. This work links microstructural characterization performed using atom probe tomography to micro-hardness of the ferrite for a wide variety of duplex steels (from cast steels with and without Mo to lean steels) aged under different conditions. An attempt to quantify the contribution of both spinodal decomposition and G-phase precipitation is made by applying linear and square superposition principle of Ardell, Orowan and a modified BKS models. The models used are shown to give an excellent estimation of the experimental values of the hardness increase of the ferrite of the cast and lean steels for a wide range of composition and temperature. This work shows that, conversely to what is said in the literature, spinodal decomposition is not systematically the main contributor to hardening. (C) 2018 Elsevier B.V. All rights reserved.