基地組織對沃斯回火球墨鑄鐵疲勞裂縫成長之影響

• Main Author: 張智為
• Other Authors: 林志光
• Format: Others
• Language: zh-TW
• Published: 1999
• Online Access: http://ndltd.ncl.edu.tw/handle/34968690774332907824

碩士 === 國立中央大學 === 機械工程研究所 === 87 === The purpose of this study is to characterize the factors that influence the fatigue crack growth (FCG) properties of austempered ductile iron (ADI). These factors include the heat treatment conditions, mean stress effect, and stress-induced martensitic transformation. FCG tests using compact tension (CT) specimens were conducted at three load ratios (R = 0.1, 0.5, and 0.7) for as-cast and two austempered ductile irons. Fractography and microstructure were analyzed by scanning electron microscopy (SEM). X-ray diffraction was conducted to determine the volume fraction of retained austenite. Experimental results show that as-cast ductile iron has lower FCG rate than ADI at low DK region. However, as-cast ductile iron has higher FCG rate than ADI at high DK region. Stress-induced martensitic transformation has been observed to occur at the blocky retained austenite near the main crack area for ADI specimens. As ADI-II (austemped at 360℃, 2 hr) has a larger volume fraction of retained austenite than ADI-I (austemped at 300℃, 3 hr), more amounts of martensitic transformation have been observed to exist in the ADI-II specimens. Such difference in the extent of martensitic transformation resulted in the greater crack closure effect and lower FCG rate at a load ratio of R = 0.1 for ADI-II in comparison to ADI-I. At higher load ratios (R = 0.5 and 0.7), the superiority of ADI-II over ADI-I in FCG resistance became less evident due to the absence of crack closure effect from stress-induced martensitic transformation. However, the difference in FCG rates at various load ratios for a given ADI can't not be explaied simply by the crack closure effect. This is inferred from the fact that when the FCG rates are correlated with the effective stress intensity range, the FCG rate is increased with an increase in load ratio.