Proceedings of the AFS Cast Iron Inoculation Conference, September 29-30, 2005, Schaumburg, Illinois
A Study of Hold Time, Fade Effects and Microstructure in Ductile Iron
E. Huerta
Gregg Industries, El Monte, CA
V. Popovski
Elkem Metals, Inc., Pittsburgh, PA
Copyright © 2005 American Foundry Society
ABSTRACT
A series of treated ductile iron ladles were inoculated with various generic inoculants and at various addition rates. The
ladles were held for up to 16 minutes after addition of inoculant and samples were extracted every two to five min. Samples
were examined for chemistry, thermal analysis properties and microstructure evolution. The investigation has shown that
there are significant metallurgical changes during holding of inoculated iron in a ladle. Time and temperature effects during
holding are found to result in potential benefits to delayed pouring. The metal shows improved ATAS® (Adaptive Thermal
Analysis System) thermal analysis graphite factors 1 and 2, reduced recalescence and better nodule count and nodularity as a
function of hold time in the ladle.
INTRODUCTION
“Fade, in ductile irons, is considered to be the loss of nodularity with time” (Gundlach, 1992). The concept of “fade” in
ductile iron (DI) includes the processes of magnesium (Mg) fade as well as inoculant fade. These two terms both refer to the
deterioration of microstructure (“loss of nodularity”) that accompanies the holding of treated and inoculated metal in the
pouring vessel for some time prior to actual pouring.
The loss of residual Mg (Mg fade) results in the reversion of spheroidal graphite to compacted or even flake graphite. This
condition has been depicted graphically in studies (Frush, 1998). As a result, foundries that feature manual pouring almost
always impose a time limit on how long iron can remain in the ladle prior to pouring. This time limit is often seven to twelve
minutes and its existence assumes that the iron will have lost sufficient Mg content after such an interval as to require pigging
of the metal. The caution that accompanies such a procedure is the natural result of the fear of producing a casting that fails
in the field.
These concerns can be depicted graphically as shown in Fig. 1. Immediately after treatment and inoculation, the structure is
assumed by many operators to be near ideal. This is because Mg fade has just begun and the established fade limit is
therefore far off in time. There is plenty of opportunity to pour off the ladle before the microstructure deteriorates and fades
to gray iron (GI). The metal will inevitably fade to GI eventually, but this model completely neglects the effects of
inoculation, inoculant fade and the possibility of negative effects of Mg content.
Inoculant fade is a lesser acknowledged condition whose importance is reflected in inoculation practices. The widespread
use of in-stream inoculant when possible, as opposed to ladle inoculant, may be as a result of fade of inoculation effect in the
pouring vessel. Castings made with faded inoculant have irregular graphite structures, just as those suffering from Mg fade.
Fuller writes, “In ductile iron the fading of inoculants … causes a reduction in the number of centres from which growth of
the eutectic occurs ... The lower degree of nucleation ... is often accompanied by a deterioration in the shape of the nodules”
(Fuller, 1979). Fuller associates this effect with a loss in nodule count and views nodule count as a “reveal(er)” of decreased
nucleation. The fading of inoculant is attributed to the “coarsening and growth of micro-inclusions, also called the Ostwald
Ripening Effect. The driving force for this coarsening is a reduction in the specific surface area of the inclusions, thus
reducing the total energy of the system” (Olsen, 2004).
Castings made under conditions with inoculant fade are also more chill prone. A principal effect of fading is “to cause
greater undercooling to take place during eutectic solidification and to lead to a greater tendency for chilling in gray and
ductile irons particularly in thin sections” (Skaland, 1992).
At the same time, it is also clear that over-treatment of DI is detrimental to microstructure, resulting in irregular graphite
shapes, carbides, and slag defects (Goodrich, 1992). Over-inoculation can similarly be a problem in that pronounced early
graphite growth depletes the iron of graphite units that are needed at the end of eutectic freezing to combat microshrinkage.
Over-treatment and over-inoculation are also excessively costly.
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Proceedings of the AFS Cast Iron Inoculation Conference, September 29-30, 2005, Schaumburg, Illinois
All DI processes include some combination of Mg loss and inoculant fade over time. If the losses of inoculation effect and
Mg content are accepted as being detrimental at some point, and it is also recognized that an excess of these same quantities
is a problem, then it is logical to ask certain questions:
• How long can the foundry wait until the iron has reverted to compacted or flake graphite?
• How long can the foundry wait until the iron exhibits the effect of the loss of inoculation (nucleation)?
• When should the foundry pour the casting to assure the most carbide-free microstructure?
• When should the foundry pour the casting to assure the most shrinkage-free casting?
• Does one inoculant stand out from others in terms of fade resistance? How is this evaluated?
• Both Mg and rare earths contribute to nodularization, but does either one offer an advantage to fade resistance?
Ultimately, the foundry is producing and selling spheroidal graphite and not residual Mg. This paper represents the
culmination of two separate but related field studies that attempted to quantify the fade effects described above.
Traditional Fade Limit
100
95
90
85
80
Nodularity of
Ladle of Iron
75
Gray
Threshold
70
65
60
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18
Time After Treatment, Minutes
Fig. 1. The graph illustrates a traditional fade model.
EXPERIMENTAL PROCEDURE
An initial set of experiments was conducted over two days, designated as Gregg Study I. A similar set of experiments was
done over a single day, four months later. This set of experiments was designated as Gregg Study II.
In the first set of experiments (Gregg Study I), magnesium ferrosilicon (MgFeSi) was added to the pocket of a 1000-lb
sandwich pocket ladle at an addition rate of 1.8 wt%. The sizing and analysis of the MgFeSi can be seen in Table 1.
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Proceedings of the AFS Cast Iron Inoculation Conference, September 29-30, 2005, Schaumburg, Illinois
Table 1. Typical Composition of the Magnesium Ferrosilicon Alloy Used in This Study
Size
Si
Mg
TRE
Ce
La
Ca
Al
3/4” x 8 mesh
46.5%
5.72%
1.26%
0.65%
0.42%
1.15%
0.64%
Cover steel (punchings) was added at an addition rate of 1.5 wt%. DI treatment was then conducted by tapping 1000 lbs of
iron into the ladle. The ladle was then split into two 500-lb pouring ladles. Inoculant was added on transfer to the pouring
ladle. In total, eight pouring ladles were made. The different inoculants applied can be seen in Table 2.
Table 2. Specifications for Inoculants Used in This Study
Inoculant A
Calcium-bearing 50% FeSi (addition rates 0.3 and 0.7 wt%)
Inoculant B
Barium, Calcium-bearing, 75% FeSi (addition rate 0.3 wt %)
Inoculant C
Cerium, Calcium-bearing, 75% FeSi (addition rates 0.2 and 0.3 wt %)
Inoculant D
Cerium, Calcium, Sulfur, Oxygen-bearing 75% FeSi (addition rates 0.2 and 0.3 wt %)
These inoculants were used at varying addition rates. For the purpose of clarity, an example is provided in Table 3 to explain
the nomenclature for the individual samples in Gregg Study I.
Table 3. Example to Illustrate Sample Nomenclature
Ladle 1B3-1, where:
the first digit “1” designates the first day of the study
the second digit “B” designates which inoculant was used
the third digit “3” designates that the addition rate of the inoculant was 0.3%
the fourth digit “1” designates that it was the first iteration of this scheme on that day
Metallurgical samples were taken from the treated and inoculated ladle immediately after inoculation. The ladle was then
covered in a refractory blanket and allowed to remain undisturbed for five to six min, whereupon a second sample was taken.
This was repeated several times, at varying lengths of hold time. Table 4 details which metallurgical tests were conducted in
Gregg Study I.
Table 4. Metallurgical Tests Applied to Test Metal
Thermal Analysis cups – at 0, approx. 5, 10, and 14 minutes after inoculation
Chemistry - X-Ray Fluorescence of all solidified cups
Quantitative Metallography - Image analysis of all solidified cups
Gregg Study II was similar to Gregg Study I, except that only one inoculant was used (Inoculant A). This inoculant was used
at four different addition rates (0.2, 0.4, 0.6, and 0.8 wt %). Samples were taken at shorter time intervals in Gregg Study II.
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Proceedings of the AFS Cast Iron Inoculation Conference, September 29-30, 2005, Schaumburg, Illinois
RESULTS FROM FOUNDRY TESTING
Gregg Study I
Figure 2, generated from data in Gregg Study I, shows that the pouring ladle temperature drops over time. The temperature
plotted here is the peak temperature logged by the thermal analysis unit. This drop in temperature appears to be linear.
Gregg Study I, Pouring Temperature
1400
1350
1A7 Pour Temp
1300
1B3 Pour Temp
1C3 Pour Temp
1D3 Pour Temp
1250
2A3-1 Pour Temp
2C2 Pour Temp
2A3-2 Pour Temp
1200
2D2 Pour Temp
1150
1100
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Time After Inoculation, Minutes
Fig. 2. Graph illustrates the pouring temperature over time, Gregg Study I.
Figure 3 shows that TeLow drops over the first six minutes of hold time. This represents a quantified measure of inoculant
fade. The metal is measurably more chill-prone after six minutes than immediately after inoculation. The results are more
erratic afterwards, with some evidence of a rise in TeLow over time.
Gregg Study I, TeLow
1143
1142
1141
1140
1A7 TeLow
1B3 TeLow
1139
1C3 TeLow
1138
1D3 TeLow
2A3-1 TeLow
1137
2C2 TeLow
1136
2A3-2 TeLow
2D2 TeLow
1135
1134
1133
1132
0
1
2
3
4
5
6
7
8
9
10
11
12
13
Time After Inoculation, Minutes
Fig. 3. The graph illustrates TeLow over time, Gregg Study I
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Proceedings of the AFS Cast Iron Inoculation Conference, September 29-30, 2005, Schaumburg, Illinois
Figure 4 shows the change in recalescence over time. Here the change over time becomes more pronounced. Recalescence
is markedly lower six minutes after inoculation. There is some minor scatter after the first six min, but the overall trend is for
a minimal change in recalescence after the first six min.
Gregg Study I, Recalescence
6
5
1A7 R
4
1B3 R
1C3 R
1R3 R
3
2A3-1 R
2C2 R
2A3-2 R
2
2D2 R
1
0
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Time After Inoculation, Minutes
Fig. 4. The graph illustrates recalescence over time, Gregg Study I
Figure 5 shows that there is clear improvement in Graphite Factor 1 (GRF1) six minutes after pouring, with a nominal
decline thereafter. The graph suggests that a casting poured six minutes into a ladle will feature considerably more graphite
precipitation than one poured immediately after inoculation. This is the case regardless of inoculant.
Gregg Study I, GRF1
120
100
1A7 GRF1
80
1B3 GRF1
1C3 GRF1
1D3 GRF1
60
2A3-1 GRF1
2C2 GRF1
2A3-2 GRF1
40
2D2 GRF1
20
0
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Time After Inoculation, Minutes
Fig. 5. The graph illustrates Graphite Factor 1 over time, Gregg Study I
Figure 6 shows a dramatic improvement in Graphite Factor 2 (GRF2) over time, again, regardless of inoculant choice. Metal
poured after a delay of six minutes will have considerably more late graphite precipitation and therefore be far more resistant
to micro-shrinkage than one poured immediately after inoculation.
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Proceedings of the AFS Cast Iron Inoculation Conference, September 29-30, 2005, Schaumburg, Illinois
Gregg Study II, GRF 2
100
90
80
70
1A7 GRF2
1B3 GRF2
60
1C3 GRF2
1D3 GRF2
50
2A3-1 GRF2
2C2 GRF2
40
2A3-2 GRF2
2D2 GRF2
30
20
10
0
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Time After Inoculation, Minutes
Fig. 6. The graph illustrates the Graphite Factor 2 over time, Gregg Study I
Figure 7 shows an improvement in nodule count, regardless of inoculant choice, for the entire period of the test. This is
consistent with the results of Fig. 5. Late nodules are smaller, so the nodule count should be higher.
Gregg Study I, Nodule Count
250
200
1A7 Nodule Count
1B3 Nodule Count
150
1C3 Nodule Count
1D3 Nodule Count
2A3-1 Nodule Count
2C2 Nodule Count
100
2A3-2 Nodule Count
2D2 Nodule Count
50
0
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Time After Inoculation, Minutes
Fig. 7. The graph illustrates the nodule count over time, Gregg Study I.
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Proceedings of the AFS Cast Iron Inoculation Conference, September 29-30, 2005, Schaumburg, Illinois
Figure 8 shows that nodularity actually increases over time, regardless of which inoculant is used, for at least the first ten
minutes of the test. This result is a logical extension of the result in Figs. 6 and 7. Late nodules are smaller and smaller
nodules are a closer approximation of a circle than are larger, more irregular nodules.
Gregg Study I, % Nodularity
94
92
90
1A7 Nodularity
88
1B3 Nodularity
1C3 Nodularity
86
1D3 Nodularity
2A3-1 Nodularity
84
2C2 Nodularity
2A3-2 Nodularity
82
2D2 Nodularity
80
78
76
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Time After Inoculation, Minutes
Fig. 8. The graph illustrates the nodularity over time, Gregg Study I.
Figure 9 shows that Mg dissipates (fades) faster than do rare earths. This graph was generated by averaging the residual
contents of Mg and rare earth (cerium [Ce] + lanthanum [La]) over all eight heats over time. A trendline was plotted on that
average. The slope of the Mg trendline is steeper than that of the rare earth trendline.
Gregg Study I, Mg and Rare Earth Fade
1A7 Mg
0.04
1A7 Ce+La
1B3 Mg
0.035
1B3 Ce+La
y = -0.0005x + 0.0313
0.03
1C3 Mg
1C3 Ce+La
1D3 Mg
0.025
1D3 Ce+La
2A3-1 Mg
0.02
2A3-1 Ce+La
2C2 Mg
2C2 Ce+La
0.015
2A3-2 Mg
2A3-2 Ce+La
0.01
2D2 Mg
y = -0.0002x + 0.0132
2D2 Ce+La
0.005
mg ave
Ce+La ave
0
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Li
(C
L
Time After Inoculation, Minutes
Fig. 9. The graph illustrates magnesium and rare earth fade over time, Gregg Study I.
After this study was complete, it was decided that a second study should be done with a few modifications, such as:
• Increased sampling frequency to better determine the timing of the effects demonstrated in Figs. 4, 5, and 6 and
• Single inoculant at varied levels to allow for a more focused study.
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Proceedings of the AFS Cast Iron Inoculation Conference, September 29-30, 2005, Schaumburg, Illinois
Gregg Study II
Figure 10 shows the drop in pouring temperature associated with the individual ladles. Figure 10, like Fig. 2, shows that
temperature is measurably falling.
Gregg Study II, Pouring Temperature
1400
1350
1300
0.2 Pour Temp
1250
0.4 Pour Temp
0.6 Pour Temp
1200
0.8 Pour Temp
1150
1100
1050
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Time After Inoculation, Minutes
Fig. 10. The graph illustrates the pouring temperature over time, Gregg Study II.
Figure 11 offers greater insight into the immediate inoculant fade effect on TeLow when compared to Fig. 3. The ladle made
with an addition rate of 0.4 wt% remained relatively steady. Beyond that, regardless of addition rate, TeLow drops
immediately after inoculation as reflected in the first two minutes of all four curves in Fig. 11. The nature of the sampling
regimen in Gregg Study I, with a six-minute gap between the first two samples, was such that it was unclear how rapidly this
deterioration occurs. Figure 11 therefore offers strong evidence of the potent effect of late inoculation on carbide tendency.
Later in the curves, TeLow seems to rebound and rise again, similar to the effect seen in Fig. 4.
Gregg Study II, TeLow
1146
1144
1142
1140
1138
0.2 TeLow
0.4 TeLow
1136
0.6 TeLow
0.8 TeLow
1134
1132
1130
1128
1126
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Time After Inoculation, Minutes
Fig. 11. The graph illustrates the TeLow over time, Gregg Study II.
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Proceedings of the AFS Cast Iron Inoculation Conference, September 29-30, 2005, Schaumburg, Illinois
Unlike Fig. 11, Fig. 12 shows varying effects on recalescence depending on inoculation rate. The two ladles made with the
higher inoculation rates of 0.6 and 0.8 wt% show rising recalescence over time while the two ladles made with lower
inoculation rates of 0.2 and 0.4 wt% more closely repeated the results found in the earlier study.
Gregg Study II, R
12
10
8
0.2 R
0.4 R
6
0.6 R
0.8 R
4
2
0
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Time After Inoculation, Minutes
Fig. 12. The graph illustrates recalescence over time, Gregg Study II.
Figure 13 mostly affirms the data in Fig. 5, with the iron (Fe) displaying a rise in GRF1 over time. Three out of four curves
suggest that that more graphite will be precipitated if the pourer waits four minutes to pour the metal. This is slightly counter
to the data in Fig. 11; after all, high TeLow suggests high inoculation effect and should mean more graphite is precipitated.
However, Fig. 13 shows a rise in GRF1 at the same time as the initial inoculation effect has faded in Fig. 11. This could be
the result of a loss of Mg over this initial period such that more carbon (C) is available to form graphite. The subsequent
gradual deterioration in GRF1 might be the result of colder metal simulating a thin section behavior and further inoculant
fade impeding general graphitization.
Gregg Study II, GRF1
120
100
80
0.2 GRF1
0.4 GRF1
60
0.6 GRF1
0.8 GRF1
40
20
0
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Time After Inoculation, Minutes
Fig. 13. The graph illustrates the Graphite Factor 1 over time, Gregg Study II.
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Proceedings of the AFS Cast Iron Inoculation Conference, September 29-30, 2005, Schaumburg, Illinois
Figure 14 consistently affirms the results shown in Fig. 6. Regardless of inoculation rate, more late graphite is precipitated
after the initial inoculation has faded. Less early graphite again means more available carbon units for late graphite
formation.
Gregg Study II, GRF2
100
90
80
70
60
0.2 GRF2
0.4 GRF2
50
0.6 GRF2
0.8 GRF2
40
30
20
10
0
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Time After Inoculation, Minutes
Fig. 14. The graph illustrates the Graphite Factor 2 over time, Gregg Study II.
Figure 15 is consistent with both Figs. 7 and 14. Nodule count rises with time regardless of inoculation rate. The formation
of late nodules is consistent with lower GRF2 values. The data suggest that waiting approximately four minutest pour a
casting will result in a casting with more late-forming nodules, characteristic high nodule count and more resistance to
microshrinkage.
Gregg Study II, Nodule Count
350
300
250
0.2 Nod Ct
200
0.4 Nod Ct
0.6 Nod Ct
150
0.8 Nod Ct
100
50
0
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Time After Inoculation, Minutes
Fig. 15. The graph illustrates the nodule count, Gregg Study II.
Figure 16 offers insight into Mg fade that was not offered in Fig. 8. Because samples were taken for a longer time period,
loss in nodularity is clearly visible in the samples taken toward the right side of the graph. Nodularity improves over time in
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Proceedings of the AFS Cast Iron Inoculation Conference, September 29-30, 2005, Schaumburg, Illinois
the first ten minutes of the test and then deteriorates. This suggests that excess Mg results in substandard nodule shape in the
very beginning of the heat, and as Mg fades, nodule shape improves. At approximately 10 min, the Mg has dissipated to a
point where the metal reverts to non-spheroidal graphite shape. This graph therefore suggests that DI improves in nodularity
over time until a threshold at which it deteriorates (in this case, ten min). In other words, the graphite is most spheroidal right
before it fades precipitously.
Figure 16 also shows that the ladle made with the lowest addition rate of inoculant suffers the most loss in nodularity and also
the earliest loss in nodularity, supporting the idea that inoculant addition rate is proportional to the ability to maintain
spheroidal graphite over time. The remaining three ladles are difficult to separate in terms of timing of nodularity fade.
Gregg Study II, Nodularity
95
90
85
0.2 Nodularity
80
0.4 Nodularity
0.6 Nodularity
75
0.8 Nodularity
70
65
60
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Time After Inoculation, Minutes
Fig. 16. Graph illustrates nodularity, Gregg Study II.
Figure 17 affirms the results of Fig. 8. Mg fades at a rate faster than do rare earths.
Gregg Study II, Mg and Rare Earth Fade
0.050
0.045
0.040
y = -0.0004x + 0.0433
0.035
0.2 Mg
0.2 Ce+La
0.4 Mg
0.4 Ce+La
0.030
0.6 Mg
0.6 Ce+La
0.025
Gregg 0.8 Mg
0.8 Ce+La
0.020
mg ave
Ce+La ave
0.015
Linear (Ce+La ave)
y = -0.0001x + 0.0099
0.010
Linear (mg ave)
0.005
0.000
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Time After Inoculation, Minutes
Fig. 17. Graph illustrates magnesium and rare earth fade over time, Gregg Study II.
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Proceedings of the AFS Cast Iron Inoculation Conference, September 29-30, 2005, Schaumburg, Illinois
CONCLUSIONS
Figure 3 suggests that the effect of inoculant fade is drastic and occurs within the first six minutes after inoculation. This
might or might not be detrimental to the foundry. Because the TeLow number has dropped some measurable amount, the
possibility exists that it could drop below the white eutectic temperature, causing carbides. If the white eutectic is still below
the TeLow value at the time of freezing, then the iron will remain carbide free, and any such drop in TeLow will be
irrelevant. The inoculant fade effect shown in Fig. 3 was largely affirmed in Fig. 11. It was further revealed to have
occurred in the first two minutes after inoculation. This suggests that the resistance to chill in a similar DI will drop more
quickly than was detectable in Gregg Study I. The data suggest that the contribution to chill resistance by inoculation is
strictly time limited.
Figure 4 suggests that the metal is far more resistant to primary shrinkage from mold wall movement six minutes after
inoculation. The metal is also more likely to have more late graphite expansion after six minutes because less early graphite
allows for the possibility of more late graphite. Figure 12 shows mixed results. Two of the four curves affirm findings in
Gregg Study I, showing a decrease in recalescence over time, therefore suggesting the metal is more resistant to shrink,
caused by mold wall movement after a delay in pouring. The other two curves do not display this condition; however, they
affirm the general idea that over-inoculation is detrimental to maintaining a desirably low recalescence.
Figures 5–8 should be read together. As Mg content falls, there will be fewer Mg units in solution during solidification. This
means the metal will be less prone to carbide formation. Because of this, carbon units that would ordinarily form carbides or
even pearlite are now free to form nodules (hence improved GRF1). These nodules will be late in forming (hence improved
GRF2). Because they form later, they will be relatively small (Fig. 7). Because they are smaller, they will be rounder.
Figures13–16 are consistent with these findings.
It is true that colder pouring temperature in and of itself should, logically, cause a microstructure that simulates a thin section
with characteristic high nodule count and nodularity. However, the effects seen in Figs. 5–8 and 13–16 are likely not simply
the result of colder metal. If so, the GRF2 curve in Figs. 6 and 14 should be more linear, such as that found in Figs. 1 and 10.
Instead, there is a clear kink in the curve shown in Figs. 6 and 14. Also, GRF2 is not simply a function of pouring
temperature (Udroiu, 2002).
The increase in nodule count over time seen in these studies is in direct contradiction to Fuller. He found that, “Nodule
numbers were highest immediately after inoculation” (Fuller, 1979).
In Gregg Study I, Inoculant D showed the best long-term performance for GRF2, while Inoculant B showed the worst.
In Gregg Study I, Inoculant B showed the best long-term performance for TeLow, suggesting a benefit with this inoculant for
late chill control. This is consistent with the Frush’s findings (Frush,1998).
The fade limit in Gregg Study II was determined to be approximately eleven minutes.
Mg fades faster than do rare earths as shown in Figs. 9 and 17. This suggests that a treatment alloy richer in rare earth
content would be more resistant to nodularity fade than one with lower rare earth content. This could be the result of either
the rare earths’ own nodularizing effect or a synergy between rare earths and inoculant. The latter condition is described by
Fuller. “The inoculating effect and fading of inoculants is changed significantly in the presence of cerium in magnesium
treated irons. Its effect is to increase nodule numbers after inoculation and significantly reduce the extent to which fading
occurs” (Fuller, 1979). It is clear that rare earths play a role both in nodularization and inoculation. Regardless, their
presence is documented in this study to be more persistent than Mg.
This study sees no relationship between high nodule count and inoculant fade. This is likely the result of the inoculant effect
being overwhelmed by the effect of relatively high Mg levels at the beginning of a ladle. It is difficult to state this as the
certain cause because it is not realistic to hold Mg steady while one analyzes the effect of inoculant fade alone.
The traditional model of fade, depicted in Fig. 1, needs to be replaced. This study shows that microstructure at the beginning
of the heat is inferior to that generated some minutes later. This model is not in contradiction to Frush and others, but rather
offers data from the time period immediately after inoculation (Frush, 1998).
The exact values depicted graphically in such a model will likely be different for every foundry because of varied
manufacturing conditions, including, but not limited to, rare earth content. It could look much like the depiction in Figure 18.
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Proceedings of the AFS Cast Iron Inoculation Conference, September 29-30, 2005, Schaumburg, Illinois
Proposed Fade Model
100
95
90
85
Nodularity of
Ladle of Iron
80
Gray Threshold
75
70
65
60
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Tim e After Inoculation, Minutes
Fig. 18. This graph illustrates a proposed nodularity fade model.
REFERENCES
1. Frush, T., Lerner, Y. and Fahmy, M., “Inoculants Selection for Counter-Gravity Casting of Thin Wall Ductile Iron,”
International Inoculation Conference Proceedings (1998).
2. Fuller, A. G., “Fading of Inoculants”, Proceedings of the Conference on Modern Inoculating Practices for Gray and
Ductile Iron, pp 141–183 (1979).
3. Goodrich, George M., “Ductile Iron Casting Defects”, Ductile Iron Handbook, p 233 (1992).
4. Gundlach, Richard B., Loper, Carl R, Jr. and Morgenstern, Bernardo, “Composition of Ductile Irons”, Ductile Iron
Handbook, p 87 (1992).
5. Lee, R. S., “Extended Holding of Treated Nodular Iron”, AFS Transactions, pp 433–444 (1971).
6. Olsen, S. O., Skaland, T. and Hartung, C., “Inoculation of Gray and Ductile Iron A Comparison of Nucleation Sites and
Some Practical Advises,” 66th World Foundry Congress, pp 891–902 (2004).
7. Skaland, T., “Fading of Inoculation in Cast Iron,” 1992 Casting Congress, Czechoslovakia (1992).
8. Udroiu, Adrian, “The Use of Thermal Analaysis for the Process Control of Ductile Iron,” Novacast ATAS® User
Conference (2002).
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