個人論文摘要



中文摘要：
	本研究主旨在探討不同熱處理狀態下平均應力與頻率效應對17-4 PH不銹鋼腐蝕疲勞性質之影響。比較在腐蝕環境中(3.5 wt% NaCl)不同的熱處理、平均應力及負荷頻率下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。在電化學實驗方面，以開路電位監測及循環陽極動態極化掃描測量材料在腐蝕環境下鈍化的情形。此外，亦利用掃描式電子顯微鏡(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，腐蝕環境下之高週疲勞性質，不論應力比為R = 0.1或R = 0.5，其H900頂時效處理都比H1150過時效處理有較高之腐蝕疲勞壽命，SA固溶處理介於兩者之間。而在疲勞裂縫成長速率方面，應力比條件雖然不同，但三種熱處理下之疲勞裂縫成長速率差距卻不大，顯示高週腐蝕疲勞壽命主要還是由裂縫起始階段所主控。在頻率效應方面，負荷頻率的改變對於高週疲勞性質及疲勞裂縫成長速率有不同的影響。在裂縫成長實驗中，保護膜會因裂縫延伸的過程而破壞，且較不易回復，頻率降低使材料有更充分的時間與環境反應，因而造成裂縫成長加速。在高週疲勞實驗中，在長壽命區1 Hz頻率條件下的腐蝕疲勞性質優於20 Hz，此現象與保護膜的回復有關，1 Hz負荷條件有較足夠的時間使已破碎之保護膜回復，減緩裂縫生成的時間而延長腐蝕疲勞壽命。由SEM觀察中得知，17-4 PH不銹鋼經H1150過時效處理及SA固溶處理，其高週軸向試棒破斷面在慢速成長區接近裂縫起始處附近可觀察到環狀孔蝕區。而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 influence of mean stress and loading frequency on the corrosion fatigue properties of 17-4 PH stainless steels in different heat treatments. In particular, the fatigue life and fatigue crack growth (FCG) rate in 3.5 wt% NaCl solution for various tempers, load ratio, and loading frequency were made a comparison. High-cycle fatigue (HCF) axial smooth-surface and FCG pre-cracked CT specimens were prepared in three different tempers, namely solution-annealed (SA), peak-aged (H900), and overaged (H1150) conditions. The effect of environmentally assisted cracking mechanisms on the degradation of fatigue resistance was characterized. Fractography and microstructure analyses with scanning electron microscopy (SEM) were conducted to determine the corrosion fatigue crack initiation and propagation modes. S-N curves showed that smooth-surface specimens in H900 temper under load ratios of R = 0.1 and R = 0.5 at 1 Hz exhibited longer corrosion fatigue lives than the H1150 ones while those in SA temper lie between them. Under the similar loading conditions, the corrosion FCG rates for all three tempers were not significantly different. This implies that crack initiation and stage I cracking stages played the major role in determining the entire corrosion fatigue life for smooth-surface specimen.The effects of loading frequency on HCF and FCG in salt water for this precipitation-hardening stainless steel are different. Passive film would rupture and hardly recover in the process of crack growth. As a result, the FCG rates at a lower frequency (1 Hz) were greater than those at a higher frequency (20 Hz) due to the longer time available for corrosive reaction at each cycle. However, at the long life region, 1 Hz cyclic loading generated longer fatigue life for smooth-surface specimens than did 20 Hz cyclic loading. This might be explained by the fact that the ruptured passive film on smooth-surface specimen could be recovered to a greater extent at 1 Hz over 20 Hz cyclic loading due to longer time available for recovery at each cycle. Therefore, the crack initiation was deferred and the fatigue life was longer for 1 Hz cyclic loading at long life region.Fractography analyses indicated that corrosion pits around crack initiation site were found in slow crack growth region for HCF specimens at H1150 and SA tempers. 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.