碳鋼可經適當熱處理後，具有高的強度、硬度、彈性與疲勞極限，常用於耐磨耗的工件與切削工具上，如中碳鋼可作為工具鋼，高碳鋼可作為鑽頭、絲攻、鉸刀等。本論文研究於不同含碳量之碳鋼經熱處理後的相變化情形，熱處理方式為加熱至1000℃後，以爐冷、空冷、水淬與淬入液態氮方式冷卻。由磁熱重分析儀(M-TGA)的分析，發現經由不同熱處理的碳鋼，得到相似的熱重溫度變化曲線：皆於475℃有磁重增加的現象。利用同步輻射臨場加熱方式的分析，亦發現於475℃附近有相變化的產生；因此推斷這些經不同熱處理的碳鋼於475℃附近是發生類似的相變化。由穿透式電子顯微鏡分析經過不同熱處理的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475℃以下時，含碳之α”相接續發生序化反應，含碳α”相會經由序化反應而轉變為新析出相(BCT)，反應式為α” →新析出相(BCT)。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’ + BCT。

關鍵字：碳鋼、spinodal相分離、序化相變化。

We have studied the phase transformations of carbon steel with different 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 475℃ from the M-TGA measurement for the steel with different heat treatment. We have also found specific phase transformation occurring at temperature near 475℃ 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 has grown by aging in slow cooling and driving force in quench. Then, 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 475°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’ + BCT

Keywords: carbon steel, spinodal decomposition, ordering reaction.

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