though ε-nitride could be the equilibrium phase below about 200°C (392°F), γ’ might instead be retained if a temperature near the transformation temperature is not held long enough. The observation of ε-nitride at room temperature agrees with at least one other set of observations by Hrivňák, although these observations are perhaps questionable because they were not peer-reviewed.23,24
ly studied nitride formation in steel, commented that Fe3 and Fe3
A paper by Jack, who extensive- C
N are isomorphous and could easily be confused.47
In light of this, the ε-nitride observed as a stable room tem- perature by Hrivňák may have been iron-carbon cementite. Notably, Enrietto also observed that dissolution of nitrides took almost as long as their precipitation, which suggests that the nitrogen solubility in ferrite will have the same value in either process.21
the equilibrium room temperature phase; which supports the phase diagram in Fig. 9a as opposed to that in Fig. 9b.
This observation is evidence that Fe4 content.14
The measured increase in strength from aging has been com- pared to the thermodynamically predicted equilibrium Fe4
N
correlation is important and suggests that for a given class of GCI, empirical results might permit development of an equation that would predict the increase in tensile strength after aging as a function of computed equilibrium Fe4
strengthening and predicted Fe4 N.
precipitates in cast irons were detected in ferritic malleable cast iron4
and, indirectly, by neutron scattering.11
The phase diagram in Fig. 9a, the data and modeling com- parisons in Fig. 10, and the other information presented as a bulleted list at the beginning of Section 3 are in agreement that the nitride responsible for fully aged behavior in cast irons should be γ’-Fe4
N. To date, the only possible nitride Character-
ization of precipitate phase and morphology in any cast iron has thus far eluded researchers. To understand the identity
A strong linear correlation was found between N, as shown in Fig. 10. This
N is
and behavior of the precipitates, work completed in less complicated microstructures such as steel and high purity Fe-N alloys can be discussed in context of cast iron. Such work will provide a model for the age strengthening of cast irons until further direct observations can be made.
In a review of precipitation processes in iron alloys, Ed- monds and Honeycombe determined that the process for ni- tride precipitation at high nitrogen supersaturation and tem- peratures below 200°C (392°F) is: Nitrogen Guinier-Preston (GP) zones → Fe16 tion of Fe16
from nitrogen GP zones is homogenous. For low nitrogen supersaturation and high aging temperatures the precipitation appears to skip the GP zones and hetero- geneously nucleate as Fe16 ceding Fe16
N2 N2 → Fe4
can be expected to occur when aging is performed above about 200°C (392°F) or at least above 250°C (482°F).49
formation were observed by M. Wada et al.50 Different temperatures have been observed where the direct precipitation of Fe4
N2 N2
N occurs, but the transition temperature One
study by Dahmen et al. showed that when aging a sample on a high voltage electron microscope stage at 285°C (545°F), Fe16
at temperatures above 250°C (482°F), in practical terms, the process starts with Fe4
Although this technically showed that Fe16 N because the Fe16
to precipitate and consume Fe16 N2 precipitate, at any temperature.20 Dijkstra claimed that Fe4
only 2.5 minutes later.19 N2
N2
can precipitate is so short lived.
N was not preceded by any other He drew this conclusion
despite the observation that other unidentified precipitates formed before Fe4
later, Jack concluded that the unidentified precipitates were a phase, which he named α’’ (Fe16
of nitride precipitation Jack notes, “Because the two differ- ent phases were precipitated on different planes of the ma-
N and dissolved as Fe4 N2
).26
N grew. Two years Regarding the order
began to precipitate after 1.5 minutes, and Fe4 N2
N began . The nitrogen GP zones pre- N.49
In this case, the precipita-
(a)
Figure 9. (a) Phase Diagram proposed by Malinov et al., where ferrite and Fe4 ture.48
are ferrite and ε-nitride. 50 International Journal of Metalcasting/Spring 10 (b) Phase diagram proposed by Du Marchie Van Voorthuysen, et al. in 2002.46
(b)
N are equilibrium phases at room temepra- Phases shown at room temperature
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89