Ductile Iron Treatment Optimization

Al Alagarsamy
Citation Corporation, Birmingham, Alabama

Introduction:
Ductile iron production has come a long way since its discovery during the 1940s. Even though magnesium and rare earths have both been shown to produce spheroidal graphite, magnesium is the element of choice commercially. There are many methods used to convert base iron to ductile iron. The result of ductile iron conversion is measured or evaluated by the following factors:

• Nodularity - maximize

• Nodule count - maximize

• Chill carbides - minimize

• Inverse chill - minimize

• Shrinkage porosity- minimize

• Inclusions - minimize

Unique properties of magnesium
Magnesium has a low boiling point, high reactivity with oxygen and low solubility in iron making reliable additions to iron difficult. To minimize Mg reactivity calcium is added to the master alloy. Magnesium content of the alloy is also restricted to lower the reactivity. To increase solubility silicon is raised in the vicinity of the alloy by using ferrosilicon as cover material. To satisfy different needs many alloys were developed with combinations of Mg and rare earths.

Different treatment process
Many different treatment processes have evolved over the years to achieve better quality ductile iron at the lowest cost. Many variables affected the selection of a particular treatment process. Some of the factors are:

• Base melt quality including sulfur content

• Temperature of treatment, holding and pouring

• Delays in metal handling

• Casting section modulus

• Ease of late inoculation

These factors affect the treatment technique due to the unique properties of magnesium and resulted in the following processes, which are still used in commercial production:

• Open ladle

• Tundish

• Flow thru processes

• Pure magnesium processes-Converters, plunging, pressure vessels

• Wire feeders

• In-mold process

Nodularity and nodule count
It has been shown by many that the nodule shape is best when the magnesium residual is just enough as too much will deteriorate the nodule shape from fully spheroidal. Nodule count can be maximized by sound base iron melting practice and good inoculation practice. Cooling rate affects both the nodule count and the nodule shape. Fast solidifying iron results in better nodule shape than slowly cooled iron for the same magnesium residuals. Larger sections require increased magnesium residual and late inoculation reduces the magnesium requirement. When rare earths are added to the iron the amount of magnesium required is also reduced. As some of the magnesium measured is in the form of magnesium sulfide, final iron sulfur level affects the magnesium needed to result in nodular graphite. All of these effects are shown in the figure 1.

Figure 1. relationship between final sulfur and magnesium affected by other factors

Combination of rare earths and magnesium
Over the years it was noticed that some tramp elements affected the graphite shape. Increasing magnesium was not able to counteract the effect of tramp elements like Pb, Ti, Bi etc. Rare earths were found to neutralize these elements and restore the graphite shape to nodular form. Hence rare earths were added either with or incorporated in master alloys. For irons treated with pure magnesium Rare Earths could be added as misch metal or rare earth containing inoculants. Rare earths, like magnesium, will combine with sulfur and oxygen. They are also additive to magnesium in nodularizing effect. Because of this, one has to look at magnesium and rare earths content in total and not separately in determining amounts needed for best properties. As the rare earths are heavier than magnesium it takes 5.8 times more rare earths than magnesium, by weight, to combine with sulfur and oxygen.

The Atomic weights of rare earths are about 140, and that of magnesium is 24. Combination of magnesium and rare earths resulting in various shapes of graphite is shown in figure 2. When rare earths are added with magnesium alloy the analyzed rare earths include that portion combined with oxygen and sulfur, similar to magnesium analysis.

Figure 2. Mg and rare earth combination and graphite shapes.

Types and amount of rare earths in the magnesium alloy varied over the years depending on the economics. As rare earths are more expensive than magnesium alloy and it takes quite bit more to combine with oxygen and sulfur it is necessary to examine the relationship of rare earths in ductile iron manufacturing. There is much anecdotal information regarding the use of rare earths and its effect on the graphite shape. A minimum amount is necessary to neutralize any residual tramp elements that may be present in the iron. It has been shown that proper addition of rare earths increase nodule count. When cerium in the magnesium alloy is used in excess resulting in a residual of over 0.01%, pro-eutectoid graphite particles tend to be larger and may result in graphite floatation and exploded graphite, especially when the sulfur is low and the CE is on the high side. When rare earths are added after the magnesium treatment along with the stream inoculant then the level of cerium that causes graphite nodule enlargement and floatation is much lower than the 0.01%. It is closer to 0.004%. The difference may be due to the fact that the rare earths in the first case are combined with sulfur and oxygen and most of the rare earths are uncombined in solution in the second case. This will lead one to envision the magnesium and rare earth relationship differently if the rare earths are added after the magnesium treatment where most of the oxygen and sulfur are tied up with magnesium. This may be represented in the revised areas for various shapes of graphite in figure 3. Obviously the levels of Mg and rare earths do not stay the same throughout the pour in a batch system due to fading of active elements with time. From figure 3 there is no vermicular graphite shown when there is no rare earths in the iron. Ductile iron with only magnesium will revert to grey iron without going through a vermicular graphite form.

Figure3. Graphite shapes, Mg and rare earth levels

Fading of magnesium and rare earths
Fading of the elements Mg and rare earths (mainly Ce, La and Pr) is due to several factors. Magnesium can evaporate due to high vapor pressure at the melt temperatures. Both Mg and rare earths will react with combined oxygen present in the iron as oxides of Si, Mn and Fe. Sulfides of these elements breakdown to form oxides and the freed sulfur will react with free Mg and rare earths. All of these mechanisms are active in any treated ductile iron melt.

The rate of fading will increase if the iron bath contains high amounts of slag, if the temperature of the melt is high and the if the melt is exposed to turbulence either in the furnace, or in the ladle when it is tilted back and forth. Protective atmospheres and clean full ladles held without disturbances minimize fading as shown in figure 4. Due to this fading event, treated iron needs to be poured before the levels of Mg and rare earths fall below that necessary for the castings being poured.

If the iron is held in a pressure pour furnace under nitrogen or inert atmosphere, the iron will have a longer life. In pressure pour furnaces sulfur tends to be lower due to the time factor hence magnesium level could be lower and still produce good ductile iron. Of course the late inoculation in the form of stream inoculation helps to improve nodularity, which helps in reducing the level of magnesium.

One of the problems seen in the industry is if pouring ladles are not emptied at the end of pouring, the remaining iron cools down and solidifies as grey iron. This may cause problems for the next time as this iron will fade faster than normal.

Inoculant fading is different from magnesium fading. Inoculant fading results in lower nodule count, chilled edges (carbides) and inverse chill. It can also result in lower nodularity even when there is adequate Mg and rare earths for the section thickness and sulfur levels.

Each foundry should establish the maximum time the iron can be poured for their circumstances.

Figure 4. Fading of Mg, Ce and La in treated iron reheated in the furnace as well as held in a covered ladle, sampled periodically in a 16 minute total time interval.

Quality of ductile iron – cleanliness
There is more to ductile iron quality than just nodularity. Inclusions resulting from sand and slag that are readily seen and recognized as macro inclusions will decrease mechanical properties. Ductile iron quality as measured by impact properties, fatigue endurance limit and machinability are affected by cleanliness of the iron.

Micro inclusions such as nitrides, and carbides are detrimental to machinability and ductility of castings.

Even though solubility of calcium is limited in ductile iron, Ca content of the magnesium alloys is limited to a low level in the production heavy section castings.

Increased Ca in ferro alloys contribute to ladle build up and increased inclusions in the castings. Dross formation in ductile iron was studied by Prof. Heine and Prof. Loper extensively and reported in the literature. They have shown that there is a temperature at which the dross formation is accelerated and iron should be poured above this temperature. This process is reversible.

If a colder iron containing dross is heated back up the iron becomes clearer as the temperature is raised above the dross formation temperature. Carbon and reactive elements in the iron combine with the oxides of silicon, manganese and iron and reduce them to their elemental form.

Industry has also recognized that pouring colder will cause excessive dross formation and may result in scrapped castings. But melting hotter has its own problems. Super heating the iron above 2750^oF results in reduction in the number of nuclei within the base iron which is finally prone to inverse chill formation. This effect is commonly referred to as ‘Monday morning iron’. Besides affecting iron metallurgically, melting at high temperatures reduces lining life as carbon reacts with silica in the lining to form silicon. Carbon-silicon temperature equilibrium curves with areas of chemistry for various cast irons are shown in the figure 5. If iron is melted above the equilibrium temperature lining wear will occur, and if the temperature of the melt is below the equilibrium temperature then dross formation is encouraged.

Hence Prof. R.W. Heine exhorted foundrymen to “Melt Cold and Pour Hot”. By following this principle least amount of damage will be done to the metallurgy of iron during melting and pouring.

Figure 5. Equilibrium diagram and areas for different grades of cast irons

To verify this axiom experiments were conducted at the Daimler Chrysler casting lab by Phil Seaton. In these experiments iron was melted at around 2500F and then treated with a 5% magnesium alloy containing about 2% balanced rare earths (Ce, La, Nd, Pr). After treatment the iron was poured back into the melting furnace and micro lug and chemistry samples were taken. The iron was then heated in the furnace to raise the temperature to around 2600F and that temperature was maintained thereafter. Every few minutes micro and spectro samples were taken. Total elapsed time from the time iron was treated to last sample (7) was poured is 17 minutes. The micro lugs were open mold rectangular bars as shown in the figure 6. The surface appearance of the micro lugs varied from severe slag inclusions in sample 1 to a very clean surface in sample 7.
At low pouring temperatures the iron is mixed with plenty of slag inclusions which do not separate easily from the melt. As the temperature is increased the slag separates from the melt more easily and the carbon, Mg and rare earths reduce oxides of Si, Mn and Fe to their elemental form. This effect can be seen in the figure 7, which shows the manganese content of the iron increase with heating of the iron suggesting that MnO in the slag has been reduced to elemental manganese. Reduction of manganese continues for at least 15 minutes. This may indicate that finely dispersed oxide particles continue to be reduced by carbon and other active elements and some of these reactions will produce gas bubbles, which may be trapped underneath freezing skin. We can expect this kind of actions taking place even in the mold as more oxides are introduced due turbulent mold filling.

Figure 6. Samples taken from furnace where treated iron is reheated to 2600^oF.

Good quality ductile iron as measured in the pouring ladle can still be damaged during the mold filling process. If the pouring stream is fragmented, especially if stopper rod and nozzles are used, the separated streams cool fast and the iron temperature can go below the dross formation temperature quickly. In the running systems and gates, iron will become colder and may spray into the mold causing an increase in oxidation and dross formation. This dross could be embedded in the iron and will reduce dynamic properties as well as machinability.

Apart form the dross formation there are other elements that affect the properties adversely.

It is important to lower residual elements that contribute to lowering the dynamic properties even if they do not affect static properties. Magnesium and rare earths in excess of that is required to form nodular graphite affect the quality of iron adversely. They tend to decrease nodularity, increase the tendency for carbides and shrinkage.

Figure7. Increase in manganese content upon reheating the treated iron.

• Economical optimization
  For making clean ductile iron with very low residuals the following procedure seems practical and economical.

• The magnesium alloy should preferably be of lower magnesium such as 4 to 5% with low rare earth content in the alloy. Steel cover may not be necessary with low temperature and low magnesium level in the alloy.

• Tap temperature should be kept as low as possible still maintaining a high enough pouring temperature (above 2500^oF).

• Post inoculation with ferrosilicon containing Ca and rare earths should be used to neutralize detrimental elements as well as to increase nodule count.

• If late inoculation (stream) is used then care must me taken to limit the rare earths contributed by the inoculant as the rare earths that are not combined with sulfur and oxygen are very potent in affecting the shape of the graphite nodules.

• Magnesium residual and rare earth levels should be optimized at the lowest levels still achieving good nodularity.

• Limit the fading time so that there will not be much difference in the magnesium level between the first and the last mold poured.

• Treatment size should be commensurate with mold pour weight so that the fade time does not exceed 12 minutes for batch processing.

Further reading
Major aspects of processing cast iron – Honorary lecture – Prof. R.W. Heine, AFS Transactions 1994.

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