Pouring Concept Extends Hold Times for Treated Ductile Iron.

A tilt-pour furnace has been used to successfully control magnesium fade in treated ductile iron, significantly increasing pouring line productivity.

The evolution of pouring treated ductile iron began with hand-pouring ladles, but when a more precise pouring method was desired, foundries began to use stopper rod pouring from unheated pour boxes. To gain better temperature control, the next advancement was the pressure pour furnace. Now that longer fade times and reduced magnesium (Mg) usage are desired, the pressure pour furnace must yield to a new type of pouring furnace.

Waupaca Foundry and Duca Manufacturing began work in 1989 on a pressure pour for treated ductile iron, and while the six original machines continue to perform well, the search for a better pouring method with longer holding times for treated ductile iron has continued.

This article discusses a new pouring concept based on a similar furnace designed and built by Georg Fischer Mattmann in Germany. Installed in the Mattmann Foundry in 1996, the furnace had proven to be a significant benefit in pouring ductile iron and provided many of the design concepts for new pouring furnaces.

Mg Fade in Molten Iron

In order to implement an efficient pouring device for treated ductile iron, it's important to understand the mechanisms that affect how Mg exists in molten iron. Mg becomes soluble in molten iron only after it has reacted and reached equilibrium with the sulfur (S) and oxygen (O). Once the Mg has reached its solubility limit, any additional Mg will exist as a gas dispersed in the molten iron.

The vapor pressure of pure Mg is 1 atmosphere (atm) at its boiling point [2043F (1117C)]. Since the pressure of pure Mg increases to 7.49 atm at a pouring temperature of 2600F (1427C), any Mg existing as a gas will readily boil out of the molten iron, causing fade.

Figure 1 shows the solubility of O and Mg in treated ductile iron. Typically, it is difficult to achieve Mg levels above 0.1% in over-treated ductile iron. When an element becomes solute in another element, the vapor pressure of each element will function according to Raoult's law (the vapor pressure of either element is proportional to its mole fraction in the solution multiplied by its normal vapor pressure at the temperature in question). For ductile iron containing 0.04% Mg at 2600F (1427C), the mole fraction of Mg is 0.000788. If the mole fraction then is multiplied by the vapor pressure of pure Mg (7.49 atm at 2600F), the vapor pressure of the Mg is 0.0059 atm, or 0.087 psi. Although this vapor pressure is considerably less than ambient pressure, Mg will still evaporate from the iron, also resulting in Mg fade.

Using N to Overcome Fade

In addition to Mg fade as a result of evaporation or as the Mg gas boils out of the molten iron, fade also occurs due to the presence of available O.

Molten iron will dissolve O from the air. This O, in turn, reacts with the dissolved Mg, forming Mg oxide that then floats out of the molten iron. This is the basic system of fade in an unheated pour box producing a characteristic white powder. The iron in an open ladle has a typical fade time of 12-15 mm.

This same fade time occurred in the early pressure pours that used air as the pressurizing media. However, when the pressurizing media is changed to nitrogen (N), the fade time increased to 2035 mm. While the inert N atmosphere was an improvement, the fade time did not improve significantly. There are three explanations:

* the so-called "inert" N atmosphere is other than inert, as N contains O in varying amounts depending on the means by which the N was produced from air. The lower the purity of the N, the more O present and the quicker the fade;

* some of the Mg in the molten iron evaporates and forms a partial pressure with the N atmosphere above the melt. As metal is dispensed from the vessel, the volume previously occupied by the molten iron is filled with more N pulling more Mg from the iron. That N/Mg atmosphere is exhausted each time molten iron is added to the vessel;

* since O entrainment occurs upon filling, the filling frequency should be reduced, which requires larger vessels and longer fade times. If the N/Mg atmosphere would never be exhausted, longer fade times should be possible.

However, some of the Mg in the molten iron will still vaporize and form a partial pressure with the N above the melt. When the partial pressure of the Mg above the melt equals the vapor pressure of the Mg in the melt, the system will reach equilibrium, where the rate that Mg evaporates and leaves the molten iron equals the rate that Mg condenses and returns to the molten iron. Equilibrium cannot be attained in a pressure pour because the atmosphere is changed each time metal is added.

Vessel Resulfurization

The mechanism of resulfurization also must be considered. When treated iron enters a pouring vessel, the suspended MgS particles also enter the vessel. While most of the suspended materials make it to the surface, some become attached to the sidewalls of the furnace lining. As long as the pouring vessel is in operation, the MgS attached to the walls remains because the residual Mg prevents the reversal reaction. However, when the vessel is opened to clean, three reactions occur:

* the O in the atmosphere will react with the MgS. Herein, the MgS is oxidized to MgO and [SO.sub.2];

* the O will dissolve in the melt and react with the residual Mg in the molten iron. When the residual Mg level nears zero, the dissolved O will increase to a level controlled by the carbon and silicon contents;

* the MgS that is in contact with the melt then will react with the dissolved O. Herein, the dissolved O will exchange places with the S dissolving in the molten iron. The amount of resulfurization depends on how much slag and buildup exists in the vessel. Typically, in a pouring vessel, the bath can be resulfurized by 0.0075-0.12%, which presents a problem when restarting the pouring vessel after a weekend shutdown. Techniques such as over-treatment with Mg or flushing the pouring vessel with low-S iron are employed to reduce residual S to acceptable levels.

Achieving Equilibrium

To address these chemistry issues, a furnace was designed to achieve equilibrium, slow down fade and result in smaller Mg additions. The first requirement was to maintain an undisturbed N-Mg atmosphere above the melt. This was accomplished by designing the body of the furnace as a pressure vessel. To allow pouring while maintaining a sealed atmosphere, the furnace was fitted with teapot fill and pour spouts, and hydraulic cylinders were employed to tilt the furnace.

Next, the furnace was fitted with both fill and pour spouts that are aligned with the tilt point or bearing centerline of the furnace so that the metal poured from or into the furnace is always at the same elevation. This allows receiving and dispensing of metal at the same time.

In addition, to allow the furnace to pour one mold at a time, an unheated pour box filled on a semi-continual basis was used. For practicality, the furnace vessel was sized 2.5 times the maximum treatment size.

Furnace Operation

The tilt-pour furnace (Fig. 2) is a vertical channel furnace with a dished-head bottom. The inductor is hung from the bottom of the furnace via a water-cooled throat extension. The lid, which is equipped with a small hatch so that the vessel can be slagged, is gasketed and bolted to the top plate of the furnace. In addition, the lid is equipped with two ports so that the inductor can be rodded, if required.

The upper case of the furnace uses an insulating firebrick against the shell, a medium-duty alumina backup brick and a castable hot face lining. The bottom refractory is dry vibratable, as is the inductor. The lid is lined with a bubbled alumina castable. The fill and pour spouts are lined with a 60% alumina-silica castable.

Inductor Use

When ductile iron is involved, the choice of inductor must be based on the treatment method. Because the treatment method uses pure Mg, a conventional throat-type inductor was chosen.

To accommodate the startup problem of cold iron (due to the converter not being at temperature), the inductor is rated at 1250 kW. This allows the first iron into the tilt-pour to be rapidly heated to the pour temperature.

The inductor case is fabricated from 1-in.-thick steel plate for rigidity and is water-cooled. A copper, water-cooled bushing with a water-cooled coil cap is employed. The core has a 14-in-diameter coil leg around which is fitted a single-wound, water-cooled coil. The inductor is side-vibrated with a dry alumina lining using a round, cast steel, loop form.

The Melting Process

The iron is melted in a hot-blast, water-walled cupola. The S level in cupola iron usually is too high for most treatment processes for ductile base iron and is lowered via a desulfurizing ladle. However, direct treatment of cupola-melted iron at 0.07% Scan be accomplished in a converter. The cupola-melted iron is held in a 150-ton horizontal channel furnace equipped with two 1000-kW single-loop inductors. The iron is tapped from the holder into a 10-metric-ton Mg treatment vessel. The treated iron then is poured into one of four 25-ton usable tilt-pour furnaces. Each tilt-pour furnace pours into an unheated box equipped with a stopper rod that dispenses the metal into a mold on a flaskless molding machine.

Performance

Each morning, this furnace is operated at 1250 kW so that the first iron into the vessel is superheated to operating temperature. Once it is up to operating temperature, the furnace is held at 350-450 kW.

The problem of temperature control is overcome since the small, unheated pour box is filled on a semi-continuous basis. The molten metal temperature control in the pour box can be maintained equal to or better than that of a pressure pour. A laser measures the metal level in the pour box while a computer-controlled proportional hydraulic valve moves the furnace up and down. Movement of the tilt cylinders maintains the metal level plus or minus 0.8 in.

A N atmosphere is maintained above the molten iron bath. The pressure in the vessel increases and decreases in accordance with the amount of metal in the vessel. Since no atmosphere is exhausted, no Mg is lost. The fade time has been reported at 120-150-min.

This article was adapted from a presentation (00-059) at the 2000 AFS Casting Congress. Conference Proceedings are available through AFS Publications at 800/537-4237 or the AFS E-Store at www.afsinc.org.

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