鋁及鉻含量對鐵錳鋁鉻碳合金腐蝕及機械性質之影響

Abstract

摘要

本篇論文的目的是經由掃描式電子顯微鏡（SEM）及穿透式電子顯微鏡（TEM）來觀察在鐵-30錳-7鋁-1.5碳及鐵-30錳-9鋁-1.5碳合金中添加重量百分比3.0wt.％, 5.0 wt.％, 6.5 wt.％ 及8.0 wt.％ 鉻含量經1100℃、2小時固溶熱處理並淬火至室溫後，其顯微結構之變化，並利用萬能拉伸試驗機（instron）、恆電位儀、X光光電子能譜分析（XPS）與鹽霧試驗機來探討成份及顯微結構對合金機械性質和抗蝕性的影響。 在本研究中，未添加鉻之鐵-30錳-7鋁-1.5碳合金，在電化學腐蝕測試後，並無明顯之鈍化區形成，但在合金中添加鉻含量後，可發現明顯之鈍化區存在。當在鐵-30錳-7鋁-1.5碳合金中添加鉻含量達6.5wt.％ 時，將在合金表面形成氧化鉻（Cr2O3）及氧化鋁（Al2O3）之緻密氧化鈍態膜，使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），且在24小時鹽霧試驗後具有最佳之抗蝕能力。然而，當在此合金中添加之鉻含量達8.0wt.％ 時，將因碳化鉻（Fe,Mn,Cr）7C3之形成，發生孔蝕之現象，造成合金抗蝕性下降。另外，我們發現在相同鉻含量之Fe-Mn-Al-Cr-C系合金中，當鋁含量由7wt.％ 增加至9wt.％ 時，合金的抗蝕性亦隨之增加，而在鐵-30錳-9鋁-1.5碳合金中添加鉻含量達5.0wt.％ 時，可使合金在動態極化曲線中呈現穩定且寬廣的鈍化區與極低之腐蝕電流密度（ip），且在24小時鹽霧試驗後仍具有極佳之抗蝕能力。然而，當在此合金中添加之鉻含量達6.5wt.％ 或以上時，將因碳化鉻（Fe,Mn,Cr）7C3之形成，發生孔蝕之現象，造成合金抗蝕性下降。 除此之外，本研究之合金經由掃描式電子顯微鏡（SEM）及穿透式電子顯微鏡（TEM）觀察下，發現其顯微結構為樹枝狀晶之顯微結構和樹枝狀枝臂間隙交錯分佈。樹枝狀晶之顯微結構為沃斯田鐵相（γ）和具有L´12結構之微細的κ´碳化物共存，樹枝狀枝臂間隙則為肥粒鐵（α）相。其沃斯田鐵相與肥粒鐵相之比值（γ/α）將隨鉻含量增加而減少，造成合金機械強度增加，延伸率下降。然而，當在鐵-30錳-7鋁-1.5碳合金中添加8.0wt.％ 鉻時，發現（Fe,Mn,Cr）7C3硬而且脆之碳化鉻在樹枝狀枝臂間隙析出，使破斷模式由延性破裂轉變為脆性破裂，造成延性急劇下降。此外，當鋁含量由7wt.％ 增加至9wt.％ 時，合金中添加鉻含量只要達6.5wt.％ 時，即可發現碳化鉻（Fe,Mn,Cr）7C3析出，且在樹枝狀枝臂間隙內之肥粒鐵相將轉變為DO3有序相，使合金更具脆性。

ABSTRACT

The purpose of this study is to examine the microstructural developments of the Fe-30Mn-7Al-1.5C and Fe-30Mn-9Al-1.5C alloys which addition 3.0wt.％, 5.0 wt.％, 6.5 wt.％ and 8.0wt.％chromium content and through 1100℃,24 hours solution heat-treatments then quenched into room temperature by means of scanning electron microscopy and transmission electron microscopy. Beside, the effect of the composition and microstructure on the mechanical properties and corrosion resistance were also made by using instron, electrochemical measurements, X-ray photoelectron spectroscopic （XPS） and salt fog test mechanism. In this study, it is clearly seen that no evidence of passivation region could be found in the Fe-30Mn-7Al-1.5C alloy after electrochemical corrosion test. With increasing the chromium content, the passivation region of the alloy could be obviously detected. The increase of chromium content above 6.5wt.％ in the Fe-30Mn-7Al-1.5C alloy, there were advanced the outmost surface of the passive films formed on the surface of the alloys which were composed of chromium oxide（Cr2O3）, aluminum oxide（Al2O3）, and it was clearly seen that the potentiodynamic polarization curves of alloys exhibited stable and broad passivation region and the minimum corrosion current density（ip）, and have the best corrosion resistance after 24 hours slat fog test. However, the increase of chromium content above 8.0wt.％ in Fe-30Mn-7Al-1.5C alloy, there was observed that the decrease of corrosion resistance caused by pitting . The pitting phenomenon of the alloy was induced by the （Fe,Mn,Cr）7C3 chromium carbides forming. In addition, when the Fe-Al-Mn-Cr alloys containing the same chromium content and the aluminum content was increase from 7wt.％ to 9wt.％, the corrosion resistance of the alloys was increase. The increase of chromium content above 5.0wt.％ in the Fe-30Mn-9Al-1.5C alloy, it was clearly seen that the potentiodynamic polarization curves of alloys exhibited stable and broad passivation region and the lower corrosion current density（ip）, and have the excellent corrosion resistance after 24 hours slat fog test. However, the increase of chromium content above 6.5wt.％ in the Fe-30Mn-9Al-1.5C alloy, there was observed that the decrease of corrosion resistance caused by pitting. The pitting phenomenon of the alloy was induced by the （Fe,Mn,Cr）7C3 chromium carbides forming. In addition, the transmission electron microscopy（TEM）and scanning electron microscopy（SEM） analysis indicated that the microstructure of the alloys were dendrite and inter-dendrite distribution. The fineκ´carbides having an L´12-type structure within the austenite（γ） matrix of the dendrite structures, and ferrite（α）of the inter-dendrite structures. It is seen that the mechanical strength could be increased and the elongation could be decreased with decreasing the ratio of theγtoα by enhance chromium content. However, addition above 8.0wt％ of chromium content in Fe-30Mn-7Al-1.5C , there were the hardness and brittle（Fe,Mn,Cr）7C3 chromium carbides precipitate in the inter-dendrite, and the fracture mechanism were transition from ductile fracture to brittle fracture and cause ductility to drop suddenly. Besides, when the alloy contained that the chromium content up to 6.5wt.％ and the aluminum content was increased from 7wt.％ to 9wt.％, it is relating not only the （Fe,Mn,Cr）7C3 chromium carbides were precipitate within the matrix but also the ferrite phase within the inter-dendrite, this will transform ordering DO3 phase caused the brittle occurring.