氣體氮化與氣體滲碳氮化對鐵-8.8鋁-30.1錳-1.73碳合金腐蝕與機械性質影響

Abstract

本篇論文是藉由觀察鐵－8.8鋁－30.1錳－1.73碳合金，經1150⁰C、2小時固溶熱處理並淬火降至室溫，再分別經由氣體氮化與氣體軟氮化處理後，其顯微組織之變化，並且利用X光繞射分析(XRD)、能量散射光譜儀(EDS)、恆電位儀與微硬度試驗機，探討兩種氮化處理方式對合金顯微結構、抗腐蝕性質和機械性質的影響。 鐵－8.8鋁－30.1錳－1.73碳合金在固溶處理並淬火後，母相為沃斯田鐵相，並有(Fe,Mn)3AlC於母相中以史賓諾多相分離的方式整合性析出。經氣體氮化和氣體滲碳氮化後，合金表面形成一層以Fe4N和AlN組成，結構為F.C.C.的緻密氮化層，此氮化層和合金母相γ之間為整合性的關係。由於氮化過程中，氮化氣氛的組成會影響氮原子活性，影響氮原子在金屬中擴散速度，故合金經氣體氮化後，其氮化層厚度約為25μm，而經氣體滲碳氮化後，厚度約為10μm。合金經氣體氮化後，其表面硬度可以達到1448 Hv，氣體滲碳氮化則為1236 Hv，硬度的差異是由於氮化處理後，兩者氮化層表面之氮含量不同所造成。合金在固溶狀態下，於極化試驗中並無觀察到明顯之鈍化區，而經過氣體氮化與氣體滲碳氮化後之極化曲線清楚呈現穩定的鈍化區，且其腐蝕電位與孔蝕電位皆有大幅的提升。藉由觀察在3.5%氯化鈉水溶液浸蝕試驗後合金表面之顯微組織，得知經氮化處理後，合金表面抗蝕性質確實有明顯的提升。而合金在氮化處理後，因為氮化處理時形成的氮化層能有效提升合金的抗腐蝕性質，於1%鹽酸浸蝕試驗所得的重量損失相較於氮化處理前皆有顯著的下降，但氣體滲碳氮化之試片在經過60小時浸蝕試驗後，其氮化層已無法完整保護合金內部，導致重量損失增加的速度開始上升。

The purpose of this study is to examine the microstructure, corrosion resistance and mechanical properties of the Fe-8.8Al-30.1Mn-1.73C alloy after being solution heat treatment at 1150⁰C for 2h and then gas nitriding or gas nitrocarburizing by using scanning electron microscopy(SEM), X-ray diffraction (XRD), electrochemical measurements, and micro-hardness tester. In addition, the microstructure, the corrosion resistance and the mechanical properties of the alloy with different nitriding processes were also investigated. After gas nitriding or gas nitrocarburzing, a nitrided layer composed by AlN and Fe4N with F.C.C. structure formed coherently in the surface of the alloy. Because the composition of nitriding atmosphere will affect the activity of nitrogen, the thickness of the compound layer after gas nitriding is 25μm, which is higher than that of gas nitrocarburzing. After nitriding processes, the surface hardness of alloy was significantly increase due to the nitrided layer composed by AlN and Fe4N. During the electrochemical corrosion test, it was clearly seen that no passivation region of the Fe-8.8Al-30.1Mn-1.73C alloy without nitrided treatment. The alloy treated by gas nitriding or gas nitrocarburizing possess a stable passivation region and high electrochemical potentials in the potentiodynamic polarization curves, and the increase of the corrosion resistance after nitride treatments can also be seen in the immersion test. The improvement of corrosion resistance was due to the nitrided layers formed in the nitriding processes.