Abstract
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}.
Highlights
Instead of describing the K-M23 C6 by a stacking of planar layers like those for ferrite or austenite, we propose to describe it by a stacking of corrugated layers and not having the same atomic density
Identification of the potential interface shared by the engaged phases with respect to the OR: the densest planes dictated by the OR and with the conserved site density, Flat and edge-on representations of the atomic planes parallel the potential interface, Gathering atomic planes to form periodic corrugated layers based on the crystal structure, the compactness of the engaged atomic planes, Correspondence between the inter-layer spacing of the corrugated layer and the inter-reticular spacing of the corresponding atomic planes
It was shown that depending on the heat treatments, isothermal or anisothermal, the K-M23 C6 precipitates as particles at the δ/δ grain boundaries or as lamellae in (K-M23 C6 /γ)-eutectoid, respectively
Summary
Duplex stainless steels (DSS) are increasingly used from the eighties of the past century as structural material in oil, chemical and power industries [1,2,3,4] This is related to the fact that their duplex microstructure (γ + δ) allows a beneficial mixture of γ-austenitic and δ-ferritic properties, leading to, on the one hand, high strength with a desirable toughness [5,6] and, on the other hand, good corrosion resistance, especially to chloride-induced stress corrosion cracking [7,8,9]. The characterization of the phase precipitation was best undertaken by an isothermal heat treatment from the fully δ-ferritic microstructure retained by water-quenching to ambient temperature, from the solutionising single domain. The various identified phases include the M7 C3 and M23 C6 carbides [11,12,13,14,15,16,17,18,19,20], the γ-Austenite with its different morphologies [19,20,21], the α’-ferrite [11,22]
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
