AISI 347不銹鋼之敏化程度及環境誘發破裂研究

The Degree of Sensitization and Environmentally Induced Cracking of AISI 347 Stainless Steel

本研究探討AISI 347不銹鋼在不同條件熱處理之微觀組織變化，以及其敏化程度的鑑定，在不同溫度和濃度NaCl水溶液中的環境誘發破裂性質，亦利用慢應變速率拉伸試驗加以分析。研究結果顯示，經650℃熱處理之347不銹鋼沒有敏化現象，而EPR（Electrochemical potentiokinetic reactivation, EPR）測試曲線則有明顯的電峰值，但並非源自晶界的鉻缺乏區的腐蝕，而是由於孔蝕的緣故。慢應變速率拉伸試驗結果顯示，在25 ℃，pH= 2之3.5wt % NaCl水溶液中，或是90℃，PH= 2之1wt% NaCl水溶液中，經650℃熱處理（5或15小時）之347不銹鋼，其抗拉強度與延伸率與經固溶處理者相似，除了在斷面外緣有限範圍內有應力腐蝕破裂跡象外，基本上347不銹鋼抵抗環境誘發破裂的性質較304不銹鋼為優。

The effect of heat treatment on the microstructural change and the evaluation for various degree of sensitization of 347 SS were investigated. The sensitivity to environmentally-assisted cracking behavior was also studied using slow strain rate testing technique. The experimental results indicated that heat treatment at 650℃ did not cause the occurrence of sensitization. The reactivation peaks appeared in EPR (Electrochemical potentiokinetic reactivation) curves were not derived from the corrosion in the Cr depleted grain boundaries, but due to pitting corrosion. Slow strain rate testing results revealed that in 3.5 wt% NaCl solution of pH=2 at 25 or in 1 wt%NaCl solution of pH=2 at 90 , the tensile strength and percentage of elongation of 347 SS heat treated at 650℃ for 5 or 15h were almost the same as that of solution annealed one. Except at the edge of the fracture surface where stress corrosion cracking was observed, 347 SS was more resistant to environmentally-assisted cracking than 304 SS.