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					                          THE EFFECT OF METALLIC CHARGE

The Effect of Metallic Charge/Melt History on Nucleation Potential in Ductile Cast
Irons

A paper presented at the Ductile Iron Society meeting, June 1998.
By James D. Mullins & Eugene C. Muratore;
Rio Tinto Iron & Titanium, Inc.

The melt history, including the type of charge material, chemistry and molten metal processing of a cast
iron melt has a pronounced and measurable effect on the final structure and properties of the castings
poured. The assessment of the nucleation potential of a graphitic cast iron before treatment and/or
inoculation has been practiced at some level or degree for a long time.

CHILL WEDGES
See Figure1
Chill wedge testing of base cupola iron gave the operating foundryman
a qualitative measurement of the graphitizing (nucleating) potential of
that iron. See figure 1. An iron that has a large chill value (tendency to
form carbides upon solidification) means that it possesses a low
nucleating potential. So by measuring the width of the chilled (carbidic)
portion of the wedge, changes to the charge amount or type of material
could be made. Pouring another wedge after the inoculation step could
assess the effectiveness of the inoculant. Before the advent of
affordable and timely chemical analysis, and certainly before the
development of computerized cooling curve analysis, the dependency upon wedge testing for the
assessment of suitability of an iron for pouring was mandatory. Melters soon realized that changes to the
charge and/or thermodynamic changes within the cupola manifested dramatic changes in the chill wedge
values of cast iron.

For example, when pig iron was introduced into the charge, the chill value typically decreased. As the
steel portion of the charge was increased, the chill value increased. As the melt conditions moved to more
oxidizing conditions, the chill value increased. Cupola well depths and iron dam heights were carefully
measured and controlled in order to maximize the nucleating effect of the coke. All of these changes to
the nucleation potential were seen even though the chemistry most often did not change.

With the increasing popularity of induction melting furnaces as primary melters, the utilization of the
wedge test has fallen out of favor. Since the chemical analysis could be much more closely controlled, it
was incorrectly assumed that the nucleation potential was also being more closely controlled.




DUCTILE IRON FINAL WEDGE
See Figure 2
Ductile Iron foundrymen oftentimes saw nothing other than white iron
(100% carbidic) fractures in their base iron wedges, typically because of
the lower content of silicon and sulfur, and also in the final wedges. So
they too abandoned the use of chill wedge testing for evaluation of the
nucleation potential.

There are a number of factors that affect the nucleation potential and
metallurgical quality of cast irons. They are: the metallic charge, the
type of melting equipment employed, melting and holding temperatures,
dwell time (holding time), chemical composition, and inoculation. Each of these factors will now be
explored further.

As I mentioned earlier with cupola melted irons, the metallic components of the charge exert a large effect
on the nucleation potential of the melt. The reason for this effect is the steel component of the charge
contributes very little in the way of nuclei for the growth of graphite. Likewise, the Ductile Iron returns
portion of the charge, being quite deoxidized during treatment and inoculation, also contribute little
nucleation.

EFFECT OF REMELTING
See Figure 3
As an example, when returns are repeatedly remelted, even just two
times, the solidifying iron can become all carbidic. To reduce this effect
and renucleate, additions of some pig irons, graphite, silicon carbide,
and other ferrosilicon alloys are made to liquid melts.

In order to produce a cast iron melt that responds well to inoculation
and exhibits the lowest potential for carbide formation during
solidification, the returns should be limited to no more than 50%, the
steel component should be limited to 40% maximum, and consideration
should be given to utilizing some pig iron in the charge. Figure 4. Effect of rusty scrap on chill in induction
melted 4.1% C.E. gray cast iron with no inoculation

EFFECT OF RUST ON CHILL VALUE
See Figure 4
The cleanliness of the charge material also plays a role in determining
the chill value. If the charge material is heavily oxidized, the resulting
iron will exhibit a much higher chill value. We have seen the opposite to
also be a problem. Several foundries have shot blasted all of their
charge materials to remove rust and sand. They found very high chilling
tendency in this iron and as a result more shrinkage defects. So having
a small amount of oxygen in the base melt is necessary.

The type of melting equipment can play a role in iron nucleation. Iron melted in a cupola is conditioned by
the nucleating effect of the intimate contact between molten iron and the coke in the cupola well and a
relatively short time at high melt temperatures. Cupola melted irons usually exhibit a lower chill value and
generally require less inoculation in order to produce carbide free microstructures. Further the presence
of adequate oxides and sulfides as nucleation sites renders cupola melted iron as one with a high
metallurgical quality.

As more experience was gained throughout the 1950's and 1960's with melting gray irons and ductile
base irons in induction and arc furnaces, note was made that these irons exhibited higher chill and more
shrinkage tendency even while having identical chemical compositions as cupola melted irons. The
reasons for this are several: In electric furnaces there is no coke contact as well as more stagnant bath
conditions, higher melting temperatures are used to dissolve carbon, and longer holding times and often
times there are lower oxide contents. This leads to higher base iron chill (low nucleation values) . For
these reasons, electric furnace melted irons generally require different charge ratios and additional
amounts and often times more potent inoculants.

I have already mentioned something about temperature, but there is more. In the case of cast iron melting
in electric arc furnaces, the temperatures attained near the arc tip may exceed 5000oF. Irons thusly
treated are called "fried" irons, because all the nuclei have been cooked out, leaving an iron that will not
have a low chill value.

EFFECT OF SUPERHEATING
See Figure 5
As the temperature of any melt is increased above the normal melt
temperature (high superheat), the nucleation is reduced. This loss of
nucleation or reduction in metallurgical quality is manifested with
virtually no change in chemical analysis. The measured chill depth may
change from an acceptable level to all white wedge over a 200oF
temperature range or less. This reduction in metallurgical quality
requires the use of greater amounts of inoculant(s) in order to produce
acceptable final microstructures. It may not be possible to correct this
iron. It is therefore advisable to melt and hold iron at as low
temperatures as practical.

The effect of long dwell or holding times on the nucleation potential of cast irons is similar to the effect of
high melting (superheating) temperature. The longer the hold times, at any temperature, the greater the
loss of nucleation. The higher the temperature during this holding period, the worse is the loss. The most
prevalent instance of this phenomenon is known in the trade as "Monday morning iron". It has long been
recognized that irons held over a weekend exhibit very different solidification behavior than normal. These
irons exhibit higher shrinkage tendency and have more carbide due to this loss of nucleation. Irons that
have not been renucleated by the addition of "fresh" iron or nucleating agents exhibit a much higher chill
level. This issue is so important, that the AFS Molten Metal Processing Committee has begun a research
project to show foundries this holding effect on iron properties/defects and what can be done to reduce or
eliminate this problem.

The chemical composition can alter the nucleating (graphitizing) tendency of cast irons to a certain extent.
As the carbon equivalent is lowered the tendency to solidify with a more carbidic microstructure
increases. As the level of carbide stabilizing elements is increased, the same effect is seen. Even at the
same carbon equivalent and residual element levels, changes in carbon/silicon ratio can alter the
metallurgical quality and physical properties. Generally speaking increasing carbon content reduces
shrinkage tendency in cast irons and increasing silicon content reduces carbide formation, but these
effects are lost due to the loss of nucleation.

Ductile Irons treated with magnesium ferrosilicon alloys often begin as base irons of very low silicon (often
times less than 1.2%) content. These low silicon base irons may exhibit an all white chill unless an
adequately large wedge test core is used. Of course, a low nucleation level may also contribute to an all
white chill value. So using the correct size wedge (see ASTM A367) is important, as is a good sampling
procedure in order to achieve the correct result. Done properly, the chill
test can be very helpful to assess ductile iron base metal to see that is
has been well processed and has a low chill value.

MAGNESIUM vs. MODULUS
In magnesium treated irons, high magnesium content acts to promote
carbidic microstructures and increase shrinkage. The magnesium level
must be controlled carefully to the cooling rate of the casting to avoid
increased chilling tendency. This cooling rate is described as the
modulus, which is a ratio of casting volume to cooling surface area.
Thus modulus is a more accurate way to describe the cooling of a casting section than just measuring the
section(s) size. See figure 6.

Of course, all of the carbide stabilizing elements should be kept to relatively low levels to minimize their
effect on chill (carbide) promotion. Doing this will then allow more of the available carbon to transform to
graphite.

Many foundries have reinstituted melt assessment through chill wedge testing and /or thermal (cooling
curve) analysis programs because they are simple and inexpensive. The wedge test can be used to verify
the results of the cooling curve.

Magnesium concentration effect on shrinkage
Inoculation is the final and the most important step in molten metal processing. Although not all of the
problems addressed above can be compensated for with inoculants, several facts stand out. Foundries
that pour thin-section castings, tapped at elevated temperatures, may be able to produce acceptable
castings with very good inoculation. Without it, this would not be possible. The correct use of inoculants
and preconditioning agents can also allow for the utilization of irons held over weekends and holiday
periods, if the iron has not deteriorated badly.

Despite the rigid control of residual elements in many foundries, some percentage of deleterious
elements is usually always present. The employment of adequate amounts of and effective inoculants
enables the seasoned foundrymen to produce acceptable castings from these irons.

ELEMENT SEGREGATION TENDENCY
The production of heavy section castings also requires adequate
nucleation and inoculation in order to shorten the intercellular spacing
so that strong segregation of carbide stabilizing elements is avoided.
Even at low concentration levels, these elements are known to
segregate to the last to freeze areas and contribute to grain boundary
carbides and deteriorate the mechanical properties, as well as
machinability.

When we look at the tendency to segregate; elements with numbers
greater than 1 tend to segregate into the intercellular regions and those elements with numbers less than
1 tend to increase their concentration around the graphite nodules. As an example, from the slide, the
molybdenum concentration can be up to 25 times more in the intercellular region than it is in the rest of
the iron. Conversely the concentration of copper around the nodule will be higher than the concentration
of silicon and neither one will have much of a presence in the intercellular regions See figure 7.

Element Segregation Factor
  Mo..........................25.3
  Ti..........................25.0
  V..........................13.2
  Cr..........................11.6
  Mn..........................1.7 - 3.5
  P..........................2.0
  Si..........................0.7
  Co..........................0.4
  Ni..........................0.3
  Cu..........................0.1

Supporting Work
The metallurgical quality or nucleation state of the iron has been studied and published by many authors.
Vern Patterson who wrote Foote Foundry Facts - devoted several issues to the importance of measuring
chill wedge values and the effects of processing variables on the nucleation level of cast irons. The
benefits of an established and practiced wedge control program are a recurrent theme throughout the
issues.




See Figure 8. Preconditioning Effect on BHN Hardness
See Figure 9. Preconditioning Effect of Elongation




SeeFigure 10.

B.C. Godsell, in his AFS Transactions paper, "Preconditioning of Ductile Iron" describes one foundry's
method to adjust the base nucleation state of ductile base iron before treatment. Before utilizing a
preconditioning program, the foundry was unable to produce castings to an acceptable hardness or
elongation range. After the institution of a preconditioning program, which normalized the nucleation state
of the iron before treatment, ductile iron castings could be produced as cast with properties consistent to
those of heat-treated castings.

J.M. Frost and D.M. Stefanescu in their paper "Melt Quality
Assessment of SG Iron Through Computer Aided Cooling Curve
Analysis" ran a designed experiment where it was shown that several
processing variables had pronounced effects on nodule count and chill
depth.

As the percentage of pig iron is increased and the superheat and
pouring temperatures are decreased the nodule count is increased.

As the superheat time or temperature is decreased, the nodule count is increased and the chill depth
decreased. See figure 11.

Further, decreasing superheat temperature and increasing pig iron
content had the effect of reducing the chill depth, while reducing the
pouring temperature had little effect. See figure 12.

In conclusion, the metallic charge and melting history of cast iron melts
have a significant effect on the final metallurgical structures obtained.
These structures affect mechanical properties, shrinkage behavior and
machinability in these castings. A base iron that has a low chill value or is preconditioned to have a high
nucleation state will tend to have less magnesium and inoculation fading. This usually means that
shrinkage problems will be reduced. Assessment of this nucleation condition is important to producing
consistently high quality castings.?

				
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