A286鐵基超合金時效處理和孔蝕行為之研究

Abstract

本研究主要探討A286合金二次相與抗蝕性之關聯，並以商用316L不銹鋼作為實驗對照組。A286合金材料製成參數有四種，分別是980°c固溶化熱處理1小時、980°c固溶化熱處理1小時 + 720°c時效處理12小時、980°c固溶化熱處理1小時+720°c時效處理24小時、980°c固溶化熱處理1小時+720°c時效處理200小時。 微結構分析使用了掃描式電子顯微鏡(SEM)，結合能量散射X射線譜(EDX)，來了解二次相分布位置，並使用背向散射電子繞射(EBSD)來了解介在物分布情形，最後使用穿透式電子顯微鏡(TEM)確認介在物之結晶結構。980°c固溶化熱處理1小時之試片經過X光繞射儀(XRD)檢測發現基地是FCC沃斯田鐵相。並從顯微結構發現有微米級的TiC、次微米級的碳化物、磷化物。980°c固溶化熱處理1小時+720°c時效處理12小時之試片發現γˈ與η析出相存在，γˈ相主要析出在基地中，而η相主要沿著晶界析出。並隨著熱處理時間增長，γˈ與η相有粗大化現象。 抗蝕性分析使用了動電位極化曲線、循環動電位極化曲線、電化學交流阻抗分析與腐蝕後表面形貌觀察。使用的是三極系統，工作電極為測試之試片、輔助電極為白金、參考電極為飽和甘汞電極(SCE)，測試水溶液為3.5wt%氯化鈉水溶液，並以鹽酸調整至pH=2。測試結果為980°c固溶化熱處理1小時之試片有最高孔蝕電位，並隨著熱處理時間增長，孔蝕電位跟著下降，而316L不銹鋼孔蝕電位低於980°c固溶化熱處理1小時之試片。循環動電位極化曲線測試結果為980°c固溶化熱處理1小時+720°c時效處理200小時之試片孔蝕敏感性最高，而當熱處理時間降低，孔蝕敏感性也跟著下降。電化學交流阻抗分析結果為980°c固溶化熱處理1小時之試片有最好抗蝕性質，隨著時效時間增長，抗蝕性有下降趨勢，316L不銹鋼抗蝕性質與980°c固溶化熱處理1小時之試片相差不大。 腐蝕後表面形貌分析發現316L不銹鋼孔時誘發位置為還有硫離子之介在物上，而980°c固溶化熱處理1小時之試片孔蝕初始位置可能為碳化物與磷化物，經過時效處理之試片，孔蝕初始位置就較為複雜，因為二次相有析出相(γˈ與η)與介在物(微米級的TiC、TiN與次微米級的碳化物、磷化物)，TiC與TiN為介穩態孔蝕的位置，而穩態孔蝕的誘發位置為次微米級的碳化物與磷化物，且可能和γˈ相有關但和η相無關，因為孔蝕位置並沒有在晶界上發生，而η相都是沿著晶界上析出，故孔蝕與η相並無相關。

In this study, the association between the secondary phase and corrosion resistance of A286 alloy was investigated, and commercial 316L stainless steel was used as the experimental control group. There are four parameters, namely, 980°C solidification heat treatment for 1 hour, 980°C solidification heat treatment for 1 hour + 720°C aging treatment for 12 hours, 980°C solidification heat treatment for 1 hour + 720°C aging treatment for 24 hours, and 980°C solidification heat treatment for 1 hour + 720°C aging treatment for 200 hours. The microstructure analysis was performed using scanning electron microscopy (SEM) combined with energy scattering X-ray spectroscopy (EDX) to understand the location of the secondary phase distribution, and backscattering electron microscopy (EBSD) to understand the distribution of the inclusion, and finally transmission electron microscopy (TEM) to confirm the crystalline structure of the inclusion. The base was examined by XRD and found to be FCC austenite iron phase. The microstructure revealed the presence of micron TiC, submicron carbide and phosphide. 980°C solid solution heat treatment for 1 hour + 720°C aging treatment for 12 hours revealed the presence of γˈ and η precipitated phases. The coarsening of γˈ and η phases was observed with the increase of heat treatment time. The corrosion resistance analysis was performed using dynamic potential polarization curves, cyclic dynamic potential polarization curves, electrochemical AC impedance analysis, and post-corrosion surface morphology observation. The working electrode was the test specimen, the auxiliary electrode was platinum, and the reference electrode was a saturated mercury electrode (SCE). The pitting potential of 316L stainless steel is lower than that of the specimen treated with solid solution heat treatment at 980°C for 1 hour. The results of the cyclic dynamic potential polarization curve test showed that the pitting corrosion sensitivity of the specimen treated at 980°C for 1 hour + 720°C for 200 hours was the highest, and as the heat treatment time decreased, the pitting corrosion sensitivity also decreased. The results of electrochemical AC impedance analysis showed that the specimen with 1 hour of solid solution heat treatment at 980°C had the best corrosion resistance, and the corrosion resistance tended to decrease as the aging time increased. The surface morphology analysis after corrosion revealed that the location of the induced pitting corrosion of 316L stainless steel is on the intermediate material with sulfur ions, and the initial location of the pitting corrosion of the specimen treated by solid solution heat treatment at 980°C for 1 hour may be carbide and phosphide, and the initial location of the pore corrosion of the specimen treated by aging is more complicated because the secondary phases are precipitated phases (γˈ and η) and intermediate material (micron TiC, TiN and submicron carbide and phosphide). TiC and TiN are the locations of the meta-stable pitting, while the locations of the stable pitting are the submicron carbides and phosphides, and may be related to the γˈphase but not to the η-phase, because the pore erosion does not occur at the grain boundaries, while the η-phase is precipitated along the grain boundaries, so the pitting corrosion is not related to the η-phase.