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Please use this identifier to cite or link to this item: http://ntour.ntou.edu.tw:8080/ir/handle/987654321/9564

Title: 304L不銹鋼鹽霧應力腐蝕裂化與防治(III)
The Evaluation of Stress-Corrosion-Cracking and the Protection on 304L Stainless Steel with Salt-Crust Deposits(III)
Authors: 開物;蔡履文
Contributors: NTOU:Institute of Materials Engineering
國立臺灣海洋大學:材料工程研究所
The nuclear waste in nuclear power plant was stored in the dry-storage nuclear-fuel canister constructed by using 304L stainless steel which was cold rolled and welded . The residual stress induced by cold working and welding, and the sensitization of the steel would lead to the degradation of the instrument in NaCl-containing environments during industrial applications. This project is observing the changes in microstructure、hardness and the mechanical characteristic of 304L stainless steel after the cold working and welding processes. In addition, the notch tensile test in air and 40wt% MgCl2 solution at 800 C was utilized to evaluate the effect of the phase transformation induced by cold working and the sensitization caused by welding、PWHT and after a period of service on the SCC characteristic. The results showed that he hardness would increased after differentlevel cold working. The higher the cold working level, the higher the degree of deformation and phase transformation is in the grain and resulted in the higher hardness. The strain-induced martensite included the non-magnetic ε-martensite (HCP) and the magnetic α'-martensite (BCT). The ε-martensite mainly formed parallel to the slip band while the α'-martensite located at the intersection of ε-martensite or slip band and along the grain boundary. After the sensitization treatment, the strain-induced martensite would recover to the γ-phase partly, and resulted in the precipitate of chromium carbide (Cr23C6), moreover, the δ-phase in the weld metal also recovered to γ-phase partly during sensitization treatment. In cold-worked weldment, the sensitization treatment resulted in the carbide precipitate at the δ/γ boundary, and the γ-phase around the carbide grew into the δ-phase. The results of 1 notched tensile test indicated that cold working increased the defect density in the material and enhanced the trapping effect of the hydrogen atoms;hence, it restricted the hydrogen atoms diffuse to the plastic zone ahead the notch and made the sensitivity of hydrogen embrittlement lower than the un-cold worked specimen. The sensitization led to the reduction in defect density induced by cold working, i.e., the trapping site decreased, and the carbides precipitated at the grain boundary resulted in the depletion of alloy composition near the boundary. Therefore, the γ-phase in the alloy depletion zone near the boundary would transform to martensite easily during notch tensile test and provided a fast diffusion path for hydrogen atoms, and increased the sensitivity of hydrogen embrittlement. The flat fracture zone of all specimens tested in air revealed the ductile fracture with dimples. In MgCl2, except the cold-worked and sensitized specimens showed intergranular fracture, the fracture appearance of the others showed the transgranular fracture with deep secondary cracks. Sensitized and unsensitized weldment also revealed the transgranular fracture with deep secondary cracks on the fracture surface tested in MgCl2, however, the cold-worked weldment revealed the quasi-cleavage fracture mixed with dimples. After sensitization treatment of the cold-worked specimen, although the α'-phase also formed at the skeletal boundary;even so, as compared to the base metal, it lacked the twin boundary which can also provide a fast diffusion path for hydrogen atoms and the fracture surface still revealed the quasi-cleavage mixed with dimples. The EBSD results indicated that the crack growth path of the secondary on the fracture surface is the location of the strain-induced martensite.
Date: 2009-01
Issue Date: 2011-06-28T07:09:32Z
Publisher: 行政院國家科學委員會
Abstract: 核電廠中之核廢料使用核廢料乾式儲存用鋼槽儲存,為304L 不銹鋼經冷加工及銲接成形後使用。冷加工及銲接所導致的應力,以及後續的敏化現象,會導致此設備在含氯鹽及濕氣腐蝕條件下產生劣化。本計畫針對 304L 不銹鋼之板材,經冷加工滾軋成形及銲接等製程後,觀察其顯微組織、硬度及機械性質等之變化。此外,利用於大氣環境與 800C 之 40 wt% 氯化鎂環境中進行缺口拉伸測試,評估冷加工導致之相變態、及銲接、銲後熱處理及後續使用時持溫所導致之敏化現象,對其應力腐蝕特性之影響。
實驗結果顯示,母材與銲道試片經不同程度之冷加工後,硬度值隨之增加。冷加工程度越高者,晶格扭曲越嚴重,相變態越多者,硬度越高。應變誘發麻田散鐵相包括不具磁性之 ε 麻田散鐵(HCP 結構)與具磁性之 α'麻田散鐵(BCT 結構)。ε 相主要形成於平行滑移帶的位置,而 α'相則位於 ε 麻田散鐵或滑移帶之交界處及沿晶界處。敏化熱處理後,應變誘發麻田散鐵將部分回復成基地相(γ 相),且會有碳化物(Cr23C6)析出,此外,銲道中 δ 相亦因敏化部份回復成 γ 相。經冷加工之銲道敏化後,碳化物易析出於 δ 相與 γ 相之介面上,且碳化物周圍之 γ 相長進 δ 相中。缺口拉伸測試結果顯示,冷加工導致材料中缺陷密度增加,並對氫原子產生捕集效應,使氫原子不易擴散至缺口拉伸之裂縫前端受應變區域,使氫脆敏感性較未冷加工者低。敏化後因回復使冷加工導致之缺陷密度降低,即氫捕集位置減少,並於晶界析出碳化物,使晶界附近合金成分下降。因此,晶界附近合金缺乏區中之 γ 相於後續缺口拉伸測試時,易生成麻田散鐵變態,提供氫原子快速擴散路徑,使氫脆敏感性提升。所有於空氣環境中測試之試片,於平坦破斷區皆呈現延性之韌窩狀破斷形貌。於氯化鎂環境測試下,除板材經冷加工再敏化後之試片呈現沿晶破斷外,其餘皆呈現具較深二次裂縫之穿晶破斷。敏化與未敏化之銲道試片,於氯化鎂環境測試下亦呈現具較深二次裂縫之穿晶破斷,而銲道經冷加工後,破斷面呈現準劈裂與韌窩狀之混合破斷形貌。再敏化後,雖於骨骼狀邊界上亦可生成 α'相,但與母材相較下,缺少亦可提供氫原子擴散之雙晶界,破斷面仍呈現準劈劣與韌窩之混合形貌。EBSD 觀察結果顯示,破斷面上之二次裂縫成長路徑皆為應變誘發麻田散鐵之位置。0
Relation: NSC98-NU-E019-001
URI: http://ntour.ntou.edu.tw/ir/handle/987654321/9564
Appears in Collections:[材料工程研究所] 研究計畫

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