465麻時效不鏽鋼之微結構及機械性質研究

Abstract

過去傳統的高強度低合金鋼，雖然在大部分的情況下皆可使用，但因為其合金成份的設計，無法有效地防止材料表面腐蝕，因此無法用於較嚴苛之環境，其發展應用領域也受到限制。而不鏽鋼因為添加了至少12wt%以上的Cr，能夠產生緻密氧化膜，以防止內部繼續腐蝕氧化，此特點若是搭配優秀的強度及延性，其可發展領域將十分廣泛。 本實驗研究之材料為465麻田散鐵系析出硬化型不鏽鋼，分成兩個部分做微結構觀察和性質上的分析。在第一部份先以熱膨脹儀做出465麻時效鋼之膨脹曲線，透過膨脹曲線了解其相變溫度以及發現465麻時效鋼內部具有恆溫麻田散鐵的存在。再以不同沃斯田鐵化溫度觀察在時效之前的初始微結構，以及基底麻田散鐵次結構中的變體，以了解在後續時效處理時，奈米析出物Ni3Ti和逆沃斯田鐵的生長行為。在第二部分將465麻時效鋼以不同溫度做時效處理，了解其析出強化曲線，再選出最具代表性之參數做微結構分析。 在時效處理之後，以TEM觀察奈米析出物Ni3Ti，使用選區繞射技術拍攝出析出物暗場影像，再用ImageJ-Fiji軟體分析其大小趨勢，發現Ni3Ti之長寬比和時效時間成正相關，且是透過差排管擴散進行生長，因此其生長方向和差排<111>方向平行。將HRTEM之影像透過軟體做傅立葉轉換後，得到局部區域之繞射圖譜，有助於了解Ni3Ti奈米析出物和麻田散鐵基底之Burger’s OR方位關係。TEM也用來了解時效處理後逆沃斯田鐵形貌和生長行為，在選區繞射後繞射圖譜中也發現逆沃斯田鐵傾向於恢復成原沃斯田鐵晶粒之方位，和母相麻田散鐵具有KS-OR方位關係，此現象為沃斯田鐵記憶效應(Austenite memory)。透過XRD和EBSD針對逆沃斯田鐵相做定量分析，了解逆沃斯田鐵和時效時間之關係，期望能夠控制逆沃斯田鐵之比率，導入相變誘發塑性變形使465麻時效鋼之延性表現有所提升。本實驗也使用了電子能量損失能譜(Electron energy-loss spectroscopy)，發現在510C下，麻田散鐵基底之差排密度變化和添加之合金元素鉬有關，發現鉬能夠有效的降低材料在時效處理下，因差排爬移回復所造成的軟化現象。本實驗最後以萬能拉伸試驗機瞭解不同參數下之機械性質表現，分析其加工硬化率曲線，是否成功導入TRIP effect。

Although traditional high-strength low-alloy steels can be used in most cases, the alloy composition is not designed to effectively prevent surface corrosion, and therefore cannot be used in harsh environments. Stainless steel with at least 12wt% Cr can produce dense oxide film to prevent internal corrosion and oxidation, and if this feature is combined with excellent strength and ductility, it can be used in lots of fields. The material studied in this experiment is 465 martensitic precipitate-hardening stainless steels, which was divided into two parts for microstructural observation and property analysis. In the first part, the expansion curve of the 465 maraging steel was made by a dilatometer to understand the phase transition temperature. The presence of isothermal martensite in 465 maraging steel was also found by expansion curves. The initial microstructure before aging and hierarchical structure of martensite were observed at different austenitization temperatures to understand the differences in microstructure after subsequent aging treatment. In the second part, 465 maraging steels were aged at different temperatures to understand their precipitation hardening curves, and then the most representative parameters were selected for microstructural analysis. After the aging treatment, the nano-precipitates Ni3Ti were observed by TEM, and the dark-field images of the precipitates were taken by the SAED technique, and the size trend was analyzed by ImageJ-Fiji software. It is found that the aspect ratio of Ni3Ti is positively correlated with the aging time, and the growth is carried out by pipe of dislocation, so its growth direction is parallel to the Burger’s vector <111> direction. After Fourier transforming the HRTEM image, diffraction pattern of the local area could help understand the Burger’s orientation relationship between the Ni3Ti nanoprecipitates and martensite matrix. TEM is also used to understand the morphology and growth behavior of reverse austenite after aging treatment. It can find that reverse austenite tends to revert to the orientation of the prior austenite grains, and has a KS-OR with the martensite matrix by the diffraction pattern, this phenomenon is called austenite memory. Quantitative analysis was performed by XRD and EBSD on the reverse austenite to understand the relationship between reverse austenite and aging time. It is expected that the TRIP effect can be achieved by controlling the proportion of reverse austenite to improve the ductility of 465 maraging steels. Electron energy-loss spectroscopy was also used in this experiment. It was found that at 510C, the dislocation density change of martensite matrix was related to the addition of the alloying element molybdenum, which was found to be effective in reducing the softening phenomenon caused by dislocation recovery under aging treatment. Finally, this experiment uses MTS to understand the performance of mechanical properties under different parameters and to analyze the work-hardening rate curve and whether the TRIP effect is successfully introduced.