| Summary: | 博士 === 國立中央大學 === 機械工程研究所 === 82 === A modified thermal analysis was presented in this study to
discuss the effect of alloy elements and surface areas of
nodular graphite on ausferrite reaction and used to observe the
transformation kinetics of ferrite in austenite matrix of
ductile irons during isothermal holding (250℃∼400℃). The
transformed ferrite fractions analyzed by the modified thermal
analysis are close to those measured by metallographic
observations of specimens austempered for different time.
Latent heats released from phase transformation were in
relation to the amounts of ferrite transformed. Addition of
alloy elements, surface areas of nodular graphite and
isothermal holding temperatures resulted in a marked influence
on the rate of volumetric heat evolution during the period of
recalescence. Synergistic effect of higher alloying elements,
which delayed substantially the carbon diffusion, shifted the
maximum rate of heat evolution to a higher temperature. The
morphologies of ferrite plates transformed during the period of
recalescence were also observed metallographically. Silicon
and suface area of nodular graphite were found to have
accelerated nucleation and early growth of ferrite in the
matrix of ductile irons during isothermal holding. Manganese
retarded the early growth of ferrite plate. Synergistic effect
of Cu ,Mo, or Ni effectively suppressed the growth of ferrite
during the period of recalescence. A model was proposed to
simulate the average rate of volumetric heat evolution in the
period of recalescence, another model was proposed to predict
the time of τe for ADIs in the austempering temperatures 250℃
∼400℃. The time for the termination of heat evolution occurs
at time for terminating the ferrite plate growth and is close
to (but less than) the time for developing the optimum
toughening properties of austempering ductile irons (ADIs).
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