; however, no observation of this has been reported. Instead, it is suggested that manganese lowers the lattice free energy in the ferrite around it, creating pre- ferred interstitial sites for nitrogen. This has been observed to delay the nucleation rate of the nitrides responsible for aging in steel21
ortho-nitride43 and possibly in cast irons.4,28
The Mn-N in-
teraction regions are known to inhibit dislocation motion which can be observed as a strength increase. Breakdown of the Mn-N complexes to provide nitrogen for nitride nu- cleation could explain the small decrease in strength and hardness at the start of the aging process. In steel samples, an increase in aging temperature reduced the effect of man- ganese on precipitation times.21
The only deviations from
the artificial aging trend were at 200°C (392ºF) and 250°C (482°F), where the apparent dual or sequential precipita- tion of Fe16
higher than those for samples aged at 350°C (662°F). This pattern argues for a dual precipitation process in GCI like that observed in some nonferrous alloys, a possibility sup- ported by the findings of Richards et al. with observation of two kinetic models depending on temperature, each with a different activation energy.13
N2 and Fe4
Accelerated aging at 182°C (360°F) and 285°C (545°F) in GCI has displayed overaging.15
Figure 7 provides an exam-
ple of such overaging. This overaging is likely associated with the growth of some precipitates at the expense of oth- ers, which is known to increase the mobility of dislocations and thus to reduce overall strength.44,45
The age strengthening process in GCI follows Avrami-John- son-Mehl kinetics and can be described by Eqns. 1 and 2. In Eqn. 1, Vf
is the activation energy, R is the universal gas constant, and T is absolute temperature.
is volume fraction of precipitate, k is a rate con- stant defined by Eqn. 2, t is time, and n is the time exponent, which is a constant. In Eqn. 2, ko
is the attempt frequency, Q
Equation 1 Equation 2
The aging process in GCI is not isokinetic.28 In the irons stud-
ied there appears to be a transition in kinetic behavior some- where between 100°C (212°F) and 182°C (392°F). This tran- sition is apparent in the Arrhenius plot in Fig. 8. The change in behavior indicates that a different precipitate forms at temper- atures above the observed transition. The temperature of tran- sition from either a dual or sequential precipitation process to a single precipitation process is somewhat lower than that observed in steel by Enrietto, who noted a transition to pre- cipitation of only Fe4
N at or above 300°C (572°F).21 His study
used a high-purity Fe-N alloy, it is conceivable, therefore, that the many elements present in cast iron, both substitutional and interstitial, could affect the precipitation kinetics and account for the difference in observations.
International Journal of Metalcasting/Spring 10 Time in Hrs
Figure 7. Curve from accelerated aging at 285ºC (545°F). There is a peak tensile strength at about 5 hours, fol- lowed by loss of strength from overaging.15 are plus/minus one standard deviation in length.
Error bars N created aging rates that were even
Figure 9 presents two of the most recent representations of the Fe-N binary phase diagram. Authors of Fig. 9b indicated that their observation of ferrite and ε-nitride were the stable phases at room temperature opposes the previous belief that ferrite and γ’-nitride (Fe4
N) are the stable room temperature phases.46 The same paper discusses the possibility that al-
1000/T(K)
Figure 8. Arrhenius plot for gray cast iron aging kinet- ics.28
Aging at 182°C (360°F) and 200°C (392°F) indicates formation of only one precipitate, whereas curves at 20°C (68°F) and 100°C (212°F) indicate the presence of at least two precipitate species and/or morphologies dur- ing aging.
49
In (k in hrs. -1
)
Tensile Strength in MPa
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