離子氮化及氣體氮化對鐵鋁錳碳合金顯微結構、機械性質及抗腐蝕性質影響之研究

Abstract

本論文主要分別研究離子氮化及氣體氮化對Fe-8.68wt.%Al-30.5wt.%Mn-1.85wt.%C和 Fe-9wt.%Al-28wt.%Mn-1.8wt. %C合金之顯微結構、機械性質與抗腐蝕性質影響。依據實驗的結果，本論文所得到的具體研究結果如下: (一) Fe-8.68wt.%Al-30.5wt.%Mn-1.85wt.%C的淬火結構為沃斯田鐵相(austenite, γ)，且在γ基地內有十分緻密奈米級(nano-sized)的(Fe,Mn)3AlC碳化物(κ'-碳化物)。緻密奈米級的κ'-碳化物是合金在固溶化淬火過程中藉由史賓諾多分解(spinodal decomposition)相變化在γ基地內形成。將在淬火狀態下的Fe-8.68wt.%Al-30.5wt.%Mn-1.85wt.%C合金，經500C、8小時離子氮化處理後，在γ基地內的κ'-碳化物會成長且量變多，而使氮化後合金能具有優異的強度和延性組合，且在合金表面可得到40 m厚的氮化層，此氮化層結構由X光繞射可知，主要組成為具有面心立方(Face-Centered Cubic, F.C.C)之AlN及少量的FCC Fe4N；藉由離子氮化處理後的Fe-8.68wt.%Al-30.5wt.%Mn-1.85wt.%C合金表面氮濃度高達20wt.% (48 at.%)。所以表面硬度可高達1860 Hv，基材硬度為550 Hv，延伸率為33.6%，且在3.5%鹽水中具有極佳抗腐蝕性質，這些特性都遠優於一般經最佳氮化處理後的高強度合金鋼，工具鋼，麻田散鐵不銹鋼及析出硬化型不銹鋼。

(二) Fe-9wt.%Al-28wt.%Mn-1.8wt.%C的淬火結構為沃斯田鐵相(austenite, γ)，且在γ基地內有十分緻密奈米級(nano-sized)的(Fe,Mn)3AlC碳化物(κ'-碳化物)。緻密奈米級的κ'-碳化物是合金在固溶化淬火過程中藉由史賓諾多分解(spinodal decomposition)相變化在γ基地內形成。在淬火狀態下， Fe-9wt.%Al-28wt.%Mn-1.8wt.%C合金經500C、8小時和50%NH3+50% H2氣氛氣體氮化處理後，表面可得到約31 m厚的氮化層，此氮化層結構由X光繞射可知，主要組成為AlN及少量的Fe4N；氣體氮化後表面氮濃度高達17 wt.% (41 at.%)；藉由氣體氮化處理的Fe-9wt.%Al-28wt.%Mn-1.8wt.%C合金，表面硬度可高達1700 Hv，基材硬度550 Hv，延伸率為33.2%，且在3.5%鹽水中具有極佳抗腐蝕性質，這些特性都遠優於一般經最佳離子氮化及氣體氮化處理後的高強度合金鋼，工具鋼，麻田散鐵不銹鋼及析出硬化型不銹鋼。經由研究發現，氣體氮化試片經拉伸測試後，表面氮化層與基材間仍有極佳的附著性，不易脫落。此外，藉由氣體氮化後試片角落無明顯的電弧效應現象產生，而此現象在一般離子氮化處理過程中始終是難以避免的。

The as-quenched Fe-8.68wt.%Al-30.5wt.%Mn-1.85wt.%C alloy was plasma-nitrided at 500C for 8h with 50% N2 and 50% H2 atmosphere under a pressure of 130Pa, and the as-quenched Fe-9wt.%Al-28wt.%Mn-1.8wt.%C alloy was gas-nitrided at 500C for 8h using an atmosphere of 50% NH3 and 50% H2, respectively. Microstructures, mechanical properties and corrosion behaviors of the plasma-nitrided Fe-8.68wt.%Al-30.5wt.%Mn-1.85wt.%C and gas-nitrided Fe-9wt.%Al-28wt.%Mn-1.8wt.%C alloys have been investigated. On the basis of the experimental examinations, some results can be summarized as follows: [1]. The as-quenched microstructre of the Fe-8.68wt.%Al-30.5wt.%Mn-1.85 wt.%C alloy was austenite (γ) phase containing an extremely high density of nano-sized (Fe,Mn)3AlC carbides (κ'-carbide). These κ'-carbides were formed within austenite matrix by spinodal decomposition during quenching. The size and the amount of the κ'-carbides increased dramatically when the as-quenched Fe-8.68wt.% Al-30.5wt.% Mn-1.85 wt.% C alloy was plasma-nitrided at 500C for 8h. Consequently, the nitrided alloy could obtain an excellent combination of strength and ductility after being plasma-nitrided. The nitrided layer obtained is 40 m-thick and composed predominantly of F.C.C AlN with a small amount of Fe4N. Due to the surface nitrogen concentration reached up to 20wt.% (48at.%), the surface hardness (1860 Hv), substrate hardness (550 Hv), ductility (33.6%), and corrosion resistance in 3.5% NaCl solution in the plasma-nitrided Fe-8.68wt.% Al-30.5wt.% Mn-1.85 wt.% C alloy are far superior to those obtained previously in optimally nitrided high-strength alloy steels, as well as martensitic and precipitation-hardening stainless steels.

[2]. The as-quenched microstructre of the Fe-9wt.%Al-28wt.%Mn-1.8wt.%C alloy was austenite (γ) phase containing an extremely high density of nano-sized (Fe,Mn)3AlC carbides (κ'-carbide). These κ' carbides were formed within austenite matrix by spinodal decomposition during quenching. The as-quenched Fe-9wt.%Al-28wt.%Mn-1.8wt.% C alloy was directly gas-nitrided at 500C for 8h with 50%NH3+50% H2 atmosphere, resulting in a 30 m-thick nitrided layer. The nitrided layer consists predominantly of nano-crystalline AlN with a small amount of Fe4N. The nitrogen concentration at surface was extremely high up to ~17 wt.% (41 at.%) . Consequently, the surface microhardness (1700Hv), substrate hardness (550Hv), ductility (33.2%) and corrosion resistance in 3.5% NaCl solution of the present gas-nitrided Fe-9wt.%Al-28wt.%Mn-1.8wt.% C alloy are far superior to those obtained previously for the optimally gas-nitrided or plasma-nitrided high-strength alloy steels, as well as martensitic and precipitation-hardening stainless steels. Moreover, it is very novel that the nitrided layer almost remained coherent and adhered well with the matrix after tensile test. Additionally, the present gas nitriding appeared to overcome the edge effects commonly encountered in plasma nitriding treatments for metals.