Prepared by:  William R. Pflug, Inductotherm Corp.

INTRODUCTION 

The competitive nature of today's global market for ductile irons castings has forced us to examine all of the different ways in which we produce our castings.  Ever increasing quality standards create new benchmarks for more precisely repeatable processes requiring us to control and automate all facets of the foundry.  And although every molten drop of production still must pass over a ladle lip or spout, truly automatic ductile iron pouring has now come of age.

The success of all automated pouring systems hinges on metering metal, which is directly related to some shape or size of orifice.  Handling a "clean" metal such as gray iron presents few problems to foundry men since critical refractory surfaces can be maintained.  The reactive nature of ductile irons, however, creates compounds not normally seen in gray iron.  Agglomeration of slag on lining sidewalls and metering surfaces can occur rapidly, depending on treatment practice, alloy, heat transfer and refractory design. Previous automated pouring technology did not compensate for these changes--- build-up had to be removed before production resumed.

New generations of automatic pouring systems have been developed to meet this challenge.  The advent of intelligent sensors and personal computers with sophisticated algorithms make it possible to adapt to the many variables associated with holding and pouring ductile iron.  A number of foundries around the world have adopted these new pouring technologies.  These systems have produced castings with a great degree of accuracy and repeatability along with being very reliable.

The ultimate success, or failure, of an automated ductile iron pouring furnace hinges on three basic criteria; furnace construction, treatment practices and furnace maintenance.

CONSTRUCTION OF DUCTILE IRON PRESSURE POUR FURNACES

Initially, ductile iron furnaces were gray iron designs that, by and large, proved to be unsuccessful.  But lessons were learned and what is available today has been developed by closely studying the operation of earlier systems and by extensive testing and evaluating new components in production situations over a period of years.

The solutions to the problems were not obviously apparent until ductile iron metallurgy and treatment processes became well understood.  These problems included: high magnesium fade rates, constriction of the receiver and pouring spouts, early power drop and eventual failure of the inductor due to rapid and tenacious slag accumulation and the inability to maintain pouring accuracy due to the same build up.

Pure magnesium boils at 2013'F.  Since its vapor pressure is well over 8 atm at 2600'F.  'F. it is in the gaseous state as it exists in ductile iron.  As the gas migrates to the surface of a ladle or furnace it aggressively scrubs the iron of oxygen and sulphur and forms primary slags of magnesium oxide and magnesium sulphide.  At the same time, it combines with any free oxygen in the refractory lining presented by the porosity inherent in the lining.  Reduction reactions with silica bearing refractories can also occur.

Retarding magnesium fade in a pressure pour furnace is currently addressed by increasing the metal depth to diameter ratio and supplying an inert gas as the pressurizing medium.

Experiments, performed at two foundries back in the 1970's by Cal Mason with Inductotherm and Bill Snow from ACIPCO, show that the metal height to diameter ratio has a marked effect on the rate of fade.

Figure 1 - Magnesium Fade Rate vs. Magnesium Level (Note higher level indicates higher fade rate)

Pressure pouring furnaces can be reasonably constructed to maximize the advantages here.  One of the easiest methods to minimize fade is to operate the furnace above half full.

Since the vapor pressure of the magnesium gas in ductile iron is many times higher than the operating pressure required within a furnace, the applied pressure has little effect on reducing the rate of loss of magnesium.  The use of an inert gas, such as nitrogen, does minimize oxidation reactions and sulphur reversion at the bath surface.

All of these slags have a lower density than the iron that produces them and have a tendency to float.  If these slags are in contact with an area with a high enough thermal gradient, they will quickly solidify.

INDUCTOR

The inductor had been an area of concern as it is the source of energy for the furnace.  Early ductile iron furnaces showed accelerated rates of clogging and reduction of power.

The location of the inductor openings must be at the bottom of the furnace and below the floor or hearth.  This removes the potential for slag laden iron from entering the inductor as it is a distance from the minimum iron level in the furnace.  It has been shown in practice that inductors mounted in areas other than the bottom of the furnace are subjected to these slags and, in general, have much shorter campaigns.

Conventional inductor channel molds have consistent refractory thickness the entire circumference of the water-cooled bushing.  This is not so with the ductile iron or "U" loop design.

Any inductor that experiences slag build up might benefit from this design.  The reasons for this concept are clear.  Slags tend to come out of solution similar to the way silt will settle out of a stream when presented to an eddy in the mainstream flow.  The lack of flow available to maintain the mixture allows the silt to collect.

The same is true for the area near the inductor/upper case flange of a conventional inductor.  The larger the cross section, the lower the metal velocity and in the standard design represents an "eddy pool" to the metal flow whereby velocities are minimal and slag has time to collect.  

A secondary effect is presented in this shape of inductor channel.  Previously, we determined that slags would begin to collect on refractory surfaces that have the highest rate of temperature loss.

A typical refractory cross section at the throat of the inductor has the same heat transfer toward the water-cooled bushing as the other sections.  But there is added transfer of energy to the bath in the upper case.  This reduces the local temperature substantially and enhances slag accretion.  

By having a thicker section at the top of the bushing, the heat transfer is reduced.  The refractory will run hotter and will be less likely to encourage a build up.

Power levels in the inductor play a role in overcoming the thermal losses from the bushing and water-cooled case.  A maximum power level roughly two times the furnace holding power will raise the channel temperature and increase stirring enough to discourage build up.

The "U" loop inductor is not a new technology.  This concept has been used numerous times with various alloys with marked reduction of build up.  

Figure 2 - Cross Section of "U" Loop Inductor

POUR AND RECHARGE ENCLOSURES

In a pressure pour vessel, the inlet and outlet openings to the main hearth must be below the metal surface to preserve the pneumatic seal.  Unlike an open pouring spout, these tubes are longer and therefore extract more energy from the iron within them. One major source of build up is created simply by the extent of heat transfer from the iron to the lining surface.  This is countered by utilizing a "zoned" area of refractory in the enclosures.             First, the insulating portion of the lining is increased significantly.  Second, the hot face lining material is selected so that it retains more of the iron's heat, or said differently, it has a lower thermal conductivity.  Silica has a lower conductivity than alumina, and by increasing the content of silica in a refractory mix; we can reduce the overall thermal conductivity.

The iron in these teapot spouts is kept molten only by the heat supplied up the tube from the furnace hearth.  As the tube diameter becomes smaller, heat transfer is reduced and two things usually happen.  Slag build up along the top of the tubes increases as the iron temperature is now closer to the solidification temperature of the slag.  And the iron eventually gets to its solidification temperature.  All ductile iron orifices constrict to some degree.  Increasing their diameter, by design, to increase heat transfer is paramount to success.  So is maintaining the openings during production so as to avoid the use of burning bars at a later time.

FURNACE HEARTH - UPPER CASE

The furnace body or upper case is, by design, a more perfect container for ductile iron.  Vertical walls greatly reduce the possibility for slag adhesion.  Sufficient energy is supplied by the inductor to maintain iron/slag temperatures above their solidification temperature.  A few details remain that enhance its ability to remain clean.

Heat transfer in the upper case, similarly to that of the enclosures, must be designed to reduce, if not eliminate, slag growth.  Not enough insulating value in the lining system and the hearth soon becomes choked off with slag that must be mechanically removed. At the other end of the spectrum, excessive insulation will not allow energy to be extracted from the hot face and rapid erosion  will occur.  Most refractory suppliers and furnace manufacturers have the means to calculate, and back up with field data, the best lining system for this application.

"Apparent porosity" is a term applied to refractory that indicates the level of open pores within the refractory lining.  Levels for porosity range from 9% to above 20%.  Let us consider only the hot face lining or the portion that is in contact with the iron.  One of the reactions that increase magnesium fade is between the refractory and the iron itself.  All operators of ductile furnaces can tell you how a new lining increases fade until it is "saturated" with magnesium or actually, magnesium oxide.

Two positive things occur when the hot face refractory is selected with the lowest apparent porosity.  First, this period of lining saturation is shortened.  Secondly, any future removal of slag is simplified since there are no good sites where the slag can "grab" onto the lining.  The one drawback associated with lowering porosity is that the overall lining cross section has a higher rate of heat transfer.  To reap the benefits that lower porosity affords us, some changes must be made to offset this higher rate of energy loss.  The designer must increase insulating values so that the overall section once again reaches that optimum rate of transfer.

TREATMENT PRACTICE

To produce ductile iron, a known quantity and quality of master alloy is added to a known quantity of ductile base iron.  The intent is to produce the highest magnesium recovery thereby reducing alloy and superheating costs.  Various methods are used:

Sandwich
Covered Tundish
Plunging
Porous Plug
Flotret
Fischer Converter

The quality of ductile iron is controlled by the base iron chemistry and its temperature, the quality of the master alloy, construction of the treatment vessel and how the alloy is exposed to the base metal.  Once quality ductile iron is produced, the role of the pouring device is to deliver the metal to the mold without reducing its metallurgical makeup.

How the base iron is treated to produce ductile iron is paramount to the success of ductile iron automatic pouring systems - not just pressure pours.  Most foundries already drive their process control in the direction of lowering costs.  Producing cleaner, more "furnace friendly" ductile does this automatically.  Here are some guidelines:

• A heated pressure-pouring vessel enables a reduction in tapping temperature, which increases magnesium recovery during treatment.  Inefficient reactions increase slag carry over and build up.

• Attempts should be made to reduce the presence of air during and after inoculation.  Covered tundishes and convertors work well for this purpose.  Magnesium sulphide will, when combined with oxygen in the air revert to magnesium oxide and free sulphur, better know as sulphur reversion.  Free sulphur recombines with the magnesium in the bath, all of which increases magnesium fade.  The photos in figure 3 shows a practice with excessive oxygen contamination.

Figure 3 - Excessive Oxygen During Treatment

• Introducing silicon or ferro-silicon into the process, either by raising the base silicon level or in the treatment, may not adversely affect chill in the end product.  Fade of the nucleating effect from ferro-silicon occurs rapidly.  Do not expect pre-furnace additions to correct carbidic tendencies.  The best method is to apply the post inoculant when the iron is being poured into the mold.
• Reduce the calcium level in the ferro-silicon-magnesium alloy to less than 1%.  Higher concentrations have the tendency to generate slag growth at a higher rate.  Calcium oxide is a byproduct of the treatment reaction.  When combined with magnesium oxide, it forms an insoluble slag with a high melting point.
• Try to be more aggressive metallurgically.  Make attempts to reduce even acceptable but high levels of magnesium.  The higher the magnesium level in the iron, the higher the fade rate and the greater the slag generation.

Figure 4 - Magnesium Fade Rates at Differing Furnace Levels

• When furnace slagging is required always do it directly after a production shift.  Since air will enter the furnace through the lid and combine with the magnesium sulphide, sulphur reversion is a reality.  Following this procedure will allow the free sulphur to react with the magnesium in solution and minimize excessive sulphur levels at the beginning of the next production shift.
• If over-treatment is required, use nickel magnesium instead of magnesium ferro silicon.  This will reduce the overall slag accumulation and maintain silicon levels within range.

Following these steps can reap the benefit of reducing overall alloy consumption (from higher recoveries), reducing melting power usage (lower tapping temperatures) and minimizing the possibility of extreme slag problems.

FURNACE MAINTENANCE

The best furnace design and treatment practice do not take the place of a simple, regimented maintenance program.  There are methods that let us clean the most critical areas of the furnace and hearth during production.

The recharge enclosure should be cleaned at least once a shift and the pour enclosure once a day.  This is done using two cast discs welded to a length of re-bar.  The two discs are different diameters.  One being roughly an inch below the nominal diameter of the tube and the other one-inch smaller than that.  These discs are run down and up the tubes--- the smaller one first and then the larger size.  Fully formed hard slag takes time to develop.  Staying one step ahead of this formation simplifies the clean up job.  Here is one foundries approach in figure 5:

Figure 5 - Recharge Enclosure Rodding/Slagging

The inductor is also an area where build up can occur.  Knowing the current condition of the inductor is invaluable.  Daily meter readings and graphs should be kept either via software-supplied programs or manually charted information.  Inductive reactance, which is an indication of the shape of the inductor loop, will show wear and/or build up.  Resistance of the loop, when compared to the reactance, can show buildup and penetration of the refractory.

When considering a fixed-voltage power supply, any build up in the inductor channel will cause the power draw by the inductor to drop.  This, initially, may not be a problem.  But, if the power should drop below the level required to maintain the iron temperature, slag growth will increase.  See figure 6 below:

Figure 6 - Inductor Readings

The only reasonable method of cleaning the inductor channel or slot is to rod it mechanically.  First, take a meter reading, as this will give a good indication as to the effectiveness of the procedure when compared to the post-rodding reading.  Usually, a piece of re-bar or angle iron is used for this purpose.  The iron bath should be dropped to the minimum level.  This reduces the length of rodding bar exposed to the bath and increases the time one can rod with a given bar.  Forcefully run the rodding bar down one side of the slot and continue to do so until the bar exits the opposite side of the loop.  This may take a number of rods.  Slag will normally accumulate on the iron bath over the inductor if the procedure is effective.  The meter readings should indicate a drop in reactance and possibly resistance.  As with the enclosures, do not allow these oxides to fully mature as they become very difficult to remove.

Most pouring units are equipped with stopper rods and nozzles.  The nozzle is an area of relatively high thermal loss and has a higher chance to become closed by oxides.  The approach here should be to reduce heat transfer by converting the nozzle refractory to a material with a lower thermal conductivity.  Many users have benefited by using fused silica, and in some cases, zirconia.  As the iron passes through the orifice of the nozzle, it loses less energy and. therefore; oxide deposits on the surface of the refractory are lessened.

Following common sense techniques for furnace and refractory design, economizing the ductile iron treatment process and providing a reasonable maintenance practice will result in producing the highest quality ductile iron with a minimum of effort.

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