Influence of Section Size on the Low-Cycle Fatigue Strength of Austempered Ductile Irons

• Main Authors: Fu, Chun-Shiang, 傅春祥
• Other Authors: Chih-Kuang Lin
• Format: Others
• Language: zh-TW
• Published: 1996
• Online Access: http://ndltd.ncl.edu.tw/handle/50747524261494711739

碩士 === 國立中央大學 === 機械工程學系 === 84 === This study investigated the relationship between low-cycle fatigue (LCF) strength of austempered ductile iron (ADI) and cast section size and position . Uniaxial LCF tests were conducted under strain control at several strain amplitudes in tension-compression cyclic loading. The effects of austempering temperatures, nodularity, nodule counts, casting defects, and morphology of retained austenite in the matrix of ADI on the LCF behavior are discussed. Fractography with scanning electron microscopy (SEM) was used to determined the LCF failure mechanisms and fatigue crack propagation modes. Results showed that as the cast section size increased, the fatigue strength decreased. The decrease in LCF 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. Iaddition to nodule morphology and defects formed during casting process, the stability of matrix after austempering heat treatment is also an important factor influencing LCF strength of ADI. LCF strength of ADIs with optimal strength is primarily affected by the defects while matrix stability is the major factor influencing LCF strength of ADIs with optimaltoughness. In general, ADIs cut from the small cast section and austempered for optimal strength showed the best LCF property. Fatigue cracks usually initiated from the surface irregular nodules and casting defects. The fracture modes are different in the crack initiation zone and final fracture zone. The brittle cleavage was obsvered in the initiation zone and dimples (ductile fracture mode) appeared in the final fracture region for ADIs tested. However, the path of crack propagation for ADIs with optimal strength and optimal toughness are different. Fatigue cracks in ADIs with optimal strength propagated in the matrix with branching around the crack tip, and fatigue cracks in those with optimal toughness propagated with larger crack opening displacement. Both types of cracks propagated toward defects and highly distributed nodules in a direction perpendicular to loading direction. Fractography results also showed that ADIs with fast solidification rate during casting process had greater toughness.