本論文主旨是在探討兩階段沃斯田鐵化對於球墨鑄鐵沃斯回火後之顯微組織及機械性質的影響。沃斯田鐵化的第一階段維持在900 ℃，而第二階段則在降到共析反應的臨界溫度。沃斯田鐵化之後再實施沃斯回火熱處理。實驗中利用SEM觀察其顯微組織，並且以硬度、拉伸、衝擊等試驗方法進行機械性質方面的評估。
實驗結果顯示：(1) 當各成份球墨鑄鐵在上、下臨界溫度區間內進行低溫沃斯田鐵化溫度後，基底組織會有初析肥粒鐵相；(2) FCD 550在經過770 ℃低溫沃斯田鐵化熱處理後，其硬度和抗拉強度均符合MADI標準規範；(3) 經兩階段沃斯田鐵化熱處理後，因為初析肥粒鐵的析出，使得各項機械性質均略低於傳統沃斯回火球墨鑄鐵，在伸長率方面則有較為提升。
此外，本文亦針對球墨鑄鐵經傳統與兩階段沃斯田鐵化熱處理後，合金元素偏析的情況及拉伸與衝擊破斷面型態等進行探討。

The main propose of this thesis is to investigate the influences of two-step austenitizing on the microstructures and mechanical properties of austempered ductile irons. For the austenitizing, in the first step, the temperature was maintained at 900 C and followed with a lower one around the critical temperatures of eutectoid transformation. After these two-step austenitizing, the cast iron was followed with an austempering. The scanning electron microscope (SEM) was used for microstructures examination; the hardness, tensile and impact tests were utilized for the evaluation of mechanical properties.
The results show that: (1) With a lower austenitizing temperature between upper and lower critical temperatures, the matrix structure shows the existence of pro-eutectoid ferrite phase; (2) The austempered FCD 550 with a lower temperature austenitizing at 770 C shows that the properties of hardness and tensile strength do meet the standard of MADI; (3) For same austempering temperature, the existence of pro-eutectoid ferrite will decrease the hardness and tensile strength but increase the elongation of the treated iron when comparing to the ADI which having no pro-eutectoid ferrite.
In this thesis, the characteristics of segregation of alloy elements and the fracture surfaces from specimens of mechanical testing are also discusses for the investigated irons.

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