中文摘要：
本研究主旨在探討不同熱處理狀態對17-4 PH與Custom 450不銹鋼腐蝕疲勞性質之影響。比較在腐蝕環境中(3.5 wt% NaCl) 不同熱處理的疲勞壽命和疲勞裂縫成長的差異性。在疲勞裂縫成長實驗方面，同時記錄裂縫閉合效應，以瞭解裂縫閉合效應對疲勞裂縫成長速率之影響。此外，亦利用光學顯微鏡(OM)和掃描式電子顯微鏡(SEM)，對金相組織與破斷面加以觀察分析，以瞭解裂縫的生成及成長模式。實驗結果顯示，兩款不銹鋼在腐蝕環境下之高週疲勞性質，其H900頂時效都比H1150過時效處理有較高之腐蝕疲勞壽命，固溶處理介於兩者之間。而在疲勞裂縫成長速率方面，兩款不銹鋼之過時效處理由於有較佳之韌性及較佳耐氫脆抵抗力，使其有比頂時效及固溶處理較低之疲勞裂縫成長速率。Custom 450不銹鋼在各種對應的熱處理條件下都較17-4 PH不銹鋼有較佳之韌性，使其在相對應熱處理下具有較低的疲勞裂縫成長速率。由高週疲勞性質及疲勞裂縫成長速率之比較顯示此兩款不銹鋼在腐蝕環境中，主導高週疲勞壽命長短的要素為裂縫起始階段，而不在裂縫延伸階段，由於頂時效比過時效及固溶處理有較高降伏強度及較佳之耐孔蝕性質，因而有較高之抗裂縫起始阻力，導致其整體疲勞壽命較高。兩款不銹鋼在同一熱處理條件下之高週腐蝕疲勞壽命互相比較亦顯示，具有較高降伏強度之材料成分與熱處理組合，亦相對具有較長的高週腐蝕疲勞壽命。由SEM觀察中得知，腐蝕疲勞的起源是由腐蝕液侵蝕的蝕孔處形成，並有羽毛狀河流紋路由起始處向外擴散延伸，裂縫延伸呈穿晶走向，在延伸中留下疲勞紋路，最後破斷則呈延性破壞模式。
Abstract：
This study investigated the corrosion fatigue properties of 17-4 PH and Custom 450 stainless steels in different heat treatments. In particular, the fatigue life and fatigue crack growth rate in 3.5 wt% NaCl solution for specimens in various tempers were compared. The effect of environmentally induced cracking mechanisms on the degradation of fatigue resistance was characterized. Fractography and microstructure analyses with scanning electron microscopy (SEM) and optical microscopy (OM) were conducted to determine the corrosion fatigue crack initiation and propagation mode. Results showed in both 17-4 PH and Custom 450 stainless steels the smooth specimens in H900 peak aged temper exhibited longer corrosion fatigue lives than the H1150 overaged ones while those in solution annealed temper lie between them. However, the overaged temper in these two stainless steels exhibited superior corrosion fatigue crack growth resistance to the other two tempers as a result of its greater toughness and lower susceptibility to hydrogen embrittlement. Custom 450 stainless steel in each temper had greater toughness than 17-4 PH one leading to a lower fatigue crack growth rate. Comparison of S-N and fatigue crack growth rate curves indicated that crack initiation stage played the major role in determining the entire corrosion fatigue life for both stainless steels in various tempers. The peak aged process which generated the highest yield strength exhibited the greater resistance to crack initiation and the longer corrosion fatigue life, as compared to the overaged and solution annealed conditions. In overall comparison, the greatest corrosion fatigue resistance of smooth specimen among these two stainless steels in various tempers is possessed by the temper with the highest yield strength. Fractography analyses indicated that corrosion fatigue cracks initiated mostly from corrosion pits followed by feathery river line features and striations as the cracks grew stably until final fracture. The corrosion fatigue cracks propagated in a transgranular mode for all tempers tested in this study.