SAE 8620合金鋼應力腐蝕特性之研究

Abstract

本研究主要是針對SAE 8620合金鋼在氯化鈉水溶液中的應力腐蝕特性做分析，藉由調整SAE 8620合金鋼之熱處理條件，觀察其抵抗腐蝕環境的能力。試驗方法包含有定速率拉伸和定荷重量測裂縫成長，另外也從顯微組織和電化學等特性加以佐證。經由慢速率拉伸的實驗結果，在常溫下隨著氯離子濃度上升，降伏強度逐漸下降，而延性下降程度則並不明顯，整體而言，還是以機械力作用造成的破壞為主；另外在高溫的氯離子水溶液中，較明顯造成延性下降的效應，但無論何種條件下拉伸，觀察到的破斷面，皆為延性破斷的特徵。 在定荷重試驗當中，預設相同長度的裂縫，觀察其裂縫成長的情形；實驗結果發現，淬火後300℃回火的裂縫成長最快，400℃和500℃回火次之，正常化在此條件下的裂縫成長幾乎趨近於0。 經由綜合比較實驗結果，可得知淬火後400℃回火的熱處理條件，能維持一定的降伏及抗拉強度，而且在產生裂縫之後，裂縫成長速率比300℃回火要緩慢許多，另外在電化學等性質方面，也有相對其他熱處理條件較佳的抗腐蝕性。

The research aims for the stress corrosion cracking characteristics of SAE 8620 alloy steel in chloride aqueous. The abilities against the corrosive environment are observed in the different parameters of heat treatment. The experiment method includes the slow strain rate test and constant K tension test for the crack propagation. In addition, it is also attested from the microstructure and electrochemical properties. From the results of slow strain rate test, in the room temperature, the yield strengths decrease with the increasing concentration of chloride ions, but the decreases in ductility are not conspicuous. In general, the mechanical force is predominant factor that causes fracture. The 80℃ chloride aqueous principally leads to the decrease in ductility, but no matter what condition of tension test, the fracture graphs appear the ductile characteristics. In the constant K tension test, the same length of pre-crack is established, and then the crack propagation is measured. In the result, the crack propagation is the most rapid in the specimen quenched and then tempered in 300℃. The crack propagation in the specimens quenched and tempered in 400℃ and 500℃ is secondary, and there is almost no crack propagation in the normalized specimen. Overall, in the heat treatment of quenched and tempered in 400℃, the SAE 8620 alloy steel maintains high yield strength and ultimate tension strength, and the crack propagation is relatively slower than the heat treatment of quenched and tempered in 300℃. Besides, there are comparatively better electrochemical properties in the heat treatment of quenched and tempered in 400℃.