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From the early non-weldable duplex grades and the more modern 22Cr duplex steels developed in the seventies,a variety of new steels have emerged on the market place with different corrosion resistance and mechanical properties.The process and alloy development is facilitated by the application of thermodynamic databases.The physical metallurgy of stainless steels,in particular the thermodynamic descriptions of phase equilibria and kinetics,is fairly well described at higher temperatures.However when such phenomena as low temperature precipitation of e.g alpha prime in the iron-chromium and higher systems are described,the extrapolation from original assessments is very large.The need of experimental low temperature data is thus crucial and unfortunately limited.Also experimental data is needed to illuminate mechanisms e.g.for the low temperature embrittlement ( 475℃ embrittlement) causing limitations,primarily by the weldment performance,in the upper service temperature of duplex stainless steels.In the presentation examples from high temperature phase transformations as well as attempts to simulate low temperature transformations are given.