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.?
 
References