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

Metals

卷:11

Antonio Manuel Mateo Garcia¹  Abdelkrim Redjaïmia² 

[1] Department of Materials Science and Metallurgical Engineering, Campus Diagonal Besòs-EEBE, Universitat Politecnica de Catalunya, 08019 Barcelona, Spain;

[2] Institut Jean Lamour UMR CNRS 7198, Université de Lorraine, F-54011 Nancy, France;

关键词: duplex stainless steels;    M₂₃C₆ carbide;    crystal structure;    (M₂₃C₆/γ)-eutectoid;    morphology;    geometrical model;   

DOI  :  10.3390/met11091340

来源: DOAJ

【 摘 要 】

This study is focused on isothermal and anisothermal precipitation of M₂₃C₆ carbides from the fully ferritic structure of the (γ + δ) austenitic-ferritic duplex stainless steel X2CrNiMo2253, (2205). During isothermal heat treatments, small particles of K-M₂₃C₆ carbide precipitates at the δ/δ grain-boundaries. Their formation precedes γ and σ-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 δ-ferritic matrix. TEM characterization of these islands reveals a particular eutectoid, reminiscent of the well-known (γ-σ)—eutectoid, usually encountered in this kind of steel. TEM and electron microdiffraction techniques were used to determine the crystal structure of the eutectoid constituents: γ-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 γ-Austenite and Kurdjumov-Sachs, on the other hand, between γ-Austenite and the δ-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 γ-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 γ-Austenite and σ-Sigma phase. For this purpose, the mole fraction evolution of K-M₂₃C₆ and σ-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 δ-ferrite ⇒ {γ-Austenite ⇔ K-M₂₃C₆}.

【 授权许可】

Unknown