| Summary: | 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.
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