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Surface degradation and creep of Inconel 617 and Haynes 230 at high temperatures

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Title: Surface degradation and creep of Inconel 617 and Haynes 230 at high temperatures
Author(s): Tung, Hsiao-Ming
Director of Research: Stubbins, James
Doctoral Committee Chair(s): Stubbins, James
Doctoral Committee Member(s): Heuser, Bretn; Shang, Jian-Ku; Uddin, Rizwan
Department / Program: Nuclear, Plasma, & Rad Engr
Discipline: Nuclear Engineering
Degree Granting Institution: University of Illinois at Urbana-Champaign
Degree: Ph.D.
Genre: Dissertation
Subject(s): Nickel base alloys Creep High Temperature Corrosion
Abstract: The high temperature gas-cooled reactor (HTGR) design has been selected for the Next Generation Nuclear Plant (NGNP) project. The helium coolant in the primary circuit has been found to contain low levels of impurities (e.g. CO, CO2, H2O, CH4, and H2) after steady-state operation. This can lead to an environmental degradation of high temperature alloys used for internals and heat exchangers. Depending on the impurity concentration and the temperature, the materials can undergo oxidation, carburization and decarburization, which can be detrimental to the mechanical behavior of the materials. In addition, the most critical metallic component of the HTGR is the heat exchanger. The materials serve in the temperature range of 760-950oC and withstand coolant pressure up to 7 MPa for a design life of 60 years. Therefore, understanding the environmental effects on the materials and the relevant creep deformation behavior is critical for the design and long-term operation of the HTGR. Inconel 617 (alloy 617) and Haynes 230 (alloy 230) are considered as primary candidate materials for serving in the HTGR, due to their exceptional combination of high temperature strength and oxidation resistance. In this study, surface degradation owing to environmental effects as well as creep deformation of the two alloys were investigated. The alloys were exposed to air, He+5% H2 and He+5% H2 +1% CH4 to study oxidizing, reducing (low-pO2 gas), and carburizing phenomena. Short-term exposures were conducted from 850 to 1000oC and long-term exposures (up to 1512 h) were performed at 950oC. For the creep deformation studies, biaxial thermal creep behavior using a pressurized tube technique for both alloys was employed. The experimental results and microstructural characterization lead to the following conclusions: 1. Short-term exposure in air showed a two-stage oxidation kinetics for both alloys. For alloy 617, the transition between the stages was related to Cr outward diffusion in the Cr2O3 and the matrix. The second stage oxidation for alloy 230 is associated with the significant formation of MnCr2O4. 2. Results of long-term exposure indicated that around 360 h exposure in air, the oxide scale grown on alloy 617 consisted of Cr2O3 and NiCr2O4. Significant spallation of oxide scale resulted in the decrease in weight after 1224 h exposure as observed for alloy 617, while alloy 230 exhibited a stable increase in weight. Both short-term and long-term exposure indicated that alloy 230 possesses better oxidation resistance than alloy 617. 3. Exposures in a reducing environment caused a faster scale growth rate in alloy 617 whereas a slower scale growth rate was observed in alloy 230. Microstructural characterization showed that much finer grains were formed in low-pO2 gas, compared with air environment for both alloys. 4. Carburization tests showed that alloy 617 had superior carburization resistance to alloy 230. X-ray diffraction analysis showed that chromium-rich carbide near the surface area is in a form of Cr3C2. 5. Residual stress measurement showed the intrinsic residual stress of Cr2O3 are tensile during the growth of the oxide for both alloys. The development of intrinsic stress with increasing exposure time in low-pO2 gas is relatively stable than in air, which may be attributed to smaller grain size formed in the reducing environment. 6. Creep properties were measured with a biaxial creep tube technique. At 950oC, alloy 617 and alloy 230 had similar creep rupture life, whereas alloy 230 exhibited better creep rupture life than alloy 617 at 850oC. Nucleation, growth and coalescence of creep voids at grain boundaries are the dominant micro-mechanism for creep fracture. At the secondary creep regime, void formation was observed along grain boundaries and at triple junction of grain boundaries.
Issue Date: 2012-09-18
URI: http://hdl.handle.net/2142/34516
Rights Information: Copyright 2012 Hsiao-Ming Tung
Date Available in IDEALS: 2012-09-18
Date Deposited: 2012-08
 

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