本研究的目的在探討鑄態及沃斯回火球墨鑄鐵在不同環境下之疲勞裂縫成長行為，比較在不同腐蝕性環境及潤滑油環境下因沃斯回火熱處理條件及負荷頻率之不同所造成疲勞裂縫成長行為的差異性。 實驗結果顯示，不論是高溫或低溫沃斯回火恆溫處理皆無明顯提升球墨鑄鐵在腐蝕環境下的耐腐蝕能力。在腐蝕疲勞裂縫成長方面，鑄態與二款沃斯回火球墨鑄鐵在負荷頻率20 Hz下，於不同室溫腐蝕環境下的疲勞裂縫成長速率因受到腐蝕產物誘起裂縫閉合效應的影響，比其在空氣中慢。扣除裂縫閉合效應後，在此高負荷頻率下，腐蝕環境因素對其疲勞裂縫成長驅動力的影響並不大，造成三款材料其有效裂縫成長速率皆與空氣中相近。換言之，在高負荷頻率條件下，鑄態與沃斯回火球墨鑄鐵的疲勞裂縫成長行為對環境的敏感性較小，造成鑄態與沃斯回火球墨鑄鐵在空氣與不同室溫腐蝕環境下的疲勞裂縫成長趨勢一致。 至於鑄態與沃斯回火球墨鑄鐵在SAE 10W40潤滑油環境下的疲勞裂縫成長行為，因該油體提供較惰性的環境降低腐蝕效應，使其裂縫成長速率比空氣中慢。此外，隨著環境溫度升高，增加裂縫成長驅動力，並加速溶液中離子的擴散速率，使得裂縫尖端氫脆效應加大，導致沃斯回火球墨鑄鐵在80℃鹽水下的裂縫成長速率比室溫鹽水下來得高。 關於頻率效應方面，因降低負荷頻率使得裂縫尖端的氫離子有較長的時間擴散進入金屬內，破壞金屬鍵結，導致裂縫尖端的氫脆效應較明顯，加速疲勞裂縫成長速率，使得沃斯回火球墨鑄鐵在腐蝕環境中於負荷頻率1 Hz下的疲勞裂縫成長速率明顯比負荷頻率20 Hz快。 The purpose of this study is to investigate the fatigue crack growth (FCG) behavior of as-cast and austempered ductile irons (ADIs) in different environments. In particular, the FCG rates in several aqueous environments and lubrication oil were made a comparison for various heat treatment conditions and loading frequencies. Experimental results show that austempering heat treatment did not improve the corrosion resistance of ductile iron in the given corrosive environments. In various room-temperature aqueous environments, all the as-cast and austempered ductile irons exhibited lower FCG rates at 20 Hz than those in air due to the crack closure effect induced by corrosion products. By subtracting the crack closure effect, the given room-temperature corrosive environments did not exert any detrimental effects on the FCG behavior at 20 Hz for the given ductile irons as the effective FCG curves of each iron in these corrosive environments were close to that in air. In other words, the FCG behavior of the given ductile irons at 20 Hz was not sensitive to the environmental effects at room temperature. The SAE 10W40 lubrication oil provided an inert environment to reduce the FCG rates for the given ductile irons as compared to atmospheric environment. An increase in the temperature of salt water from room temperature to 80℃ caused a remarkable increase in FCG rate for ADI due to the enhanced diffusion rate and hydrogen embrittlement (HE) effect at the crack tip. As the cyclic loading frequency was reduced, more time was available in each cycle for the corrosive environments to interact with the crack tip and enhance the HE effect. As a result, the FCG rates at a lower frequency (1 Hz) were greater than those at a higher frequency (20 Hz) in corrosive environments.
