中文摘要
本論文主要探討22%Cr雙相不銹鋼在含氯離子水溶液中之應力腐蝕破裂(SCC)及腐蝕疲勞裂縫(CFC)生長性質，同時比較氮含量的不同對22%Cr雙相不銹鋼之應力腐蝕破裂敏感性及腐蝕疲勞裂縫生長速率的影響。此外，以316L沃斯田體系不銹鋼及430肥粒體系不銹鋼作為對照組，藉以探討不同的晶體結構組織，於應力腐蝕破裂及腐蝕疲勞裂縫生長行為上所扮演的角色。
實驗結果顯示22%Cr雙相不銹鋼中氮含量由0.103wt%增加至0.195wt%，其拉伸強度及延性皆會隨之增加。再者，雙相不銹鋼中的 相的硬度會較 相為高，且兩相的硬度亦隨氮含量的增加而增加，這主要是因為氮所提供之固溶強化的效果。此外，雙相不銹鋼中 相的比例及大小皆會隨著氮含量的增加而增加
22% Cr雙相不銹鋼在100 C、相同莫耳濃度之不同陽離子氯化物水溶液中，其Enp值大小依序為NaCl CaCl2 MgCl2。然而，雙相不銹鋼在高溫氯離子溶液中的Enp值皆會隨著氮含量的增加而增加，且隨著溶液中氯離子濃度及溫度的提高，其增加的幅度越不明顯。至於雙相不銹鋼的蝕孔形態及位置，在NaCl水溶液中傾向成核於 相或 / 相界，而往 相來成長；在MgCl2水溶液中則傾向成核於 相。然而，在CaCl2水溶液中，雙相不銹鋼的局部腐蝕形態則有別一般的孔蝕，而是以 相的選擇性腐蝕形式出現。
慢應變拉伸試驗的結果顯示：22%Cr雙相不銹鋼在80 C、3.5wt% NaCl水溶液中，並無SCC的出現，但卻發生動態應變時效(DSA)的現象。然而，當氮含量為0.153wt%時，雙相不銹鋼於100 C、40wt% CaCl2水溶液中，應變速率為4.1 10-7 s-1下，其抵抗SCC的能力最差，再者，氮含量對SCC敏感性的影響主要與滑移溶解機制及肥粒體相的選擇性溶解有關。此外，陽離子對各個不銹鋼在氯離子水溶液中之SCC敏感性的影響程度，對22%Cr雙相不銹鋼而言，依序為Mg2+＞Ca2+＞Na+；但對沃斯田體系不銹鋼及肥粒體系不銹鋼而言，則依序為Mg2+ Ca2+＞Na+。
由疲勞裂縫生長試驗結果可發現，在80 C、3.5wt% NaCl水溶液中，腐蝕疲勞生長速率以430肥粒體系不銹鋼最高，而其腐蝕疲勞破裂以劈裂形態的脆性破裂為主。對22%Cr雙相不銹鋼而言，腐蝕疲勞裂縫生長主要與肥粒體相的脆性破裂現象有關。當負荷頻率為1Hz時，不論在空氣中或在3.5wt%NaCl(80 C)水溶液中，氮含量的改變對雙相不銹鋼的疲勞裂縫生長速率影響並不顯著。然而，在3.5wt%NaCl(80 C)水溶液中，負荷頻率為0.1Hz時，雙相不銹鋼的疲勞裂縫生長速率，於氮含量為0.103wt%時，會明顯較其他氮含量之雙相不銹鋼來的大。

Abstract
Stress corrosion cracking (SCC) and corrosion fatigue cracking (CFC) behaviors of 22%Cr duplex stainless steel (DSS) were investigated. Slow strain rate testing (SSRT) and fatigue crack growth (FCG) rate measurement were employed in chloride solutions at high temperature to evaluate the SCC and CFC susceptibility. The effects of nitrogen content, phase, and environment in DSS on SCC and CFC behaviors are also discussed in this study. In addition, AISI 316L austenitic stainless steel (SS) and AISI 430 ferritic SS were also used for comparison purpose. The experimental results showed that the tensile strength and ductility of 22%Cr DSS increased with increasing amount of nitrogen content (in range of 0.103 to 0.195 wt%). Furthermore, nitrogen also caused increase of the microhardness of both ferrite and austenite. Similarly, nitrogen addition also increased the volume percentage and phase size of austenite in DSSs.
The pitting nucleation potential (Enp) of 22%Cr DSS after potentiodynamic polarization measurement in three chloride solutions with different cations at 100°C varies in following order: NaCl CaCl2 MgCl2. In 40wt% CaCl2 solution at 100°C, selective corrosion of ferritic phase in 22%Cr DSS was observed. Besides, pits in NaCl solutions could initiate in phase or at the boundary between and phase, and propagated in phase. On contrary, in 34wt% MgCl2 solution at 100°C, pits initiated preferentially in phase. However, the Enp increased with increasing nitrogen content in DSSs in all hot chloride solutions employed.
The slow strain rate testing results indicated that 22% Cr DSSs were resistant to SCC in 3.5 wt% NaCl solution at 80°C under two strain rates employed. But, dynamic strain aging occurred in 22% Cr DSSs during SSRT in preceding solution at higher strain rate. In hot 40wt% CaCl2 solution at lower strain rate, 22%Cr DSS with nitrogen content of 0.153wt% was more susceptible to SCC. However, slip dissolution and selective corrosion of ferrite participated in SCC process of 22%Cr DSSs. The ability of cation to promote SCC in chloride solution changes in the following order: Mg2+ Ca2+ Na+, for 22%Cr DSS; Mg2+ Ca2+ Na+, for 316L austenitic SS and 430 ferritic SS.
The FCG rate measurement results indicated that the FCG rate in 3.5wt% NaCl solution at 80°C was found to be the highest for 430 ferritic SS. The high FCG rate in 430 ferritic SS was associated with cleavage-like brittle failure. For 22%Cr DSS, accelerated FCG in hot NaCl solution was mainly attributed to brittle failure of the constituent ferrite phase. Furthermore, the results revealed that nitrogen in the range of 0.103 to 0.195 wt% did not significantly affect the FCG rate of 22% Cr DSS in air and in 3.5 wt% NaCl solution at 80°C, at 1 Hz. However, at a frequency of 0.1 Hz and in 3.5 wt% NaCl solution at 80°C, a higher FCG rate was found in the low-N DSS (0.103 wt% N) as compared with those with high nitrogen content (> 0.153 wt% N). As the nitrogen content increased, the content of ferrite phase, which was susceptible to hydrogen embrittlement, in DSS decreased. As a result, the corrosion fatigue crack propagation rate was reduced.

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