Transformation phenomena in a super duplex stainless steel

Abstract: Duplex stainless steels consist of about equal fractions offerrite and austenite and are attractive due to their highcorrosion resistance against pitting corrosion and stresscorrosion cracking. The pitting corrosion resistance can bedescribed by the PREN value which is defined asPREN=%Cr+3.3'%Mo+16'%N. A higher value correspond to higherpitting corrosion resistance. Duplex stainless steels having aPREN value above 40 are denoted super duplex stainless steel.One such steel is SAF 2507 which is a 25%Cr-7%Ni-4%Mo-0,27%Nsteel. Improper heat treatment can cause embrittlement oftendue to precipitation of s-phase and other intermetallic phases.Furthermore, 475°C embrittlement can occur. Chromiumnitrides might also precipitate. In addition to embrittlement,the precipitation of various phases can reduce the corrosionresistance. Owing to the microstructural instability of thissteel, it is important to understand the phase transformations.Calculations, such as using Thermo-Calc, in combination withexperiments can improve this understanding.The thermal history of duplex stainless steel was found tobe important, because the equilibrium fraction of ferrite andaustenite varies with temperature. Heat treatment thereforechanges this balance and thereby changes the chemicalcomposition of these two phases. Precipitation of s-phase wasfound to be strongly dependent on the composition of ferrite.In order to avoid s-phase precipitation, information on thekinetics of formation is essential. In the present study, atheoretical model was developed for continuous coolingtransformation diagrams based on experimental time temperaturediagrams. This model can be used to describe the kinetics ofs-phase precipitation. It can also be used to extrapolate theprecipitation behaviour to low volume fractions that arenormally to small to be accurately measured.