The Study on Stress Corrosion Cracking of 304L Stainless Steel and Its Welds with Salt-Crust Deposits

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Project title

Code/計畫編號

NSC101-NU-E019-001-NU

Translated Name/計畫中文名

304L不銹鋼及銲件之鹽霧應力腐蝕研究

Project Coordinator/計畫主持人

Wu Kai

Funding Organization/主管機關

National Science and Technology Council

Co-Investigator(s)/共同執行人

蔡履文

Department/Unit

Department of Optoelectronics and Materials Technology

Website

https://www.grb.gov.tw/search/planDetail?id=2382350

Year

2012

Start date/計畫起

01-01-2012

Expected Completion/計畫迄

31-12-2012

Bugetid/研究經費

586千元

ResearchField/研究領域

原子能工程
機械工程

沃斯田鐵系不銹鋼具有優良之耐蝕性及高經濟效能，因此一般核電廠廣泛使用沃斯田鐵系不銹鋼作為管路材料，並以銲接製程進行配管或組裝構件。此系列不銹鋼構件或管材於核電廠連續使用一段時間後，常在銲道及鄰近區域發生裂紋。一般認為沃斯田鐵系不銹鋼因碳化鉻析出產生敏化現象，將導致腐蝕及應力腐蝕破裂；此外，近年來多項研究結果顯示，銲接或機械加工現象，尤其是大量冷加工後，將引發鋼材之塑性變形，是誘發不銹鋼發生破裂之主要肇因。已知介穩型沃斯田鐵系不銹鋼如AISI 304L 及316L，會因塑性變形而引發麻田散鐵變態，而於使用過程中，持續加熱將導致冷加工之304L 不銹鋼，加速產生碳化鉻析出，進而影響其耐蝕性，甚至造成應力破裂。鑑於核廢料之乾式儲存用鋼槽將採304L 不銹鋼，且須經冷加工及銲接成形後使用，因銲接及冷加工所導致的應力，以及後續的敏化現象，恐導致此一設施在含氯鹽及濕氣腐蝕條件下引發鋼材設施的劣化，因此，本研究工作主要針對304L 不銹鋼板材及銲件，進行一系列鹽霧應力腐蝕劣化與防治之評估，將以加熱鹽霧模擬乾式儲存槽在溼熱環境下，評估測試不同程度之沈積鹽層(salt crust)，對不銹鋼及其銲接試片鹽霧應力腐蝕龜裂行為與劣化速率，建立乾式儲存用核廢料密封不銹鋼筒環境效應，促進其完整性劣化之評估方法，探討防治氯鹽誘發不銹鋼應力腐蝕龜裂之最佳銲接程序，與其他防治措施效益。此外，建立完整的鹽霧應力腐蝕劣化機制，並評估最佳的防治對策，進而提升不銹鋼材抗鹽霧應力腐蝕劣化的能力及效益，亦是本研究之重點。Austenitic stainless steels (SSs) have been extensively applied to petrochemical and power industries because of their excellent general corrosion-resistance and low material costs. The line-pipe systems in the nuclear power plant are constructed mainly using austenitic stainless steels (SSs) including AISI 304L and 316L. Generally, arc welding processes are used to join the infrastructure and assembly those isolated components. Thus, cracks can be found in the regions near the weld metal after a certain periods of service. It is well known that the sensitization of SSs by forming chrominum-carbide precipitates is considered to deteriorate the corrosion resistance and enhance stress corrosion cracking (SCC). Furthermore, numerous results have reported recently that the residual stress induced by welding or machining (especially cold working) plays an important role in inducing SCC in SSs. Commercial SSs, such as AISI 304L and 316L, are metasatble and easy to transform to martensite steels during plastic deformation, typically caused by cold working in practical applications. Continuous heating also leaded to enhance the SCC of cold-worked 304L by forming larger amounts of chromium carbides near grain boundary (so called sensitization), which reduced the corrosion resistance of SSs and caused to the stress-induced fracture. A typical application of 304L was to use it in the dry-storage nuclear-fuel canister, in which the steel may suffer severe cold-worked deformation and subject to a welding procedure. Very likely, the mechanical cold-working and welding processes will induce residual stress, and result in sensitization of the steel, thereby leading to the degradation of SSs in NaCl-containing environments during industrial applications. Thus, the main aim of this proposal is to systemically study the SCC degradation and its prevention of 304L in dry-storage nuclear-fuel canister with salt-crust deposits under heating at 80 - 120oC. The mechanism of SCC and the degradation of SSs will also be explored. The entire evaluations and possible corrosion-prevention methods for the SCC of 304L will be proposed. It is of essence to wholly establish the corrosion mechanisms of 304L under SCC environments with salt-crust deposits and the possible ways to enhance the corrosion resistance. The analyses of phase constitution after cold rolling and the effect of sensitization on the SCC susceptibility of 304L SS deposited with salt-crust will be performed in this study. The final goal of this research is not only to submit all results to be published in international journals but also to wholly establish the systematically high-performance database for the industrial applications.

Keyword(s)

304L 不銹鋼
鹽霧應力腐蝕
銲接程序
腐蝕防治
304L stainless steel
stress corrosion cracking with salt-crust deposits
welding process
corrosion prevention

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The Study on Stress Corrosion Cracking of 304L Stainless Steel and Its Welds with Salt-Crust Deposits

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