Practical Methods for the Control of the Inclusion Content in Ductile Iron

Based on the findings from Ductile Iron Society Project No. 20, which is concerned with the subject of ductile iron machinability; it has been determined that as the inclusion content in ductile iron increases, machinability declines and tool wear is substantially reduced. In addition to reductions in machinability the presence of inclusions reduces the overall mechanical properties. While these conditions have been known to exist for many years, the Research Committee of the Ductile Iron Society held Project No. 18 in abeyance for approximately two years pending verification of the influence of the various types of inclusions on the machinability and mechanical properties of ductile iron. Currently, it is the considered opinion of the DIS Research Committee that the fact that all inclusions have a detrimental influence on the machinability and mechanical properties of ductile iron. In view of this fact, a different approach to the influence of inclusions on the machinability and mechanical properties will be studied.

Inclusion control in ductile iron castings is not necessarily a new or unique factor. In the past, there were many casting producers who did not consider it important to their customer. It was simply considered as an extra cost, which was not given any consideration unless the casting customer made it an issue, When this occurred, casting producers always have made an effort to renegotiate casting prices upward based on the increased costs to cover the additional expense.

It must be pointed out that all graphitic cast irons, which include ductile iron, are not homogenous metals in the same sense as non-ferrous metals, steel, and unannealed malleable iron (white-iron or hard iron). They are in fact mixtures containing graphite more or less evenly distributed in a metallic iron or steel-like matrix. 1n the case of ductile iron, the graphite is present in the spheroidal form. The basis for nodule or spheroid formation is the presence of large quantities of heterogeneous nuclei. These consist of micron size nonmetallic inclusions and other stable micron size inclusions in the form of inter-metallic compounds. As a result, graphitic cast iron contains relatively large quantities of what can be teamed inclusions. In fact, if these inclusions are not present in sufficient quantities, flake or spheroidal graphite will not form. These nuclei come from many different sources, which Include furnace charge materials, magnesium ferrosilicon treatment alloys, and inoculants. Under certain conditions, when an excess of these materials is present they will coagulate to form slag. Refractories, both from furnace linings and ladles, can be another source of nuclei and slag. In addition, there is always the magnesium vapor, which continually comes out of treated molten ductile iron to form magnesium oxide and magnesium silicate. All of these different compounds which are considered as sources of heterogeneous nuclei not only contribute nuclei to the ductile iron process, but also act as a source of inclusions in the metal as well as a source of slag in castings, if present in excess. In addition to products of melting and the treating reaction, as well as inoculation, there are inclusions in the form of inter-metallic compounds and carbides of chromium, molybdenum, vanadium, and carbides, formed by the presence of rare earth metals, which collect in the grain boundaries if the solidification rate extends over sufficient or critical length of time (the time required to pass through the interval between LIQUIDUS and SOLIDUS phases) which is reduced as the level of residual elements in ductile iron increases. It appears that the increased accumulation of these extraneous materials in the grain boundaries substantially reduces the mechanical properties of ductile iron. This is one of the least understood phenomena related to the production of heavy section ductile iron castings.

While inclusions and slag from treating material, inoculants, and other sources will always be present in ductile iron even under the most ideal processing conditions; it is possible to minimize this condition by taking certain precautions. These practices can be introduced in any foundry producing ductile iron castings. The main objection is the apparent increase in casting cost, which inevitably occurs. As a result, such practices are never introduced except at the demand of the customer. This attitude has always prevailed, in spite of the fact that these practices can improve casting quality and simultaneously, actually reduce processing costs. It is a condition which exists primarily in those companies where the purchasing department's only consideration is price alone, and where management is not sufficiently sophisticated to understand the value of improved machinability and casting quality on the overall cost of the finished components.

There are a series of steps, which can be taken in any foundry to substantially reduce, and control, the levels of excess inclusions present in ductile iron castings. Basically, they involve the quality control of furnace charge materials, and an understanding of Stoke's Law, relative to dross and slag removal, from base iron melting furnaces and treatment ladles. Descriptions of the steps and procedures needed to minimize the presence of excess inclusions are as follows:

1. One of the initial sources of inclusions is the melting material. This includes excess rust (iron oxides) and dirt (silicon dioxide), which is always present. The main source of slag in the melt is the molding sand, which adheres to returns.
2. One more significant source of inclusions is alumina, and also silica slag from furnace refractory. The quality of refractory used in melting and pouring requires serious consideration.
3. One of the most overlooked slag and inclusions source, is the slag that is present in under-Cooled blast furnace pig irons. This includes charcoal-reduced pig iron, coke-reduced pig iron, and on occasion, submerged arc-furnace pig iron. Slag from the pig irons described here, contain substantial amounts of iron oxide, and are very fluid. As a result, they require the use of dry slag coagulant such as unexpended vermiculite. Even then it is not always possible to overcome the dilatory effects this type of slag.
4. Another source of slag in ductile iron base irons is the oxides and silicates contained in sponge iron pellets and pre-reduced iron pellets from various sources. The unreduced oxides contained in these materials usually ranges from 8.0% to 15%, which is a significant volume.

In order to remove or minimize the presence of these and other sources of slag and inclusions from the base iron, it is necessary to utilize the concept of the Brownian Movement to allow the sources of inclusions to float to the surface of the metal in the furnace. This can be calculated, based on time, temperature, and slag density. Once these oxides come to equilibrium, they will float out of liquid metal at the rate established by Stokes Law:

V =  2gr\2 (d1 -d2)
 9/n

Where V is the rising velocity in cm per sec, if g, the acceleration due to gravity, is cm. Per sec\2, r is the radius of the particle in cm, d1 the density grams in per cubic cm. of the liquid, d2 the density grams per cubic cm of the suspended particle, and n, the viscosity of the liquid in dyne-sec. Per sq. cm. or poises. In liquid iron, or steel, the formula can be reduced to:

V=8.38 x 10 (6.94 - d) r\2

For example a particle of 30 microns in diameter will rise 5 ft. in 30 minutes, while a particle 300 microns in diameter particle will rise 5 ft. in 21 seconds, and a 1 mm particle will rise 2 ft. per sec.

In a typical coreless induction furnace base iron melting operation, the time for power-off allowed for slag flotation, is usually from four to ten minutes. In a situation where extremely rusty scrap and or large quantities of sponge iron are used in the furnace charge, longer times may be required. When all of the slag from melting has floated to the surface, the furnace should be tilted back, and the slag skimmed off. If slag coagulants are required, the quantity used should be kept to the absolute minimum required for effective slag removal, and no more.

In the case of desulfurized cupola melted base irons, regardless of the desulfurization practice, slag removal required the following steps:

1. Refractories used in desulfurizing ladles, must be extremely resistant to the corrosive action of the desulfurizing media.

2. After desulfurization, using a porous plug or other mixing device, sufficient time must be allowed for the sulfide slag to float to the surface of the base iron in the desulfurizing ladle. This usually involves time intervals ranging from three to fifteen minutes, depending on the ladle size and metal temperature.

It is very important that the time interval involved be held to an effective level, which will minimize temperature loss and sulfur reversion, and at the same time provide optimum sulfur removal.

3. Slag removal and cleaning of the desulfurizing vessel is absolutely necessary, otherwise the channel in the holding furnace will become plugged in a very short time, and there also can be a serious sulfur reversion problem in the holding furnace.

Up to this point, the subject discussed has been the melting and preparation of the base iron. Based on our collective experience; it is very obvious that while the major part of the slag generated in base iron melting and preparation, will float to the surface of a still or non-active molten metal bath. Small particles of slag, which are 20 microns or less in diameter float up to the surface of liquid metal at a relatively slow rate, and in some cases, based primarily on size and density, remain in the liquid metal, and cannot be removed except by filtration. The use of clean, oxide free, charge materials, along with proper slag coagulation and removal techniques, minimize the accumulation of dross and slag in base irons used in ductile iron production.

The last and major source of dross and slag inclusions, are the treating material and treating practices, including inoculation practices. There are a substantial number of treating procedures using magnesium-ferrosilicon, nickel-magnesium, and magnesium metal. The cleanest, and most effective procedures are those, which require large quantities of treating alloys, and produce ductile iron castings with high casting scrap levels.

The on-going viability of any practice is determined by the acceptance or rejection, by the casting purchaser. This can be based on quality or cost. The last considerations, and possibly the most important, are the treating materials and inoculants used in the ductile iron process. These ferroalloys, magnesium metal, and inoculants, can, and do, have a profound influence on the cleanliness of the ductile iron castings produced. The following is a list of quality factors, which should be taken into consideration when treating materials, and inoculants are chosen:

1. All magnesium ferrosilicon alloys contain magnesium oxides, magnesium silicates, and various forms of dross, which are the result of magnesium metal cleanliness, and the method by which the magnesium metal is introduced into base 50% ferrosilicon. The most effective procedure is to plunge virgin magnesium ingots into the liquid 50% ferrosilicon under an inert gas cover in a covered ladle. When scrap magnesium is stirred into the ferrosilicon, the result is always large quantities of oxides and dross. In general, the oxides present in a 5% to 6% magnesium ferrosilicon should never exceed 0.5%. The effective magnesium level in the form of magnesium silicide must be in the range of 5% to 6%.

2. When magnesium ferrosilicon alloys are produced, it is very important to consider the method of solidification. For example, when magnesium ferrosilicon is poured in thick sections in open molds, a heavy layer of oxides and silicates occur on the surface exposed to the atmosphere. This oxide and silicate layer, if not removed, becomes a major inclusion source.

3. The overall structure and solidification pattern has a significant influence on the reactivity of magnesium ferrosilicon treating alloys regardless of the calcium level of the alloy. For example, the magnesium ferrosilicon alloys cast in an open mold having a thickness of 5 to 6 inches or 25 to 150 mm will solidify with large columnar grains of magnesium silicide and iron silicide, which are plainly visible. The presence of these large magnesium-silicide grains, cause increased levels of violence and oxidation in the treating reaction, regardless of the quantity of calcium or barium present in any magnesium ferrosilicon-treating alloy. Chilled thin cast materials with thickness in the range of 0.75 - 1.25 in. results in magnesium ferrosilicon alloys with a relatively fine-grained mixture of iron silicide and magnesium silicide, which reacts at a relatively moderate rate.

4. Another important consideration is the violence of the treating reaction. Calcium and barium suppress the violence of the treating reaction, and their presence, in sufficient quantity, can increase magnesium recovery very dramatically up to as high as 75%. The optimum level calcium in a 5% magnesium ferrosilicon alloy is between 2% and 2.5%. The barium level for suppressing reactivity is between 5% and 6%. This is also the level at which maximum nucleation occurs.

5. In processes where pure magnesium metal is used to introduce magnesium into the base iron, every possible effort must be made to minimize the violence of the treating reaction and thus reduce the presence of oxides and silicate slag produced in the process. This can be done by creating back-pressure in the treating vessel by having a silicon content of approximately 2.0% or more in the base iron, and by adding calcium silicon and other high silicon material, such as silicon metal and/or 75% ferrosilicon along with the magnesium metal addition.

6. It is common knowledge that both crushed and sized treatment alloys and inoculants oxidize when exposed to the atmosphere starting at the time of crushing and sizing. Sea voyages are particularly detrimental in this respect because of the high levels of moisture and chlorides present in the atmosphere. These are factors that can cause reduced magnesium recovery, and at the same time significantly increase the presence of inclusions in ductile iron castings.

In the case of inoculants crushed to relatively small sizes for use in stream injection, can have losses in effectiveness up to 70% due to a few months exposure during storage. This is particularly true of crushed ferrosilicon fines, which are available at very attractive prices. This problem of erratic stream inoculation chill control can be minimized by the use of fresh crushed inoculant.

7. One source of very destructive massive inclusions is undissolved inoculant. This material has been known to seriously damage cutting tools and machining fixtures resulting in hundreds of thousands of dollars in scrapped castings. The use of exothermic inoculants may become a mandatory part of ductile iron production. This is necessary along with a pouring temperature exceeding 2450^oF depending of the section size. For example, 50% ferrosilicon is endothermic having a cooling effect on the iron. The exothermic range starts at approximately 62% silicon, which is exothermically neutral and ending with approximately 90% silicon, which is strongly exothermic. Ductile iron requires an exothermic inoculant with at least 75% silicon in the form of iron silicide and metallic silicon. Such inoculants will go into solution and generate their own heat of solution simultaneously. After giving this matter thoughtful consideration, in-stream ferrosilicon inoculants with silicon levels in the range of 85% will probably be the most effective, and have a highest practical exothermic level. Inoculants in the endothermic range below 60% silicon will not go into solution on hitting the stream. Such inoculant can only be used for ladle addition.

Conclusions and Recommendations
While there may be additional sources of inclusions in the ductile iron process other than those described here, these are the primary sources. When the level of inclusions present in ductile iron castings increases, the mechanical properties decline. Their presence can be reduced substantially by taking the corrective actions described in this report. In general, any operating ductile iron foundry can improve casting quality without the purchase of any costly capital equipment, however; there may be some increase in the cost of the melting and treating material used in the process. These will be compensated for by overall reductions in casting scrap. In addition, there will be fewer scrap castings returned by customers. This alone will promote customer confidence in the casting producer.

The production of ductile iron castings suitable for A.D.I. requires the use of every possible precaution that will minimize the presence of all types of inclusions. This can only be done by careful adherence to recommendations outlined here along with the application of low stress casting design criteria to all Austempered Ductile Iron castings.