| Summary: | 碩士 === 國立臺灣科技大學 === 機械工程系 === 106 === High carbon steel has high strength, hardness, elastic limit and fatigue limit after proper heat treatment, and is suitable for tool steel. Phase transformations play an important role on the high carbon steel. We have studied the phase transformations of a high-carbon steel with 1.0 wt.% C. The methodology for the processes of the steel includes heating the steel samples to high temperature, 1000℃, and cooling to room temperature via furnace cooling, air cooling, water quenching and liquid nitrogen quenching. We have discovered similar curves showing significant magnetic weight gain at the vicinity of 485℃from the M-TGA measurement for the steel with different heat treatment. We have also found specific phase transformation occurring at temperature near 485℃ by applying the synchrotron radiation with in-situ heating. After examination of the phase transformations of the steel after various heat treatment by TEM, we have concluded that ferrite phase has undergone unknown phase transformations during cooling. Upon cooling from high temperature, ferrite has undergone the spinodal decomposition and decomposes into two other low temperature ferrite phases. One is carbon-free ferrite (α’), and the other is carbon-contained ferrite (α”). The spinodal decomposition is as follows. α→α’+α”.The ordering reaction occurs at temperature below 485°C and the ” phase transforms into new precipitate, i.e. α” → new precipitate. The overall reactions occur in the ferrite during cooling are as follow. α→α’+α”→α’ + new precipitate
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