中文摘要：
	本研究的目的是探討不同斷面尺寸對沃斯回火球墨鑄鐵高週疲勞性質的影響。實驗材料共分為兩批，第二批材料多加上合金元素鉍(Bi)，以了解鉍對改善厚斷面鑄件機械與疲勞性質的影響。熱處理條件分為最佳韌性與理想強度、硬度兩種，進行疲勞試棒的沃斯回火處理。經由掃描式電子顯微鏡觀察破斷面及金相組織，並以X-ray繞射儀分析不同熱處理 條件下殘留沃斯田鐵量與疲勞強度間的關係。 實驗結果顯示，當斷面尺寸增加時，鑄件澆鑄凝固的速率降低，凝固時間增加，導致疲勞強度降低。這是因為鑄件尺寸增加，球墨尺寸增加，球墨形狀劣化，發生縮孔及可能在晶界產生偏析的關係。在厚斷面 鑄件取樣位置不同時，由於澆鑄凝固時間不同，疲勞強度也不同。這與球墨變數和縮孔及可能發生偏析有關。合金元素鉍會增加厚斷面鑄件的 球墨數和球化率，因此會影響到疲勞強度。疲勞限與平均球墨直徑大小及衝擊韌性值有密切的關係，在對數刻度時，當球墨直徑增加，衝擊韌性值下降時，疲勞限成線性下降，本文提出一個可預測疲勞限的關係式。藉由掃描式電子顯微鏡觀察破斷面，對沃斯回火球墨鑄鐵的疲勞破壞機構亦做了分析與討論。
Abstract：
	This study investigated the relationship between high-cycle fatigue strength of austempered ductile iron (ADI) and cast section size. Two types of ADIs were prepared. The difference between these two materials is that the alloy element Bi was added in the second material. Two austempering heat treatments were applied to generate optimum toughness and optimum strength and hardness. Fractography and microstructure were analyzed by scanning electron microscopy (SEM). X-ray diffraction was conducted to determine the volume fraction of retained austenite. Results showed that as the cast section size increased, the fatigue strength decreased. The decrease in high-cycle fatigue strength is attributed to the increasing graphite nodule size, the deterioration in nodule shape, the increasing of microshrinkage pores, and possibly the increased cell boundary microsegregation present as the cast section size is increased. Microstructure analysis showed significantly consistent change in nodule counts and possibly the numbers of microshrinkage pores present from the bottom to the top of heavy-section casting. This change was attributed primarily to the large change in local solidification time. Fatigue strengths of specimens cut from the bottom of heavy-section casting were superior to those from the top. The nodule counts and nodularity were improved, when alloy element of Bi was added in heavy-section casting, so as the fatigue strength. The fatigue limit of ADI depended strongly on graphite nodule size and impact toughness. On the logarithmic scales, the fatigue limit decreased linearly with increasing mean nodule diameter and decreasing impact toughness value. A prediction equation for the fatigue limit of ADI was introduced in terms of the nodule size and impact toughness. Fractography results were also discussed to determine the fatigue fracture mechanism for ADI.