本研究主旨在探討不同環境參數對不同熱處理狀態下的Custom 450不銹鋼腐蝕疲勞性質之影響，探討的環境參數包括鹽水溶液之酸鹼度、溫度及氯離子濃度。比較三種熱處理狀態在空氣及四種鹽水溶液中之高週疲勞及疲勞裂縫成長的差異性，並以電化學試驗觀測在不同環境下其電化學行為之變化情形。此外，亦利用掃描式電子顯微鏡(SEM)觀察疲勞破斷面，以了解裂縫的生成及成長模式。實驗結果顯示，Custom 450不銹鋼在空氣及四種鹽水溶液中之高週疲勞行為，皆為H900頂時效處理比H1150過時效處理具有較長之疲勞壽命，SA固溶處理介於兩者之間。而在長裂縫成長實驗(stage II)方面，H900頂時效處理皆比H1150過時效處理具有較快之裂縫成長速率，SA固溶處理介於兩者之間。由此顯示其高週疲勞壽命主要花在裂縫起始階段，而不在裂縫延伸階段。就同一種熱處理的高週疲勞而言，在四種鹽水溶液中之整體疲勞壽命皆明顯比空氣中來的短，但對長裂縫成長而言，在室溫的二種鹽水溶液中及空氣中之裂縫成長速率卻相差不大，由此顯示其整體疲勞壽命下降的主因為鹽水溶液縮短了疲勞裂縫起始所需的時間。降低鹽水溶液的pH值會使氯離子活性增加而導致腐蝕速率上升，進而加劇蝕孔的生成並縮短裂縫起始所需時間，因此對整體疲勞壽命有很大之影響，但對長裂縫成長的影響則不大，這可能是由於在腐蝕溶液中之裂縫尖端自行酸化所造成。對裂縫成長而言，提高溶液溫度會增加裂縫之表面活化能以及腐蝕速率，並促使氫脆之發生，因此會明顯提升長裂縫之成長速率。而提高氯離子濃度會使腐蝕速率上升，對裂縫成長也有不利之影響。而在高週疲勞方面，提高溶液溫度或氯離子濃度，皆對材料之鈍化行為有不利影響，並使腐蝕形態傾向均勻腐蝕，但反而減低了因孔蝕所產生之應力集中效應，因此對整體疲勞壽命的影響較小。由SEM觀察中得知，空氣中之疲勞裂縫起始處通常由塑性變形所造成，且較難辨識其明確之起始位置，而在腐蝕環境中之裂縫大多從蝕孔處或因腐蝕作用衍生之表面缺口起始，但有時也會由介載物周圍起始。 The aim of this study is to investigate the influence of environmental factors, including pH value, temperature, and NaCl concentration, on the corrosion fatigue properties of Custom 450 stainless steels in different heat treatments. In particular, the high-cycle fatigue (HCF) and fatigue crack growth (FCG) behavior in air and several salt water environments were made a comparison for 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. The electrochemical properties for these tempers in various aqueous environments were also made a comparison. Fractography and microstructural analyses with scanning electron microscopy (SEM) were conducted to determine the corrosion fatigue crack initiation and propagation modes.Results showed that, in air and corrosive environments, smooth-surface specimens in H900 temper exhibited longer fatigue lives than the H1150 ones while those in SA temper lie between them. However, the H1150 temper exhibited superior FCG resistance to the other two tempers in air and corrosive environments. 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 environmental effects exerted more detrimental influence on the HCF behavior than on the stage II cracking behavior for the given alloys. A reduction in the pH value of salt water increased the activity of chloride ion and its influence on the crack initiation stage leading to a decrease in fatigue strength. An increase in the temperature of salt water from room temperature to 80℃ caused a remarkable increase in the FCG rate due to the enhanced activation energy of crack growth and diffusion rate of hydrogen which promotes hydrogen embrittlement (HE) effect at the crack tip. In addition, increasing the NaCl concentration of salt water resulted in a slightly higher FCG rate. However, an increase in the temperature or chloride ion concentration of salt water caused negligible effects on the HCF properties for the given alloys.Fractography analyses indicated that fatigue crack initiation sites could not be clearly indentified for HCF specimens tested in air while, in the corrosive environments, fatigue cracks initiated mostly from corrosion pits.
