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Title: | Effect of Thermal Ageing on the Corrosion and Stress Corrosion Cracking of Type 310S Stainless Steel in Supercritical Water |
Authors: | Jiao, Yinan |
Advisor: | Kish, Joseph Zheng, Wenyue |
Department: | Materials Science and Engineering |
Publication Date: | 2018 |
Abstract: | This thesis documents a multi-faceted investigation of the effects of thermal ageing-induced microstructure instability on the corrosion/oxidation and stress corrosion cracking (SCC) susceptibility of Type 310S stainless steel in supercritical water (SCW). The motivation for this study is the exiting knowledge gap associated with these two material performance aspects, which are considered essential to help guide the selection of a fuel cladding material for application in the Canadian Gen IV Supercritical Water-Cooled Reactor (SCWR) design concept. The focus was placed on the pressurized light water coolant outlet conditions (25 MPa SCW at 500-550 °C) considering that the likelihood for corrosion and SCC in combination with microstructure instability resulting from thermal ageing is presumed to be relatively high. The working hypotheses being that (i) Cr-rich intermetallic precipitate formation would serve to bind alloyed Cr, thus preventing the formation of protective Cr-rich oxide scales required for suitable corrosion/oxidation performance and (ii) Cr-rich grain boundary precipitate formation would serve to enhance SCC susceptibility by either sensitizing (chemical factor) or embrittling (mechanical factor) grain boundaries. Two limiting thermal ageing treatments were evaluated. The ‘S’ treatment, designed to ‘sensitize’ the microstructure, induced significant Cr-rich M23C6 carbide precipitation on the grain boundaries concomitant with the development of adjacent Ph. D. Thesis, Yinan Jiao McMaster University, Materials Science and Engineering III Cr-depleted zones. This limiting case represented a short-term exposure condition for the fuel cladding. The ‘TT’ treatment, designed to ‘mechanically embrittle’ the microstructure, induced significant Cr-rich sigma (σ)-phase precipitate formation on the grain boundaries and within the grains. This limiting case represented a long-term exposure condition for the fuel cladding. The ‘SA’ treatment, designed to solution anneal the microstructure, served as the baseline microstructure exhibiting a low extent of microstructure instability. Long term thermal ageing of Type 310S austenitic stainless steel at the maximum expected fuel cladding temperature operation temperature range of Canadian-SCWR concept to produce the TT material induced significant σ phase precipitate formation within the grains and on grain boundaries, all without the concomitant adjacent Cr-depleted zone formation. Such microstructure instability did not have a major effect on the general corrosion/oxidation susceptibility since the thermally-aged material exhibited a similar weight gain and oxide scale structure after exposure in 25 MPa SCW at 550 °C as the solution annealed material, albeit after relative short exposure time (500 h). The likely reason for this was the discontinuous nature of the σ phase precipitate formation. The remaining research conducted was aimed towards investigating the effect of thermal ageing on the intergranular SCC susceptibility of Type 310S stainless steel in SCW. A ‘first approximation’ attempt to isolate and assess the potential contribution of Ph. D. Thesis, Yinan Jiao McMaster University, Materials Science and Engineering IV the likely chemical factor (sensitization) and mechanical factor (embrittlement) to the overall cracking mechanism. The overall ranking in terms of an increasing degree of sensitization, as determined by Double Loop – Electrochemical Potentiokinetic Reactivation (DL-EPR) testing, of three heat-treated materials studied is: SA ~ TT << S. The ranking reflects the controlling role played by Cr-depleted zones, which were only observed in the S material. The absence of a Cr-depleted zone accompanying the σ phase grain boundary precipitates in the TT material indicates that the treatment time of a 1000 h was sufficient to “self-heal” the Cr-depleted zone, which was expected to have formed during the early stages of the treatment at 800 °C. Bulk uniaxial tensile measurements revealed both the S and TT material were embrittled relative to the SA material. The associated fractography revealed that the grain boundaries in the S material were sufficiently embrittled to cause intergranular fracture, whereas the grain boundaries in the TT material were not sufficiently embrittled to cause intergranular fracture. Classic micro-void formation involving a M23C6/matrix decohesion process occurs on the S material grain boundaries. Intergranular facture occurs as these micro-voids coalesce on the grain boundaries. In contrast, micro-void formation involving cracking of the large hard σ phase precipitates occurs on the TT material grain boundaries. Transgranular ductile fracture of the softer austenite matrix occurs as these micro-voids coalesce across the grains as the softer grains need to accommodate the stress after fracture of hard σ phase Ph. D. Thesis, Yinan Jiao McMaster University, Materials Science and Engineering V precipitates. The effect of thermal ageing on the SCC susceptibility of Type 310S in SCW was evaluated using the Slow Strain Rate Test (SSRT) technique, straining samples to 10% in 25 MPa SCW at 500 °C. The results were interpreted in light of the likely contributions of the chemical (sensitization) and mechanical (embrittlement) factors as revealed by the above mentioned research. The sensitized material exhibited well-developed intergranular SCC after straining to 10% (about 7.5% plastic strain). The TT material exhibited neither intergranular nor transgranular SCC susceptibility under the same exposure conditions. The localized deformation model for SCC initiation was combined with a void-assisted film-cleavage model for SCC propagation to explain the SCC susceptibility of thermally-aged Type 310S stainless steel in SCW. |
URI: | http://hdl.handle.net/11375/22689 |
Appears in Collections: | Open Access Dissertations and Theses |
Files in This Item:
File | Description | Size | Format | |
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Jiao_Yinan_201712_PhD.pdf | 5.96 MB | Adobe PDF | View/Open |
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