An Experimental and Theoretical Evaluation of Microstructure Coarsening in Duplex Stainless Steels

Abstract

The coarsening of duplex stainless steel microstructures at temperatures above 10008C was investigated experimentally and theoretically. Hot-rolled duplex 2205 and superduplex 2507 samples were heat treated isothermally at 11008C up to 700 h, and also between 1000 and 12508C for 24 h. The ferrite content and the microstructure coarseness, quantified by the measured austenite spacing, AS, were evaluated. It was found that the observed coarsening could be described using Ostwald ripening theory expressed as AS n À AS n 0 ¼ kt, where AS 0 is austenite spacing of the as-delivered material, k the rate constant, n the exponent, and t the heat treatment time. Using regression analysis, n and k were evaluated and the resulting predictions compared with literature theories and results. The results were also calculated according to the original Lifshitz, Slyozov, Wagner (LSW) theory with parameters taken from literature, as well as with the Dictra software for diffusion and growth in multicomponent systems. The calculations indicated that while the classical LSW theory failed to give a good description of the experimental results, the Dictra simulations proved very accurate, even though systems of equal ferrite and austenite content are considerably outside the intended range of applicability of the software.

Development of Thermodynamic Criteria for Determining the Composition of Duplex Stainless Steels with High Corrosion Resistance

“…Further increasing the solution annealing temperature to 1200 • C led to an increase in the δ-ferrite fraction to 82 vol.%. After this treatment, colonies of small austenitic grains completely disappeared, and the remaining grains decreased in size, losing their sharp angularity due to simultaneous processes of dissolution and Ostwald ripening [20]. With an increase in the solution annealing temperature to 1250 • C, the δ-ferrite fraction increased to 90 vol.%, and the austenite islands further reduced in size, approaching a more rounded shape.…”

Evolução microestrutural do aço inoxidável dúplex UNS 532205 durante laminação a frio e posterior recozimento

“…De fato, mesmo depois da recristalização total de ambas as fases, a microestrutura resultante continua com esta morfologia bandeada (KEICHEL; FOCT; GOTTSTEIN, 2003b;WITKOWSKA;ZAID;BHATTACHARJEE, 2014). Entretanto, para tempos mais longos de recozimento, a microestrutura evolui para uma estrutura com morfologia mais globular devido à diminuição da energia associada às interfaces /α, via engrossamento de Ostwald (KEICHEL; FOCT; GOTTSTEIN, 2003b;WESSMAN et al, 2013).…”