EFFECT OF ELEMENTS IN GRAY & DUCTILE IRON

By

Roy Lobenhofer

The following is a handout I developed for people who were involved in the inplant classes I would hold about the basics of iron metallurgy. The information and comments are not sourced, but come from my experiences and memory over my 50 years in the industry.  I would love to hear comments or questions on it.

It's important that foundry workers understand the effects of the various elements so that they will have a better understanding of the possible results of their decisions.

What are elements?

Before going into their effect, it's a good idea to review what elements are. Most of us remem­ber science classes in school when they talked about the ele­ments and the periodic table. When I went to school there were less than 100 elements that had been found. Now there are a few more, but, needless to say, since the new ones were so hard to find, they aren't common enough to be of concern here.

What we most likely don't remember is the definition of an element. There are many, but the one I like best is: An element is one of the fundamental substances that consist of atoms of only one kind and that cannot be separated by ordinary chemical means into simpler substances.  

In simpler terms, elements are what are put together to make everything. The elements hydrogen and oxygen combine together to form pure water. The air we breathe is made up primarily of the elements oxygen and nitrogen.

Both water and air contain other elements that can affect them greatly. We all know that water from some wells have high iron contents and that affects the way it smells and/or tastes. Similarly, the additions of other elements to iron can greatly affect the metal made by a foundry.

What elements are in cast iron?

Some may remember that iron is an element. However, the cast iron made in foundries is a mixture of many elements.  Most of them have a significant effect on the properties of the iron.

Atoms of the element iron make up about 95% of all of the atoms in cast iron. The chart below gives typical percentages of some of the other elements found in gray and ductile iron. (This chart is given as an example only and should not be used as a reference to "good" iron. The order of the elements comes from the sequence obtained from the spectrometer of the foundry that supplied the data.)

Categorizing the elements

There are many different ways of categorizing the ele­ments and their effects on cast iron. Perhaps the sim­plest way is to list whether an element strengthens or weakens the iron.

One of the most common ways to cate­gorize them is by the way they affect the microstructure of the iron.

!          Some of the ele­ments increase the graphite content in the iron. These are called graphitizers.  

!          Some increase the likelihood that pearlite will ap­pear in the ma­trix. These are called pearlite formers.  

!          Some strength­en any pearlite that is present in the ma­trix. These are called pearlite strength­eners.  

!          Some increase the likelihood that car­bides will ap­pear. These are called car­bide for­mers.  

!          There are some ele­ments that affect the shape of the graphite. These are called dele­teri­ous elements

Another common way of cate­gorizing the elements is by listing the source of the ele­ments. Basically, there are only two primary sources of the elements (melting stock and purchased additions); however, other sources of specific elements can be of extreme importance in certain cases.

Specific Elements

As much we like to categorize things, what is important are the effects of the specific elements. Categorization leads to generalization that can be misleading. As an example, aluminum, when too high in content can make iron prone to pinholes; howev­er, without aluminum, inoculants would not be effective. Another, example is tin which up to a certain level is an excellent pearlite former; however, if too much tin is present, a film forms on graphite which makes the iron extremely weak. Almost every element has exceptions to the categorizations; therefore, we have to look at the specific elements.

Before discussing the individual elements, it is important to remember that the microstruc­ture, and therefore, the properties of gray and ductile iron are determined by a balance between the chemistry of the iron (amounts of the various elements con­tained in the iron), the nucleation, and cooling rate of the iron. Typically, in any foundry and for any specific casting, a balance of these items is developed which give the desired properties. The discussion of the effects caused by the individual ele­ments will be made from the point of their amounts changing from the established norm.

                                                                     CARBON

Sources

Carbon is present in almost all charge materials. While there is very little carbon in steel, there is enough that it must be taken into consideration when calculating a charge. Pig iron, purchased cast scrap, and returns all have considerably higher percentages of carbon than steel does.

Carbon is also purchased for addition. The addition materials are called graphite or carbon raiser. Graphite is a crystalline material that when added to a ladle may have an inoculating effect. Carbon Raisers, on the other hand, are amorphous are not believed to have any inoculating effect.

Certain alloy additions can contain significant amounts of carbon and must be taken into consideration when calculating a charge. In particu­lar, silicon carbide is usually considered a source of silicon, but it con­tains a significant amount of carbon.

Effects and Comments

Carbon has long been recognized as one of the most important elements in effecting the microstructure and strength of gray and ductile iron. This has led to great efforts in controlling it. It is a rare iron foundry that doesn't have some control of their carbon in order to minimize effects from changes. The quality of the control will vary from foundry to foundry. Those foundries that don’t have good control of their carbon can expect significant changes in their iron’s properties.

In general, carbon is the most potent of graphitizers. The more carbon in the iron the more graphite will be in the matrix. Also, the more carbon there is in the iron the greater the probability that the matrix will have ferrite in it. If that is the case, that also means that there will be less chance to have carbides.

Higher carbon irons are less likely to shrink and have better fluidity than lower carbon irons. In addition, higher carbon leads to less likelihood of producing massive carbides. If a foundry is producing thin castings it is likely that they will run a higher carbon than a foundry

On the other hand, attempting to correct shrink with higher carbons can lead to other problems, especially in big castings. High carbons and slow cooling rates (thick castings) can lead to a condition called carbon flotation. Graphite typically forms first during solidification and is lighter than iron. If the solidification of the entire casting is slow enough, the graphite floats toward the surface of the casting.

Recovery of carbon when added to ladles is usually less than 50%.

Sources

Silicon is found in almost all charge materials. Like carbon, there is very little in most steels, but it should be accounted for in charge calculations. It is also possible to buy pig iron with relatively low silicon; however, most pig irons, cast scrap and returns have higher silicon contents than steels. (It is sometimes possible to get a source of high silicon steel scrap. This can be an economical charge material.)

Silicon is also purchased as an addition. Care should be used when selecting the addition material. Inoculating grades of ferro-silicon should only be used when inocula­tion is desired. The aluminum and calcium in those grades can cause add­itional slag when introduced in induction furnaces.

Effects and Comments

Silicon is like carbon in many respects. It has been long recognized as an important element, and, therefore, controls are typically adequate. While silicon control in a cupola can be difficult, in induction melting it is relatively easy.

Also like carbon, the higher the silicon the more likely larger graphite will occur as well as more ferrite in the matrix. This, of course, will generally be a weaker iron but with less likelihood of having carbides to degrade machinability.

When silicon becomes very high it hardens the ferrite and can increase the hardness of the iron.

In ductile iron the impact transition temperature is affected by the silicon content. The impact transition temperature is the temperature at which ductile iron changes from being a ductile material to a brittle material. With silicon contents around 2.40% the temperature is about -40^o F. With silicon contents over 3% that temperature can be raised so high that castings will be brittle at room temperature.

Recovery of silicon alloys when added to a ladle is usually around 90%.

                                                                      SULFUR

Sources

There are small amounts of sulfur in virtually all charge materials. Certain pig irons and most steels have very low amounts of sulfur. Purchased gray iron scrap and gray iron returns have significant amounts.

Sulfur can be purchased for additions usually in the form of iron pyrite.

Some carbon raisers have high amounts of sulfur, and care must be used in selecting a carbon raiser for ductile iron production.

Sulfur will also come from the coke used in cupola melting. It is the reason that almost all foundries melting with a cupola and producing ductile iron go through a desulfurization process.

Effects and Comments

When making ductile iron, the object is to keep the sulfur as low as possible before treatment. The magnesium added to the iron first combines with any sulfur present before it will begin changing the graphite shape. (Some experts have maintained that sulfur can be too low {somewhere under .001} in ductile base iron; however, I’ve never seen that documented to my satisfaction.)

Conversely, in gray iron, if the sulfur is too low, problems can arise. Low sulfur can cause the inoculants to behave erratically. While some "experts" say that the sulfur should be above .03 and others say it should be above .05, I would rather see it targeted at .07

Gray iron can also have sulfur that is too high. Gray iron specifications frequently limit sulfur contents to .12 or .15. Sulfur in gray iron that isn't tied up with manganese will weaken the iron drastically.

Sulfur is also reported to be one of the more difficult elements for a spectrometer to read.

                                                                 MANGANESE

Sources

Manganese is present in almost all charge materials. Most steels have rather high manganese contents. Certain pig irons can be purchased with relatively low manga­nese.

Since manganese is needed in certain quantities in gray iron, it is also available in alloy form.

Effects and Comments

Manganese is needed to tie up the sulfur in gray iron. It forms MnS. As mentioned in the section on sulfur, if it isn’t tied up with manganese the iron will be severely weak­ened. There are numerous formulas printed which purport to calculate the amount of manganese needed to tie up the sulfur. The equation MnS tells us that in a perfect world it would be necessary to have only 1.72 times the weight of the sulfur in manganese. However, in order to make sure that the sulfur is tied up extra manganese must be present; therefore, I like to use the equation that says that

                                                                Mn= 1.8 * S + .2

At one time it was thought manganese strengthen the iron when there was more than needed to tie up the sulfur; however, recent research has shown that manga­nese over the amount needed to tie up the sulfur weakens the iron. Many foundries still have not reduced the manganese content of their iron since this new research has been conducted.

In ductile iron, manganese forms pearlite. This can be advantageous if trying to make some of the stronger grades, but a disadvantage if trying to make 65 or 60.

Recovery of manganese when added to a ladle is usually 90% or better.

                                                                  MAGNESIUM

Sources

Magnesium is highly oxidizable; therefore, the only source is very carefully made alloy additions.

Effects and Comments

Magnesium is what makes ductile iron ductile. It is desirable to have at least .035 to create the nodules. If the magnesium content gets too high, it can cause carbides. It is also said to cause "inverse chill." "Inverse chill" is a phenomenon where carbides form in the center of the section instead of the edge. High magnesium contents are also blamed for causing “exploded graphite.” (“Exploded graphite” is a detrimental graphite form that weakens the strength of ductile iron.)

Recovery of the magnesium is very process dependent. Magnesium recovery can be as low as 25% or as high as 100% depending upon the amount of iron to which it the magnesium is being added, the temperature of the metal, and, most importantly, the method by which the magnesium is added. As a general rule of thumb the recovery is better when less flash and/or smoke is observed. Also, the colder the iron the better the recovery will be.

                                                                      COPPER

Sources

Some steels and purchased cast scraps have significant amounts of copper, but if copper is to be used as an alloy addition, it will most likely come by using copper shot.

Effects and Comments

In both gray and ductile, copper is a strong pearlite former and a mild pearlite strength­ener. It is a common alloying agent because it is not a carbide former. Therefore, fairly large additions can be made without fear.

According to the Cupola Handbook there is danger of separating free copper if there is more than 1.5% copper.

Recovery of copper as ladle addition is 95 to 100%.

                                                             MOLYBDENUM

Sources

While some purchased scrap may contain some molybdenum, significant additions usually come from alloys. Typical alloys contain 58 - 64% Mo.

Effects and Comments

Molybdenum significantly strengthens the pearlite in cast irons. It used to be said that it was a mild carbide former but that contention has been disputed recently.

Molybdenum doesn't form pearlite and there must be pearlite present in order to get the strengthening effect of Molybdenum. That's why it is almost always used in conjunction with a pearlite former. Typical pearlite formers are tin and copper.

Recovery of molybdenum when added as a ladle addition is usually 90-95%

TIN

Sources

 Tin is usually added as an alloy. The typical addition material is "commercially" pure tin.

 Some work has been done using “tin cans" as a charge material. While the cans are mainly steel there is some tin in their coatings; therefore, use of them will cause a small rise in the tin level in the iron.

 Effects and Comments

Tin is a strong pearlite former. According to the Cupola Handbook, .08% is needed to insure a pearlitic matrix.

Over .1% may cause embrittlement through formation of an inter-cellular precipitation.

The pearlite created by tin cannot be annealed out.

Recovery of tin is usually 100% whether it is added in a ladle or in charge material. It does not oxidize out even when remelted in a cupola; therefore, if the amount of tin is too high the only way to reduce it is to dilute it with a purer material.

                                                                   CHROMIUM

Sources

Stainless steels and ni-resist and other alloyed cast irons can contain significant amounts of chromium. Typically additions come from alloys. Alloys can contain from 35% to 70% chrome.

Effects and Comments

Chromium is a strong carbide former. When used in good control it is a very econom­ical way to strengthen cast irons. The danger is that loss of control can have dramatic negative effects on machinability.

Automotive foundries typically use chrome as strengthener. Those foundries usually have good enough control and with the amount of iron they melt the savings from using chrome can be significant. (I usually like to avoid using chrome because in most smaller foundries the savings is not worth the risk of degrading machinability.)

Recovery of chromium when added to ladles is usually in the 90-95% range.

ALUMINUM

Sources

Inoculants require aluminum and/or calcium to be effective. Steels also have alumi­num from the "killing" process to which they are subjected. In addition, aluminum parts (pistons, bearing housings, and pop cans) are sometimes mixed with or attached to ferrous secondary scrap.

Effects and Comments

The small amounts of aluminum associated with inoculants have little effect other than inoculating. If aluminum gets too high, the iron becomes susceptible to hydrogen pinholes.

                                                                     TITANIUM

Sources

Pig irons generally contain titanium. Some of the newer steel alloys also have titanium in them. If an effort is made to control titanium, it is done with alloy additions.

Effects and Comments

Small additions of titanium are said to increase machinability; however, larger additions are said to degrade machinability. Some have found that additions of titanium reduce the strength. It is believed that is caused by negating the strengthening effect of nitrogen.

Titanium ties to nitrogen to form titanium-nitride particles. This facility is used to prevent nitrogen pinholes.

Titanium is used to form vermicular (compacted) graphite when it is added to ductile. Rare earths will combat this effect.

Recovery of titanium, when properly added to ladles, is about 60%

                                                                       NICKEL

Sources

Nickel can be found in some steels and purchased cast scrap. Nickel is added, most commonly, by using purchased alloys containing nickel.

Typical nickel alloy is 92% nickel and 5-6% silicon.

Effects and Comments

Nickel is a graphitizer and thus reduces carbide-forming tendencies. It exerts a mild strengthening and hardening influence.

Recovery of nickel is approximately 100%.

 LEAD

Sources

Since lead is a very deleterious tramp element, the only way it is going to get into the iron is by mistake. The primary source of contamination is steel. With obsolete scrap the primary sources are lead wheel weights, leaded paints, leaded steels and steel sheets with a layer of lead sandwiched between. With prompt industrial scrap the primary danger is leaded steels. (Leaded steels are steels that have been alloyed with lead. Generally this is done to improve machinability.)

It also has been theorized that automotive cast scrap may contain lead from the old leaded gasoline.

Effects and Comments

In both gray and ductile iron, lead has very deleterious effect on graphite. It creates fine growths of existing flakes or nodules. These growths greatly reduce strength.

Rare earths will combat the effects of small amounts of lead in ductile iron.

Sources

High nitrogen amounts are usually associated with absorption from cores containing high nitrogen binders. Steel also contains 

Effects and Comments

Nitrogen pinholes are the result of high nitrogen contents. Before the nitrogen reaches the level to cause pinholes, it is said to strengthen gray iron. Titanium negates the effects nitrogen.

Sources

Hydrogen is not desirable; therefore, it comes from obscure places. Generally, hydro­gen comes from liquid iron coming in contact with water. Wet ladle lips and wet molding sand are two of the more common sources of this unusual event. In cupola melting, if a water-cooled tuyere starts to leak, it can put hydrogen in the iron.

Effects and Comments

Hydrogen leads to hydrogen pinholes in the castings. These pinholes are usually close to the surface. The presence of more aluminum than normal will make the iron more susceptible to hydrogen pinholes.

                                                              PHOSPHOROUS

Sources

For special circumstances ferro-phos can be purchased as an alloy. All other phos­phorous comes from charge materials.

Effects and Comments

Phosphorous in ductile iron can cause brittleness. An effort must be made to keep the phosphorous low.

Actually, there are some gray iron, specifications which impose limits on the amount of phosphorous. These are the result of the times when steel was made in the south and had a very high phosphorous content. These specifications are far higher than foundries typically make today.

If the phosphorous is high in gray iron it will result in phosphides in the microstruc­ture. These phosphides look and behave like dispersed carbides. They will make machining difficult and, if in sufficient quantity, will weaken the iron.

                                                                       BORON

Sources

Boron can come in charge materials and can be leached from new linings in coreless induction furnaces. There are also ferro-alloys available for deliberate ladle additions. These were primarily used in the production of malleable iron.

Effects and Comments

Boron typically does not appear in sufficient quantities to be of concern. In gray iron it promotes type “D” graphite in smaller amounts. In larger amounts, it will form carbides. Recent investigations have shown boron in small quantities may prevent the formation of pearlite by copper additions in ductile iron. This is of concern if pearlitic grades are to be produced from a freshly lined furnace or a furnace with a large patch.