Microstructural characterization of cobalt-containing super-bainitic steels

在鋼鐵中添加約2wt%的矽( Si )，並在大過冷度之環境下可以生成無碳化物析出之變韌鐵，此種組織主要由變韌鐵與富含碳的殘留沃斯田鐵所組成，性質優於傳統上、下變韌鐵，藉由大量的次晶界與差排強化，因此具有良好的韌性與強度，各方向上具有均勻的之組織及性質，此類高碳高矽變韌鐵合金鋼稱為超級變韌鐵。不用太複雜的製程，亦不需要高量的固溶合金，但熱處理往往耗時長久才能到達理想性質，為工業應用上的缺點。 本實驗在超級變韌鐵中添加鈷元素，降低沃斯田鐵變態至肥粒鐵的自由能，增加相變態驅動力，以期得到快速的生長速率及較高的變韌鐵體積分率，並利用光學顯微鏡、電子顯微鏡及X光繞射儀作為主要分析工具。此外鈷也具有加速波來鐵生成的特性，在沃斯田鐵化後空冷進行200℃恆溫熱處理時，高溫停留的期間已足夠在晶界上進行擴散型相變態生成波來鐵。超級變韌鐵相變態過程為成核控制，主要成核位置是在沃斯田鐵晶界上，在熱處理初期一成核就可以生成很長的次平板，新生的變韌鐵會借助於先前變韌鐵的尖端或是晶界成核，大幅提升成核速率，因此在其他條件不變一下，設法加速相變態初期能夠使較快到達穩定硬度。因此波來鐵的生成讓部分合金元素如碳、矽及鈷分配，形成更佳的成核點，加速了整個變韌鐵相變態，因此鈷添加的效應也包括了這部份。採用小試片及油浴冷卻的方式，讓晶界上的初析雪明碳鐵或是波來鐵來不及生成，而單純鈷對整個變韌鐵相變態的影響的確存在，添加了2wt%的鈷可以使到達最低硬度及穩定硬度的熱處理時間縮短，且最終超級變韌鐵所佔據的體積百分率增加，提高了穩定硬度約HV20的量。Adding ~2wt% silicon to the carbon steel can produce carbide-free bainitic steels under proper heat treatments, which consists of a mixture of bainitic ferrite and carbon-enriched residual austenite. This kind of steel has excellent properties in tensile strength and toughness due to ultra-fine grain and high dislocation density, so it is called “super-bainite”. The heat treatment for super-bainite is very simple, which conducts isothermally at low temperature but very long processing time. That means more energy and money will be wasted and can’t be applied in industrial processes. In this research, the topic is to study the method about accelerating bainite transformation by adding ferrite-promoting elements, cobalt. We expect the whole transformation time would be shortened by cobalt additions which lower the free energy change of ferrite transformation, and also increase the volume fraction of bainite sub-unit. The experiments results show that the cobalt additions exactly accelerate the transformation but also induce pearlite nucleation and growth. The pearlite structure can be observed along the grain boundary which might influence the mechanical properties. The oil bath treatment was chosen to cool samples from austenite temperature for preventing the transformation of pearlite. This result showed that the existence of pearlite would shorten the bainite transformation time. The pearlite transformation would create carbon-depletion regions nearby so that they would be superior nucleation sites of bainite. Since the super-bainite transformation rate is nucleation control, the additional nucleation sites from pearlite transformation will also accelerate bainite transformation. By XRD analysis, it can be seen that the cobalt additions increase the volume fraction of bainite sub-unit about 3%. The increase of bainite phase effectively strengthened the matrix, and final hardness was lifted up about 20 Hv