Phase-Field Modeling of Sigma-Phase Precipitation in 25Cr7Ni4Mo Duplex Stainless Steel
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teels with roughly equal amounts of the phases, austenite and ferrite, are called duplex stainless steels (DSS). DSS are used in applications where a combination of excellent corrosion resistance with very good mechanical properties is needed, e.g., tubing for chemical industries, heat exchangers, seawater applications, and as construction material in applications such as bridges and storage tanks. For DSS with a pitting resistance equivalent over 40 in both austenite and ferrite the material is denoted as super duplex stainless steel (SDSS),[1] but this enhanced corrosion resistance comes with an increased susceptibility for
AMER MALIK, JOAKIM ODQVIST, LARS HO¨GLUND, and JOHN A˚GREN are with the Department of Materials Science and Engineering, KTH Royal Institute of Technology, 100 44 Stockholm, Sweden. Contact e-mail: [email protected] STAFFAN HERTZMAN is with the Outokumpu Stainless Research Foundation, KTH Royal Institute of Technology, 100 44 Stockholm, Sweden. Manuscript submitted November 2, 2016.
METALLURGICAL AND MATERIALS TRANSACTIONS A
precipitation of harmful intermetallic phases, e.g., sigma phase and chi phase, during production and welding.[2,3] Due to its large negative impact on both the corrosion resistance and the mechanical properties, the precipitation of sigma phase in DSS and SDSS has been studied extensively.[4–12] In line with the principles of Integrated Computational Materials Engineering (ICME),[13,14] where modeling and simulations are combined with critical experiments in order to accelerate materials development, accurate modeling of the sigma-phase precipitation in DSS and SDSS allows for process control and prediction of microstructure evolution. The kinetics of sigma-phase precipitation in DSS has been predicted using Avrami-type models.[12,15] Modeling of the precipitation of sigma phase in an austenitic stainless steel has been performed by Schwind et al.[16] using the DICTRA software. In their treatment, the nucleation stage was neglected. Sieurin and Sandstro¨m[17] modeled sigma-phase precipitation in the DSS 2205 using a nucleation model based on classical nucleation theory and a quasistatic growth model.[18] Quite recently, Wessman and Pettersson[19] also studied the growth of
sigma phase in SDSS using Thermo-Calc and DICTRA softwares. Despite the relative success of these treatments, in order to study the influences of grain morphology and concurrent grain growth and precipitation, the phasefield method is preferred.[20] The method stems from the diffuse-interface concept: the one introduced by van der Waals for the smooth change of the density between a liquid phase and a gas phase, and independently proposed by Landau and Lifshitz[21] and by Cahn and Hilliard[22,23] for the excess free energy of the wall between two magnetic domains in a ferromagnetic material and for the liquid–gas surface respectively. The microstructure is described with one or more field variables, e.g., density or magnetic order, time evolutions of which are governed by a set of kinetic equations.[2
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