不同環境下之Custom 450不銹鋼腐蝕疲勞性質研究

Corrosion Fatigue of Custom 450 Stainless Steel in Different Environments

本研究主旨在探討不同環境參數對不同熱處理狀態下的汽機葉片用Custom 450不銹鋼腐蝕疲勞性質之影響。實驗結果顯示，在四種鹽水溶液中之高週疲勞壽命皆較空氣中來的低，可能原因為氯離子所造成的蝕孔加速了裂縫的起始，因此縮短了整體疲勞壽命。而在不同環境中皆為H900頂時效具有較高疲勞強度，顯示了疲勞壽命主要由強度所主導。降低鹽水溶液的pH值會使氯子活性增加並導致腐蝕速率上升，使蝕孔在更短時間內生成，因此對整體疲勞壽命有很大之影響。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，但有時也會由介在物周圍起始。

The aim of this study is to investigate the influence of environmental factors, including pH value, temperature, and NaCl concentration, on the high-cycle fatigue (HCF) properties of Custom 450 stainless steels in different tempers. Results showed that, in air and various corrosive environments, smooth-surface specimens in H900 temper exhibited longer fatigue lives than the H1150 ones while those in SA temper lie between them. 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 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 identified for HCF specimens tested in air while, in the corrosive environments, fatigue cracks initiated mostly from corrosion pits.