English  |  正體中文  |  简体中文  |  Items with full text/Total items : 26988/38789
Visitors : 2314410      Online Users : 26
RC Version 4.0 © Powered By DSPACE, MIT. Enhanced by NTU Library IR team.
Scope Adv. Search
LoginUploadHelpAboutAdminister

Please use this identifier to cite or link to this item: http://ntour.ntou.edu.tw:8080/ir/handle/987654321/52163

Title: CF8鑄造不銹鋼及模擬熱影響區組織應力腐蝕特性研究
The Effect of Simulated Microstructures on The Stress Corrosion Creaking of CF8 Stainless Steel
Authors: Chang, Hao Han
張顥瀚
Contributors: NTOU:Institute of Materials Engineering
國立臺灣海洋大學:材料工程研究所
Keywords: 鑄造不銹鋼;熱影響區;肥粒鐵;應力腐蝕
cast stainless steels;HAZ;ferrite;SCC;Gleeble
Date: 2018
Issue Date: 2019-05-23T06:19:35Z
Abstract: 本研究是針對不同肥粒鐵含量CF8鑄造不銹鋼,使用Gleeble熱模擬機模擬覆銲後熱影響區組織,從高溫急冷至室溫產生明顯相變態,探討覆銲對CF8熱影響區肥粒鐵含量及形貌的影響,並評估此一相變態對拉伸、衝擊及水環路應力腐蝕下性能之影響。 實驗包含兩種不同肥粒鐵含量鑄造不銹鋼,各為含8% (M)及21% (H) 肥粒鐵試件,經過Gleeble熱模擬加熱處理後分別命名為MG及HG試件,實驗結果顯示:熱模擬加熱後H試件的肥粒鐵含量從原本21%上升到了47%,M試件則從原本的8%增加為18%,顯示銲接熱循環促使CF8鑄造不銹鋼肥粒鐵量大幅增加,尤其以原始高肥粒鐵鑄件更為明顯。硬度量測結果同於材料強度變化,較高強度對應較高硬度。 使用氬銲覆銲不同肥粒鐵含量鑄造不銹鋼,以308L不銹鋼作為填料銲於鑄造不銹鋼表面,比較銲後顯微組織的差異,308L銲道形貌主要呈蕾絲狀肥粒鐵與母材的蠕蟲狀肥粒鐵不同,在靠近熱影響區位置皆會新生成大量肥粒鐵,H試件為分解為多小島塊狀肥粒鐵,而M試件則為蕾絲狀肥粒鐵。 拉伸試驗結果顯示:四種不同肥粒鐵含量試件,除M試件外其餘三者具相當之抗拉強度,延伸率高低依序為M> MG> H> HG,降伏強度則呈現與延性相反趨勢,其高低依序為HG> MG> H> M,試驗結果為肥粒鐵含量影響試件延性與降伏強度。衝擊試驗結果顯示: 衝擊值高低依序為MG> M> HG> H,值得注意的是H試件在經過熱模擬加熱後衝擊值反而是大幅上升。熱模擬試片雖具有較原始鑄件為高之肥粒鐵含量,但細化之顯微組織反而有利衝擊韌性但不利延性。 使用掃描電子顯微鏡觀察拉伸試件破壞面,M及MG試件主要呈現延性孔洞(dimple)的形貌, H和HG試件在破斷面處可以看到有許多二次裂縫,應是低延性高肥粒鐵試件受應力時,易在肥粒鐵與沃斯田鐵界面處產生裂縫,破斷面有類似台階狀平坦破壞,結果顯示高肥粒鐵試件的脆性程度較低肥粒鐵試件還高。而觀察衝擊試件破壞面,H試件出現較多的劈裂形貌,HG試件則呈現低延性之類劈裂之形貌,M與MG試件主要皆為窩穴狀破壞特徵,但M試件的窩穴形貌主要較深,而MG試件窩穴形貌則是大且淺。 於模擬BWR水環路環境應力腐蝕試驗結果顯示:應力腐蝕試件巨觀斷裂外觀較為平整,而空氣中拉斷之試件形變量較大、斷面縮率較大,證明在應力腐蝕環境下會使鑄造不銹鋼延性下降,而MS試件可以看到有明顯劈裂形貌。從微觀來看,H、HG試件成低延性撕裂形貌,破斷面邊緣處可看到有多處二次裂縫,M、MG試件脆性破壞區均呈現平坦劈裂破壞,且可看到有延肥粒鐵邊界延伸的二次裂縫,與在空氣中的窩穴特徵截然不同。鑄造不銹鋼因應力腐蝕,其延性、最大斷裂強度皆會下降。最大斷裂強度下降的幅度會受到肥粒鐵含量的影響,肥粒鐵含量愈高、強度下降的幅度就越少;肥粒鐵含量愈低,強度下降的幅度就越大。高肥粒鐵試件受應力腐蝕作用較早斷裂,以致其破斷面應力腐蝕特徵較不明顯。 從EBSD分析得知,在空氣中拉伸試件會有大量的滑移帶出現,而M試件則會因滑移產生大量的相變態,H試件則較少滑移變形,在應力腐蝕環境下,裂縫成長機制主要為沿肥粒鐵與沃斯田鐵邊界延伸,相較於空氣中沒有大量的滑移與相變態的存在。 實驗第二個部分是將鑄造不銹鋼分別透過時效、回復和固溶處理,評估CF8鑄造不銹鋼於不同熱處理狀態下之肥粒鐵形貌的改變與對機械性能的影響,經SEM與EBSD的觀察,瞭解四種鑄造不銹鋼其肥粒鐵形貌與相變態在空氣中與在應力腐蝕環境的機械特性。 SEM觀察發現固溶後的肥粒鐵形貌與其他試件差異最大,從原來的細長骨骼狀肥粒鐵變為球狀的肥粒鐵。而時效處理後的試件肥粒鐵相硬度最高,固溶處理後的硬度則為最低;四個試件在沃斯田鐵相中的硬度則差異並不大。經由SEM與EBSD分析比較,鑄造不銹鋼在室溫空氣中受到應力作用後,呈現大量的滑移與麻田散鐵的相變態,尤其在固溶處理下最為明顯;而在應力腐蝕環境下相變態的情形則大幅的減少,僅在固溶處理下較為明顯,斷裂面邊緣處也較為平整,呈脆性斷裂形貌。
In this study, Gleeble thermal simulator was used to generate thermal cycle on the CF8 cast stainless steels (SSs) with different ferrite contents to generate the microstructures formed near the weld fusion boundary. All the specimens were subjected to microstructural examinations, tensile, impact, and stress corrosion cracking (SCC) tests. The effects of ferrite contents and morphologies on the mechanical properties were investigated. The SCC susceptibility of simulated specimens was evaluated by using a modified smooth tensile specimen, which was strained at constant extension rate tensile (CERT) tests in high temperature water in a nuclear reactor. The results indicated that an increase in ferrite content in a CF8 substrate will lead to a minor increase in hardness. The thermal cycle provided by Gleeble thermal simulator caused a marked rise in ferrite contents, especially for the CF8 substrate with inherently high ferrite content. Near 50% ferrite was obtained in the high ferrite substrate after being subjected to Gleeble simulation. The yield strength of the test samples increases with increasing ferrite content obviously, however, the ductility varies in reverse manner. It seems that the ultimate tensile strength of the samples reaches a plateau, if the ferrite content is greater than 18%. The impact toughness of low ferrite substrate was minor affected by the Gleeble thermal treatment. For the high ferrite CF8 substrate, it was noticed that Gleeble thermal treatment causes a great increase in impact toughness in comparison with the as-cast substrate. The results indicated the samples with high ferrite content were of shorter rupture life and greater ductility loss than the samples of low ferrite content. Moreover, cleavage-like fracture was seen in all of the samples suffered from SCC in simulated nuclear water condition with 10 ppm dissolved oxygen.
URI: http://ethesys.lib.ntou.edu.tw/cgi-bin/gs32/gsweb.cgi?o=dstdcdr&s=G0010555017.id
http://ntour.ntou.edu.tw:8080/ir/handle/987654321/52163
Appears in Collections:[材料工程研究所] 博碩士論文

Files in This Item:

File SizeFormat
index.html0KbHTML4View/Open


All items in NTOUR are protected by copyright, with all rights reserved.

 


著作權政策宣告: 本網站之內容為國立臺灣海洋大學所收錄之機構典藏,無償提供學術研究與公眾教育等公益性使用,請合理使用本網站之內容,以尊重著作權人之權益。
網站維護: 海大圖資處 圖書系統組
DSpace Software Copyright © 2002-2004  MIT &  Hewlett-Packard  /   Enhanced by   NTU Library IR team Copyright ©   - Feedback