本論文研究含碳量為0.45 wt.%的商用中碳鋼經熱處理後的相變化現象。熱處理方法為加熱至1000°C後，以水淬、空冷或爐冷方式冷卻至室溫，以及後續的低溫恆溫處理。由磁熱重分析儀(M-TGA)的分析發現經由不同熱處理的中碳鋼，得到相似的熱重溫度變化曲線，皆於485℃有磁重增加的現象，因此推斷這些經不同熱處理的中碳鋼於485℃附近是發生類似的相變化。經由穿透式電子顯微鏡分析經過不同熱處理的中碳鋼內發生的相變化情形，推論當肥粒體(α)於冷卻過程生成後，肥粒體會先經由spinodal相分離而相轉變成低溫無碳肥粒體(α’)與含碳肥粒體(α”)，其反應式表示如下：α→ α’ + α”；當冷卻溫度低於485℃以下時，含碳之α”相接續發生有序化反應，含碳α”相會經由有序化反應而轉變為新析出相，反應式為α”→新析出相。故於冷卻過程中，當肥粒體相形成後的總反應式如下：α→ α’ + α”→ α’ + 新析出相。由於快速冷卻的中碳鋼內生成的新析出相的含碳量是遠高於緩慢冷卻的中碳鋼內生成的新析出相，我們亦將其表示為新析出相’。

Phase transformations of an Fe-0.45 wt.% C alloy have been studied. 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, and water 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 treatments. 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 α” 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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