This study is focused on isothermal and anisothermal precipitation of M23C6 carbides from the fully ferritic structure of the (¿ + d) austenitic-ferritic duplex stainless steel X2CrNiMo2253, (2205). During isothermal heat treatments, small particles of K-M23C6 carbide precipitates at the d/d grain-boundaries. Their formation precedes ¿ and s-phases, by acting as highly potential nucleation sites, confirming the undertaken TEM investigations. Furthermore, anisothermal heat treatment leads to the formation of very fine islands dispersed throughout the fully d-ferritic matrix. TEM characterization of these islands reveals a particular eutectoid, reminiscent of the well-known (¿-s)—eutectoid, usually encountered in this kind of steel. TEM and electron microdiffraction techniques were used to determine the crystal structure of the eutectoid constituents: ¿-Austenite and K-M23C6 carbides. Based on this characterization, orientation relationships between the two latter phases and the ferritic matrix were derived: cube-on-cube, on one hand, between K-M23C6 and ¿-Austenite and Kurdjumov-Sachs, on the other hand, between ¿-Austenite and the d-ferritic matrix. Based on these rational orientation relationships and using group theory (symmetry analysis), the morphology and the only one variant number of K-M23C6 in ¿-Austenite have been elucidated and explained. Thermodynamic calculations, based on the commercial software ThermoCalq® (Thermo-Calc Software, Stockholm, Sweden), were carried out to explain the K-M23C6 precipitation and its effect on the other decomposition products of the ferritic matrix, namely ¿-Austenite and s-Sigma phase. For this purpose, the mole fraction evolution of K-M23C6 and s-phase and the mass percent of all components entering in their composition, have been drawn. A geometrical model, based on the corrugated compact layers instead of lattice planes with the conservation of the site density at the interface plane, has been proposed to explain the transition d-ferrite ¿ {¿-Austenite ¿ K-M23C6}.
