氣體氮化及熱滾軋對鐵鋁錳碳合金機械性質與微觀結構之研究

Abstract

本論文主要分為二部分，第一是研究氣體氮化熱處理對鐵鋁錳碳合金(Fe-9Al-30Mn-1.8C ,wt.%)的顯微結構、機械性質與抗腐蝕性質影響；第二是研究鐵鋁錳碳合金 (Fe-9Al-30Mn-1.0C&1.5C)僅經過熱滾軋處理後，合金的顯微結構與機械性質分析。綜合實驗結果，本論文具體研究結果整理如下: (一) Fe-9Al-30Mn-1.8C合金淬火後的結構為沃斯田鐵相 (austenite, γ)，且加上γ基地內有固溶淬火後藉由史賓諾多相分解(spinodal decomposition)整合型(coherent)的緻密析出物-(Fe,Mn)3AlC碳化物 (κ′-碳化物)，κ′-碳化物為奈米級的結構。接著將Fe-9Al-30Mn-1.8C合金進行550℃、4小時的氣體氮化處理，顯微結構結果發現，κ′-碳化物會在γ基地內成長且量也會增加，此氣體氮化過程也同時兼具了時效作用。因此使得合金在具備優異強度與延性的同時，合金表面又會有約莫30μm的氮化層。藉由低掠角繞射量測(grazing incidence X-ray diffraction ,GIXRD)技術來進一步分析氮化層的組成。結果發現，氮化層主要是由面心立方(Face-Center Cubic, F.C.C.)的氮化鋁(AlN)以及少量的氮化鐵 (包含:F.C.C.之Fe4N和六方最密堆積(Hexagonal Closed-Packed, H.C.P.)之Fe3N)。結果也顯示，在靠近內部基地的氮化層之氮化鐵含量較多。由表面成分分析結果可知，氮化層表面氮濃度高達19.2 wt.% (31.4 at.%)。相對的，表面硬度高達1780 Hv，而基材硬度亦有540 Hv，且優異的強度與延性結果為最大抗拉強度(UTS)=1356 MPa、降伏強度(YS)=1228 MPa 及伸長率(El)=32.3%，氣體氮化的抗腐蝕能力為Epit=1860 mV。且拉伸試驗後，由於氮化層與基材有極佳的附著性，氮化層不僅沒有剝落，而且其破裂深度僅有3~4μm左右。因此，拉伸後的氣體氮化試片進行極化實驗後，亦保有很好的抗腐蝕能力(Epit=890 mV)。這在應用上有很好的優勢，且上述的這些特性都優於一般經最佳氮化處理後的高強度合金鋼、工具鋼、麻田散鐵鋼及析出硬化型不銹鋼。 (二) 鐵鋁錳碳合金在經過熱滾軋後而無時效處理，即可具有良好機械性質表現。藉由顯微結構研究，來觀察其強化機制。結果顯示，熱滾軋Fe-9Al-30Mn-1.0C (1.0C)合金的顯微結構為單一沃斯田鐵相(γ)，有容易滑移的差排線(dislocation lines)在基材出現，亦有微量細小的析出物在差排線上產生。所以，1.0C合金表現出良好的延性和韌性，其最大抗拉強度(UTS)=968 MPa、降伏強度(YS)=655 MPa且伸長率(El)=52.0%。而1.5C合金除了沃斯田鐵相外，還有L′12結構的κ′-碳化物析出，其體積比約為36%左右，平均尺寸約為22nm。且κ′-碳化物於大量差排包(dislocation cell)內析出，這些κ′-碳化物與差排包組織使得合金有強化作用。因此其最大抗拉強度與降伏強度提升至1090MPa與982MPa，而延伸率仍保有38.5%，合金破裂結果也顯示仍然為有大量酒窩狀組織的延性破裂模式。

The thesis is divided into two parts: one is the study on of gas nitriding treatment effect on mechanical properties, microstructure and corrosion behavior of Fe-9.0Al-30Mn-1.8C (in wt.%) alloy. The other is the research on hot rolling treatment influence on microstructure and mechanical properties of Fe-9.0Al-30Mn-1.0C and Fe-9.0Al-30Mn-1.5C alloy. On the basis of the experimental examinations, the results can be summarized as follows: [1] The as-quenched Fe-9.0Al-30Mn-1.8C (in wt.%) alloy is the austenite (γ) phase and fine (Fe,Mn)3AlC carbides (κ′-carbides) are coherently precipitated by spinodal decomposition in the γ matrix. The κ′-carbides are nano scale structure. Then, the as-quenched Fe-9.0Al-30Mn-1.8C was gas-nitrided at 550℃ for 4h, resulting in a ~30 μm nitrided layer. And, it can be observed clearly that the volume fraction and size of κ′-carbides would increase in the matrix. The gas nitriding treatment can achieve the aging effect simultaneously. Therefore, the Fe-9.0Al-30Mn-1.8C can possess high strength, high ductility and excellent corrosion resistance at the same time. The nitrided alloy shows UTS=1356 MPa、YS=1228 MPa and El=32.3%. Then, the nitrided layer would be detected by GIXRD technique. Consequently, the nitrided layer is composed predominantly of F.C.C AlN with a small amount of Fe3N and Fe4N. Evidently, Fe3N and Fe4N distributed in the nitrided layer have higher concentration near to the matrix. Owing to the high surface nitrogen concentration (19.2 wt.% (31.4 at.%)), the surface hardness is 1780 Hv and substrate hardness is 540 Hv. The nitrided alloy shows excellent corrosion resistance, Epit=1860 mV. Due to the well adhesion between the nitrided layer and the matrix, the nitrided layer would not be peeled off easily after tensile test. And the shallow fracture depth is about 4μm. Therefore, the corrosion resistance of present gas-nitrided and then tensile-tested alloy is superior to those optimally gas-nitrided or plasma-nitrided high-strength alloy steels, as well as martensitic stainless steels. The nitrided and then stretched alloy still retains a satisfactory corrosion resistance (Epit = +890 mV; Ecorr = +10 mV). Furthermore, only nanoscale-size pits were observed on the corroded surface after being immersed in 10% HCl for 24 h. [2] FeAlMnC alloy underwent hot-rolling treatment without aging can also show well mechanical properties. The strengthening mechanism would be revealed by microstructure researches. Fe-9.0Al-30Mn-1.0C hot rolled alloy is a single γ phase and a large amount of dislocation lines could be observed in the matrix. Therefore, 1.0C hot rolled alloy shows well ductility and toughness and its UTS=968 MPa、YS=655 MPa and El=52.0%. In high carbon content alloy, Fe-9.0Al-30Mn-1.5C hot rolled alloy reveals κ′-carbides (in L′12 form) precipitated in γ matrix. And the volume fraction and average size of κ′-carbides are about 36% and 22nm, respectively. Evidently, the fine κ′-carbides would improve precipitation strengthening dramatically. Hence, Fe-9.0Al-30Mn-1.5C hot rolled alloy would reach higher strength, UTS=1090 MPa, YS=982 MPa. However, the ductility would remain in a satisfactory state (El=38.5%), the fracture surface still reveals dimples structure.