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BASIC DUCTILE IRON ALLOYING

byJames D. Mullins, Sorelmetal Technical Services

One of the characteristics of alloying is that it affectsthe diffusion rate of the carbon, which is the ability of

the carbon to move within the matrix. Upon coolingand depending upon the rate of cooling, aftersolidification, but before we reach the point wheretransformation stops, we find that the carbon is able tomove/diffuse. The amount of this movement will varyand as a result if it is not fast enough (limited byalloys) to clear all of the matrix structure before theend of transformation; this is one of the major reasonswhy we get various mixtures of matrix microstructuresin Ductile Irons.

Ductile Irons form ferrite much more easily than grayirons even with the same alloy content. Graphitemorphology is the reason for this. Graphite nodulesare encased within an austenite shell. Thecomposition of the austenite affects the microstructureand subsequent iron properties, again because ifaffects the ease of carbon to move away from pearliteleaving ferrite, as mentioned earlier. Ferrite containspractically zero carbon. The average thickness of theferrite shell around a nodule of graphite does notchange whether the structure is from an as-cast orheat treated sample. The other reason is that whenthe nodule count is increased the amount of ferrite isalso increased, since the shell thickness remainsessentially the same but the number of places wherethe carbon can diffuse to is increased. And so byincreasing the nodule count we see changes in thestructure and properties.

Another factor that we must consider is that during

solidification some elements are pushed into orsegregate to the last areas to freeze. These areas arelocated between the graphite nodules and are calledthe intercellular regions. Elements that have hightendencies to segregate are P, Mn, Cr, V, Ti and Moin increasing order of tendency to segregate. (Forfurther information about this see Chapter 4, TheSorelmetal Book of Ductile Iron). See Figure 1. Evenwhen the concentration of an element appears to below or in a normal range we find that they canconcentrate up to extremely high levels in thoseintercellular areas. And when they do, they influencewhat happens there. So we must be careful which

elements that we use for alloying to avoid formingcarbides in these intercellular regions. They increase

the hardenability in those areas, and the structureschange becoming harder, less ductile and more brittle.Increasing the nodule count will reduce this effect andthen it may not make as much difference or it willmake less difference.

SEGREGATION OF VARIOUS ELEMENTS

Element Segregation FactorMoTiVCrMnPSiCoNiCu

25.325.013.211.61.7 3.52.00.70.40.30.1

Figure 1.

There are some alloying elements, specificallyantimony, copper and tin, which surround the graphitespheroids and act as barriers to the diffusion ofcarbon. The use of these elements gives us the abilityto form nearly 100% pearlite in the matrix, stabilizing itto keep it from transforming further.

Most of the time when we are talking abouthardenability we are talking about the ability to formmartensite. The term hardenability by itself means theresponse to heat treating. But in cast irons we are

primarily interested in pearlitic hardenability. Theability to form pearlite is the function of those itemspreviously mentioned. Matrix structures are functionsof cooling rate, nodule count and alloy content.

Looking at what some of the individual elements do.Silicon has an effect on carbon equivalent. Its a solidsolution strengthening agent, so increasing silicongreatly effects the embrittlement of Ductile Iron,particularly at lower temperatures. For these reasonsthe overall amount must be controlled. However,silicon reduces chill and undercooling and promotesgraphite. It segregates negatively, which means thatthe highest concentration is near the graphite nodule.

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Nickel is also a graphitizing element and acts similarly

to silicon except that it does not embrittle, but ratherincreases the toughness of the matrix. It contributesto the carbon equivalent in the same way that silicondoes but not as effectively. It promotes pearlite but itdoes not stabilize pearlite. With nickel you get morepearlite but the pearlite breaks down more easilyduring heat treatment. It increases hardenability andthrough hardenability, it promotes the formation ofaustenite and it also makes it easier to heat treat toform martensite or ADI structures. It segregatesnegatively in the same way that silicon does. Itdoesnt cause problems.

Copper is similar to nickel in many ways, except thatis a much stronger pearlite promoter. At a 1% level,usually a 100% pearlitic matrix can be obtained. It isthe most widely used alloy for pearlitic Ductile Ironproduction, because of its effectiveness and cost. Itonly gives a modest increase in hardenability (SeeFigure 2).

Manganese is generally a pearlite former/stabilizer,and forms carbides. It segregates strongly to grainboundaries and increases hardenability.

Chromium is not a strong pearlite former, but is acarbide stabilizer and can cause many problems withrespect to segregation as mentioned earlier.

Molybdenum is a pearlite stabilizer and promoteshardenability. It is most often used in combinationwith copper or nickel, but at a much lower additionrate, because of its serious segregation tendency.

Looking at some of the other elements that we mightuse. Antimony is a fantastic pearlite former, but isused only in heavy sections with balanced ceriumadditions, otherwise it can certainly change the nodulestructure/shape towards flake. Arsenic is another verypotent pearlite stabilizer, but for some reason oranother people are not enthusiastic about havingarsenic around in the foundry, but it is very good. It isanother one like antimony in that with a combinationwith rare earth, it increases the nodule count andproduces better nodule shape.

The last major alloying element is tin. It is used in

small amounts because it is a strong pearlite formerand stabilizer. It can form flake graphite if theadditions are too large. Tin also strongly segregates ifyou have excess tin. If there is not enough room forthe tin atoms to line up around the graphite spheroids,the excess tin segregates into the last areas to freezeand it greatly deteriorates the toughness of thematerial.

See Figure 3 for the relative pearlite promotingstrength of some elements.

All the allowing/trace elements that have been

mentioned above are recovered upon remelting, sowhen calculating subsequent additions, the additionmust be reduced by the amounts recovered from thereturn scrap.

Figure 2. Comparison between the pearlite promoting effects of Mn and Cu (TC: 3.85%, Si: 2.00%. Unalloyed Mn: 0.25%).

Cerium and magnesium are not used specifically asalloying elements in Ductile Iron, but they can act tochange the structure and promote carbides. It is bestto add only the necessary amounts. Excessive ceriumcauses graphite shape problems especially when youhave high purity charge materials. You must alwaysbalance these things out.

If you want to change the mechanical properties youmust change the microstructure. With no otherprocess changes this can be done easily with alloys.The alloying elements can affect both the solidificationprocess and the cooling from red heat to black heatwhere changes to the structure occurs.

RelativeElement Pearlite Promoting Effectiveness

SnMoPCuTiMnNi or Cr

39.00 7.90 5.60 4.90 4.40 0.44 0.37

Figure 3.

Rev March 2006