Abstract
Phase-field modeling is used to simulate the formation of sigma phase in a model alloy mimicking a commercial super duplex stainless steel (SDSS) alloy, in order to study precipitation and growth of sigma phase under linear continuous cooling. The so-called Warren–Boettinger–McFadden (WBM) model is used to build the basis of the multiphase and multicomponent phase-field model. The thermodynamic inconsistency at the multiple junctions associated with the multiphase formulation of the WBM model is resolved by means of a numerical Cut-off algorithm. To make realistic simulations, all the kinetic and the thermodynamic quantities are derived from the CALPHAD databases at each numerical time step, using Thermo-Calc and TQ-Interface. The credibility of the phase-field model is verified by comparing the results from the phase-field simulations with the corresponding DICTRA simulations and also with the empirical data. 2D phase-field simulations are performed for three different cooling rates in two different initial microstructures. A simple model for the nucleation of sigma phase is also implemented in the first case. Simulation results show that the precipitation of sigma phase is characterized by the accumulation of Cr and Mo at the austenite-ferrite and the ferrite-ferrite boundaries. Moreover, it is observed that a slow cooling rate promotes the growth of sigma phase, while a higher cooling rate restricts it, eventually preserving the duplex structure in the SDSS alloy. Results from the phase-field simulations are also compared quantitatively with the experiments, performed on a commercial 2507 SDSS alloy. It is found that overall, the predicted morphological features of the transformation and the composition profiles show good conformity with the empirical data.
Highlights
STAINLESS steels with roughly equal amounts of the phases, austenite and ferrite, are called duplex stainless steels (DSS)
The results presented are from numerical simulations performed on 2507 DSS alloy under continuous cooling
A multiphase and multicomponent phase-field model is presented based on the WBM model to study the formation of sigma phase in an alloy mimicking a commercial stainless steel alloy
Summary
4914—VOLUME 48A, OCTOBER 2017 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.[4,5,6,7,8,9,10,11,12]. Villanueva et al.[41] used the WBM approach and developed a multicomponent and multiphase model with fluid motion to study reactive wetting They used arbitrary phase diagrams and assumed ideal solutions for Gibbs energies. Cogswell and Carter[42] suggested a new approach and developed a thermodynamic phasefield model to study the microstructural evolution with multicomponents and phases. Their simulations were based on an arbitrary case, and the parameters used in the phase-field formulation did not correspond to a physical system. The model is applied to the precipitation of sigma phase in a SDSS 2507 alloy (Fe-25Cr-7Ni-4Mo, wt pct) under continuous cooling
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