環境因素對沃斯回火球墨鑄鐵高週疲勞之影響; Influence of Environment Factors on High-Cycle Fatigue Behavior of Austempered Ductile Cast Iron

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题名:	環境因素對沃斯回火球墨鑄鐵高週疲勞之影響;Influence of Environment Factors on High-Cycle Fatigue Behavior of Austempered Ductile Cast Iron
作者:	楊章豪;Chang-Hao Yang
贡献者:	機械工程研究所
关键词:	腐蝕疲勞破裂;腐蝕環境;高週疲勞;沃斯回火球墨鑄鐵;麻田散鐵;Austempered Ductile Cast Iron;High-Cycle Fatigue
日期:	2001-07-18
上传时间:	2009-09-21 11:35:41 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	本研究主旨在探討環境因素對沃斯回火球墨鑄鐵高週疲勞性質之影響，暸解其在不同腐蝕及中高溫環境下之高週疲勞行為，探討的腐蝕環境參數包括溶液之溫度、氯離子及pH值的影響，並比較在100oC、180oC、220oC及250oC中高溫空氣中之疲勞強度。此外，亦利用掃描式電子顯微鏡(SEM)觀察疲勞破斷面，以了解裂縫的生成及成長模式。實驗結果顯示，由於腐蝕環境能協助短裂縫克服在微結構上所遭遇的障礙物，加速短裂縫成長，故在蒸餾水、3.5% NaCl、80oC 3.5% NaCl與硫酸溶液中的高週疲勞壽命皆比空氣中還低。而提高溶液溫度、加入氯離子及降低pH值，皆會加劇短裂縫的成長速率，進一步縮短高週疲勞壽命；其中，又以降低溶液pH值的影響最大。而潤滑油因其惰性環境降低了腐蝕效應，使得其高週疲勞壽命比空氣中長。對ADI疲勞性質而言，要造成所謂腐蝕疲勞破裂的現象必須要有應力與腐蝕環境的同時存在作用，只有單純的腐蝕環境先作用再受疲勞負載，並不會影響ADI的高週疲勞壽命。 ADI在室溫至300oC間的抗拉強度，並不會因環境溫度的改變，而有明顯的不同。此外，ADI在中高溫及高應力(低壽命)區的疲勞壽命明顯會隨著溫度的上升而下降；但在低應力(長壽命)區，220oC與250oC的疲勞壽命，並不如預期般的減少，反而會接近180oC的疲勞壽命值，這是由於其較高的溫度，使得碳化物更易析出，相對使得麻田散鐵的變態點(Ms點)上升，此時殘留沃斯田鐵較容易在循環應力的負載下，產生應力誘發變態成為麻田散鐵，而在變態過程所引發的體積膨脹，會在裂縫尖端產生殘留壓縮應力阻礙了裂縫成長，延長壽命。 The purpose of this study is to investigate the influence of various aqueous solutions and ambient air temperatures on the high-cycle fatigue (HCF) behavior of austempered ductile iron (ADI) . The effects of presence of chloride, pH value and temperature in aqueous solution on the HCF resistance were characterized. HCF results obtained in air at temperature ranging from room temperature to 250oC were made a comparison to characterize the temperature effect on HCF strength of ADI. Fractography and microstructural analyses with scanning electron microscopy (SEM) were conducted to determine the fatigue crack initiation and propagation modes. Experimental results show the HCF lives in room-temperature water, 3.5% NaCl, 80oC 3.5% NaCl, and sulfuric acid solutions were shorter than those in room-temperature air for the given ADI. In addition, increasing solution temperature, adding chloride, and decreasing pH value would further decrease the HCF life as compared to room-temperature water. Among these factors, the decrease of pH value generated the most detrimental effect on HCF resistance. The SAE 10W40 lubrication oil provided an inert environment to increase the HCF life of ADI as compared to atmospheric environment. The synergism between corrosive environment and cyclic stresses was primarily responsible for the reduction of HCF life as the effect of prior corrosion did not significantly change the fatigue life when tested in air. At temperatures from room temperature to 300oC, the tensile strength of ADI was invariant with temperature. The fatigue life of ADI would decrease with increasing temperature at high stress levels. However, the fatigue lives in 220oC and 250oC at low stress levels were comparable with those in 180oC. This is due to the fact that a higher temperature would cause more carbide precipitation and increase the temperature of Ms for martensitic transformation. Therefore, the unstable retained austenite could transform to martensite more easily under cyclic loading at higher temperatures. The volume expansion resulting from this transformation would produce residual compressive stress at crack tip to retard fatigue crack growth and extend fatigue life.
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