On the M23C6-Carbide in 2205 Duplex Stainless Steel: An Unexpected (M23C6/Austenite)—Eutectoid in the δ-Ferritic Matrix

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