A new technique for producing as-cast ductile iron.

An innovative melting process is proving to be consistent, simple to operate while offering technical and economic benefits.

To produce ductile iron castings with a ductility exceeding 2%, the iron's matrix must contain some ferrite. For high levels of ductility, the matrix must be almost entirely ferrite.

Heat treatment after casting can be used to attain the ferrite levels required for ductility, but for the more ductile grades of iron there are economic advantages in producing the required properties in the as-cast condition. This is done by using charge materials that contain low levels of carbide-stabilizing elements (e.g., manganese) to reduce the tendency for pearlite formation.

Some steel scrap, particularly "new-production" material, can have acceptably low levels of trace elements for the production of the more ductile grades of iron castings, but they usually contain excess manganese. For this reason, iron foundries normally use some low manganese pig iron in the charge, but this raises the cost of the charge materials.

Manganese Reduction

In conventional electric melting operations, manganese in charge materials is retained by the molten metal. One of the advantages of electric melting is the efficient recovery of charged materials that allows an accurate chemical analysis.

Liquid iron produced in an induction furnace generally contains low levels of oxygen that is governed at low temperatures by the reaction Si + 20 = Si |O.sub.2~ (slag) and at high temperatures by the reaction C + O = CO (gas).

Research, however, at the Univ. of Wisconsin-Madison indicates that the silica-formation reaction continues to control melt oxygen levels even at higher temperatures. This produces bath oxygen contents far in excess of those expected if the carbon monoxide formation reaction were to control oxygen contents at higher temperatures.

Because manganese oxide is more stable than iron oxide, the formation of manganese oxide is feasible as long as additional stable oxide-forming elements are absent or present at low levels. Stable oxides, including silica and silicon-bearing additions, should be avoided until the manganese content has been reduced to a desired level and the resulting manganese oxide removed from the system.

Studies show that the reaction between iron oxide and manganese theoretically reduces manganese to acceptable concentrations of less than 0.1% if the manganese oxide content in a MnO-FeO TABULAR DATA OMITTED slag is maintained below 30% at a reaction temperature of 2730F (1500C). Even more significantly, such manganese levels are possible as long as the manganese oxide concentration in the slag is kept below 90% at a reaction temperature of 2370F (1300C).

This indicates that acceptable manganese concentrations are obtainable without the need for excessive slag removal, assuming that the reaction kinetics are satisfactory. Manganese removal from iron held at 2370F depends upon sufficient rapid replenishment of oxygen into the slag or directly into the molten iron to provide FeO for the reaction: |Mn~% + Fe|O.sub.s~ = Mn|O.sub.s~ + F|e.sub.1~

Experimental Investigation

Avoiding the presence of silicon in the molten iron to enhance manganese loss leads to the consideration of whether medium frequency coreless furnace melting is appropriate. Such furnaces usually melt from empty, eliminating the energy costs necessary to retain a molten heel of metal.

Experimental investigations were carried out using a 400 kg, 300 kW, 1000 Hz furnace that allowed steel scrap and graphite to be melted to assess the variables controlling manganese loss. The resultant slag from each trial melt was carefully removed before adding further graphite, ferrosilicon or foundry returns to produce a base iron suitable for ductile iron production.

The results of these trials yielded a new melting system known as the Wilman Process. The patented process is a melting routine that allows the manganese content in steel scrap to be reduced during the production of a base iron that is suitable for producing the more ductile grades of ductile iron.

After melting a suitable steel scrap with a controlled graphite addition, the melt is deslagged at a recommended temperature. Further graphite additions and foundry returns are added and the melt is super-heated to the foundry's preferred tapping temperature.

Experimentally, the process is capable of reducing the manganese content of steel scrap from 0.87% to 0.28%. Tensile ductilities were recorded as high as 25%.

Mechanical Properties

The tensile properties of ductile iron test bars (British Spec. 2789: 1985) produced in two foundries are shown in Table 1. The foundries conventionally melted ductile irons and compared them to irons produced using this new technique. The elimination of low-manganese pig iron shows a significant cost advantage.

More recently, EA Technology has installed a one-ton furnace powered at 1mW at any of three frequencies: 250 Hz, 500 Hz and 1000 Hz. It has been used to melt steel scrap commonly available in the U.K. and known as chopped steel plate. Iron properties attained using an all steel scrap charge are shown in Table 2.

Trials also indicated that producing ductile iron by this process produced acceptable fatigue and impact properties.

Microstructure 

Early experimental work, the results of industrial trials and the experience of licensed users of the process indicate TABULAR DATA OMITTED that ductile iron produced with this process has a higher nodule count than conventionally produced irons in the microstructures.

The high graphite nodule count in castings produced by this process provides an explanation for the unexpectedly high ferrite content and the associated improved ductility. High nodule counts reduce carbon diffusion distances during solidification. Regression analysis of samples made from one series of trial melts indicated the following relationships:

* percent ferrite in matrix = C + 31.9 (%Si) + 0.19 (nodule count) - 63.4 (%Mn) where C is a constant and nodule count is number per m|m.sup.2~;

* elongation % = -4.36 + 0.29 (% ferrite in matrix).

This type of ductile iron usually has a particularly good combination of strength and ductility due, partially, to the nature of the graphite nodules. Quantitative metallography indicated that a Wilman melt using steel scrap contains fewer coarse nodules than a pig iron (conventional) melt. Further, the graphite nodules are more spherical, as indicated. The shape is defined as |(perimeter).sup.2~ divided by area and has a value of 4|pi~ for a perfect sphere. There is no current explanation for this improvement.

Present Status of Process

Several U.K. foundries are licensed by EA Technology to use the process. EA holds the patent rights in Europe as of January 20, 1992. Midwest Instrument Co., Inc., Hartland, Wisconsin, holds the U.S. patent and is licensed to sell the process outside the U.K. A few U.S. foundries are evaluating the process, among them Wells Manufacturing Co., Woodstock, Illinois.

Licensed users confirm superior casting quality over conventional practices and a reduction of undesirable trace elements when steel scrap is melted in this manner. Higher final manganese levels than previously thought for a given level of ductility may be allowed.

The process was conceived for operation with medium-frequency coreless melting furnaces of the type used by many European foundries because of the ability to empty the furnace after every melt. This allows the next melt to be operated at a low silicon content, which improves the manganese reduction. The final addition of silicon-containing foundry returns has the advantage that any remaining high manganese slag is rendered safe to the normal silica lining via the reaction: 2 MnO + Si = 2 Mn + Si|O.sub.2~

The manganese resulting from this reaction reverts into the melt, thus, deslagging must be as effective as possible before the returns are introduced.

The process has been operated successfully with mains, low-frequency (60 cycles) coreless furnaces using a hot heel practice. However, in this case, two furnaces are required and deslagging has to be thorough to prevent manganese oxide-rich slags from contacting the silica lining at high temperatures.

Users of this process gain a significant cost advantage by replacing low manganese pig iron or premium quality grades of (low manganese) steel scrap by more readily available steel scraps. Control of the scrap and the way the process is operated can allow the foundry to produce a range of grades of ductile iron castings using steel scrap charges.

References

1. S.C. Ghorpade, et al, "Oxygen Probe Measurements in Cast Irons," AFS Transactions, pp 193-198, (1975).

2. C.J. Edgerley, et al, Cast Metals, vol 1 (4), (1989).

For additional information on the Wilman Process circle No. 337 on the Reader Action Card.