Thermodynamic modeling and kinetics simulation of precipitate phases in AISI 316 stainless steels
Introduction
AISI 316 austenitic stainless steels are widely used reactor materials in conventional nuclear power plants because of their outstanding mechanical formability, good high temperature strength and oxidation resistance [1]. However, these reactor materials are subjected to degradation due to thermal aging and other external factors (irradiation, stress, temperature, coolant media, etc.), which could affect the reliability of components [2], [3], [4]. Phase instability under thermal aging is one of the degradation phenomena that influence material mechanical behavior, subsequently determining whether the reactor fleet can be extended to a longer life. However, there is currently a lack of understanding of long term (40–60 years or beyond) thermal aging effect on the microstructure of reactor materials. It limits the deployment of the extension plan of the current reactor fleets. Phase stability in multicomponent materials such as AISI 316SS under long term thermal aging is a result of interplay of many variables such as alloy compositions, temperature, time, and processing history. To understand such interplay exclusively through experimental methods is extremely time-consuming and impractical. This work aims at utilizing modern computational microstructural modeling tools to accelerate the understanding of the stability of precipitate phases in austenitic steels under extended thermal aging. The first step of this study was to develop a thermodynamic property database for reliable modeling of precipitate phases in AISI 316SS, and then apply this database towards thermodynamic input for precipitation kinetics simulation. The simulation results demonstrate the potential applications of the current methodology to the understanding of phase stability in structural materials at low temperature regime as in light water reactors, where experimental study is difficult to be implemented.
Section snippets
Computational microstructural modeling
Computational microstructural modeling based on the CALPHAD (CALculation of PHAse Diagram) computational thermodynamics [5] and the mean-field approach of precipitation kinetics simulation [6], [7] was used in this work. The CALPHAD approach is to simultaneously evaluate all available thermochemical and constitutional data for the system and then obtain one set of self-consistent thermodynamic models for the Gibbs energies of all phases. From these models, the phase equilibrium and phase
Validation of thermodynamic calculation using commercial austenitic steels
The Fe–C–Cr–Mn–Mo–Ni–Si–Ti database was developed using the CALPHAD approach based on the phase diagram and thermodynamic properties of constituent binary and ternary systems. The validation of the database was carried out in two steps. One was to compare the calculated phase equilibria with experimental data for key quaternary systems; the other was to compare the calculated phase equilibria with experimental data obtained from commercial 316SS. Systematic experimental study on phase stability
Conclusions
A thermodynamic database OCTANT (ORNL Computational Thermodynamics for Applied Nuclear Technology) that can satisfactorily describe precipitate phases in 316 austenitic steels has been developed using the CAPLPHAD approach. The calculated temperature-dependent equilibrium phase properties are in a good agreement with experimental observations in long-term thermal annealed alloys at intermediate and high temperatures. The phase equilibria in 316SS at low temperatures are complicated because of
Acknowledgements
This research was supported by the US Department of Energy (DOE), Office of Nuclear Energy, Nuclear Engineering Enabling Technology (NEET) Reactor Materials, under contract DE-AC05-00OR22725 with UT-Battelle, LLC. Pandat software from CompuTherm LLC is acknowledged. Discussion with Dr. P.J. Maziasz from ORNL, Dr. Ernst Kozeschnik from Vienna University of Technology is also acknowledged.
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