離子氮化對鐵-30 錳-9 鋁-1.8 碳合金腐蝕及機械性質之影響

Abstract

本篇論文的目的是藉由掃描式電子顯微鏡（SEM）來觀察在鐵-30 錳- 9鋁- 1.8碳合金，經過1200℃、兩小時固溶熱處理(SHT)後淬火至室溫再分別以450℃、12小時時效熱處理或離子氮化處理；500℃、8小時時效熱處理或離子氮化處理，其顯微結構之變化，並且利用恆電位儀、X光繞射分析（XRD）、X光能量散射光譜儀（EDS）與微硬度試驗機，探討表面氮化處理過後其顯微結構以及其對合金抗腐蝕性質和機械性質的影響。 在本實驗中，未經過氮化處理之鐵-30 錳- 9 鋁- 1.8 碳合金，在電化學腐蝕測試後，並無明顯之鈍化區形成，但經氮化處理過後，可發現明顯之鈍化區生成。經由電子顯微鏡（SEM）以及X光繞射分析（XRD）進行觀察，可發現經過離子氮化處理後之合金試片，其合金表面形成一層緻密的氮化鋁（AlN），使得合金在動態極化曲線中呈現穩定且寬廣的鈍化區與最低之腐蝕電流密度(ｉp)，並且在以1％ HCl溶液做為腐蝕液的浸蝕測試中，其每單位面積損失的重量有顯著的下降。而以3.5％ NaCl溶液極化測試後之試片，經由電子顯微鏡（SEM）觀察其合金腐蝕狀況，可發現未經氮化處理過之試片，從晶界大量向下腐蝕，而晶粒內的母相產生許多渦狀腐蝕以及腐蝕的小孔洞，其腐蝕情況相當嚴重。而經過氮化處理後之試片其抗蝕能力大幅提升，經過極化測試過後，其在晶界開始造成腐蝕，而在晶粒內產生一些微小的孔洞，但其腐蝕情況相較於未氮化之試片有顯著的改善，其晶界腐蝕的程度以及晶粒內腐蝕之規模大小有明顯之差異。且試片經過氮化處理過後，其表面氮化層組成為AlN與少量之Fe4N，微硬度隨氮化後深度變化之情形，氮化層的表面硬度最高為1800 Hv，而後往內部逐漸遞減，相較於未氮化片有顯著的表面強化效果。

The purpose of this study is to examine the microstructural development of the Fe-9 wt.%Al-30 wt.%Mn-1.8 wt.% C after being solution heat treat (SHT) quench to room temperature and then aging or plasma nitriding which processes were carried out at temperatures 450℃ for 12h and 500℃ for 8h by scanning electron microscopy(SEM). The corrosion resistance and mechanical properties of the alloy were also examined by electrochemical measurements, X-ray diffraction(XRD) and micro-hardness tester. In addition, the effects of microstructural development of plasma nitrided alloy on the corrosion resistance and mechanical properties were also investigated. In this study, it is clearly seen that no evidence of passivation region could be found in the Fe-30Mn-9Al-1.8C alloy without nitrided treatment after electrochemical corrosion test. After plasma nitrided, the passivation region of alloy could be obviously observed. The outmost surface of the nitride layers was a compact AlN layer after plasma nitrided. It is also seen in the potentiodynamic polarization curves that alloy possesses a stable and broad passivation region and a minimum corrosion current density(ｉp). The plasma nitrided sample had the lowest weight loss after immersion test in 1% HCl water solution. By SEM examination of the surface of corrosion tested, the untreated samples was heavily corroded from grain boundary and matrix of the samples became rough with large amount of pits after electrochemical corrosion test. In contrast, the corrosion surface of a nitrided sample was slightly corroded from grain boundary and corrosion was just occurred few little pits. The main phases of the nitride layer were AlN with few Fe4N. The hardness was decrease from surface 1800Hv rapidly to matrix about 550Hv near the nitride layer boundary after 500℃ plasma nirided for 8h. It is to say that plasma nitrided present significantly hardness enhance effect.