ESSC & DUPLEX 2019

The prediction of alloying effects on phase equilibria and transformations in advanced stainless steels and in particular duplex stainless steels is complicated. This makes computational thermodynamics and kinetics an indispensable tool for the materials industry. It has been successfully deployed in the alloy development of steels for decades and continuously developed towards use in more and more applications. In this work, the performance of the current state-of-the-art commercial thermodynamic database was improved specifically for stainless grades containing high contents of Cr, Ni, N, W, and other elements. Several binary, ternary, and quaternary systems were re-evaluated, and the thermodynamic models for certain phases were improved. The database thus provides a better and more reliable tool for designing new alloys, optimizing heat treatments and studying precipitation of secondary phases at intermediate temperatures. The description of secondary phases which are typically detrimental for mechanical and corrosion properties was revised including the Epsilon-nitride, Sigma, Chi, Laves, Pi-nitride phases. In addition, a nitride phase (Eta) which has proven to be a critical equilibrium phase in super-austenitic stainless steels but previously was not included in any commercial steel databases, was modelled and added to the database. Another key improvement for the high-temperature phase equilibria reflects that several grades of duplex stainless steels crystallize by fully ferritic solidification, supported by new experimental evidence. Furthermore, the balance of matrix phases, ferrite and austenite, in the database has been improved in the critical temperature region for duplex stainless steels.