Controlling Carbon, Sulfur and Metal Penetration of Induction Furnace Linings.

By following these recommendations, foundries can increase melting efficiency by reducing gas and metal penetration of the refractory lining

Coreless induction furnace refractory linings (Fig. 1) have two common characteristics that drive gas and metal penetration. The first is porosity (in the range of 15-25%). It is comprised of closed pores and cracks in the grain and interconnected closed-pore cells as a function of density/grain size distribution. With dry linings such as silica, loss of density is critical because it increases interconnected porosity proportionately and, more importantly, opens up cellular pores closed by compaction, thus radically increasing access for metal and slag.

The other major factor affecting metal and gas penetration is moisture. It is found in trace amounts on or in the grain; in natural hydrates; and in hydrated bond materials such as boric acid or boron oxide with silica. Although a perfectly dry lining doesn't exist, even silica bonded with boron oxide will retain natural moisture, with the degree of moisture retention dependent upon the grain's moisture, humidity in operation and bag integrity/time of storage.

This article takes a look at the interaction of carbon (C), sulfur (S) and molten metal with these two characteristics and the resultant effect on coreless induction furnace linings. Recommendations then are provided for reducing gas and metal penetration to improve melting efficiency and productivity.

C Penetration

C is an integral part of cast iron, copper-base alloys and aluminum. With iron, it is an alloying element; with copper, it is a reducing agent that provides atmosphere as a melt cover; and with aluminum, it is the principal lubricant base/mold release as well as the primary connection in reduction cells. This means that C is present in some form as a charge material in all coreless induction melting applications. Although this article focuses on the interaction of iron and silica furnace linings, the same principals apply with brass-, copper- and aluminum-based alloys.

As a new coreless furnace lining is put into service, moisture in the refractory and from dehydration of the bond fraction raises the dew point within the pore structure to full saturation. Little moisture (a few milligrams per cu. foot of refractory material) is required to accomplish this, as water vapor acts as a true gas and expands in all directions, including toward the hot face, when it comes in contact with molten iron. Then, the water vapor dissociates into hydrogen (H) and oxygen (O). The H, being a small and light atom, quickly diffuses out through the bath and ignites, exhibiting a blue flame that is sometimes visible around the edge of the melt. The highly reactive O left behind interacts with the molten iron, manganese, silicon and C to form a glassy slag and carbon monoxide (CO) gas.

CO is the stable high-temperature form and is always what results when C is oxidized in a limited atmosphere at high temperatures. This limited atmosphere is provided by the small quantity of moisture that dissociates at the interface between the refractory and the liquid iron, which presents essentially an infinite source of C.

The CO then diffuses through the melt (like the water vapor) and back into the lining. The CO seeks to escape through any aperture available, which means it usually exits through cracks in the coil grout as well as around the top and bottom ends of the coil. (It is not unusual for coil grout materials to have small cracks, which is all that is really needed to provide egress.) As the diffusion of CO into the lining occurs, the temperature and stability of the CO is reduced, and the trace amounts of iron present in or on the refractory (as an impurity or from crushing, screening, etc.) catalyze it to [CO.sub.2]. The iron does not participate in the reaction other than as a "trigger" (thus it is not depleted), and consequently remains available to continue to catalyze the reaction as more CO passes. Thus, the combination of low temperature and steep thermal gradient created by the coil in combination with the iron catalyst in the refractory and ordinary cracks (not indicating poor workmanship) create ideal con ditions for the reaction to occur. A byproduct of the reaction is the creation of C that is essentially pure with high thermal and electrical conductivity.

This C deposits in the cracks of the coil grout and on the coil turns, reducing the turn-to-turn resistivity and promoting arcing, pinholes and, ultimately, coil failure (Figs. 2 and 3). C production and deposition can take place at any time, but is particularly rapid during and shortly following furnace startup because the moisture in the lining provides a concentrated source of O in the right place.

Recommendations for reducing carbon penetration are listed later this page.

S Penetration

S is typically present on charge materials as a component in lubricants. These lubricants break down in exactly the same way as hydra-carbon oil components and form sulfur monoxide (SO) gas, which hydrolyzes with moisture to form highly corrosive, dilute sulfuric acid vapor.

In a furnace, it takes only a pinhole in the coil coating material for the sulfuric acid to start to react and lift the coil coating material from substantial areas. This takes place in the same location as the deposition of C in the reaction described above. The result is a synergistic effect where the electrically conductive sulfuric acid exposes the bare coil, and the C deposits provide a solid conductor that will hasten turn-to-turn arcing/coil failure. Though dilute, hot sulfuric acid is highly reactive and difficult to protect against.

Reducing C and S Penetration

To avoid C and S penetration, the following actions are recommended:

1. Utilize the full pre-fire recommended by the refractory supplier/furnace manufacturer and don't take short cuts or rush the startup of a new lining. Consider using extra time if the coil grout has been replaced or repaired or other wet lining materials have been utilized in addition to the working refractory such as cast rings, dome, extensive top cap/spout or surface patch.

2. If possible, use already-alloyed cast iron returns for the first 3 days of melting (which minimizes the charging of steel/C additions) and avoid high C/low silicon iron such as ductile base. Avoid charging oily scrap for the same 3-day period.

3. Consider using an impervious slip plane such as mica paper between the coil grout and lining.

4. Patch the coil grout when the lining has been removed to minimize avenues for CO and SO to escape and deposit. Remove deposits of C in cracks and patch.

5. If agreeable with your refractory/equipment supplier, consider venting the form with about 0.06-in. holes to allow for better venting of water vapor during the early stages of startup.

6. If milliamp-to-ground readings remain high during operation, run the furnace up to the sintering temperature, which will help drive off the bulk of water and interrupt the CO/SO development. Then, drop back to normal operating temperature.

Metal Penetration

As described earlier, the O created by the breakdown of water vapor at the liquid metal-refractory interface also creates a number of other oxides, the aggregate of which is a slag composed of varying amounts of iron oxide, manganese oxide and silica all derived from the iron itself. The nature of this slag material is to "wet," adhere to and penetrate the refractory surface. It also reduces the surface tension of the iron, making it possible for molten iron to pass through tiny pores it otherwise would not enter.

As the slag penetrates the refractory coating the grain, iron follows (driven by capillarity), creating a network of metal saturation (Fig. 4) that can electromagnetically couple with the coil, pull power, superheat and cause the refractory to erode and wash out, If the refractory is properly sized, installed and sintered, the network that develops will be too fine and contain too little metal to support sufficient electromagnetic coupling to cause refractory damage. Under these circumstances, the network stabilizes in balance with the thermal gradient and the lining performs well. If, on the other hand, porosity is unacceptably large or continuous, saturation will be dense, superheat will result and performance will be severely curtailed (Fig. 5).

Silica linings in coreless furnaces are unique because the bonding agent creates a glass (boron silicate), which, if properly developed with a well-sized, installed and sintered lining, will seal the hot-face surface with the imperviousness of a bottle and prevent the saturation mechanism described above. This is critical to good silica performance because sealing the surface forces the erosion to take place on a single plane, rather than three-dimensionally in a penetration zone. As a result, the refractory erodes more slowly as reactions are confined to the hot-face surface. In some situations, it is advantageous to go through the high-temperature sinter cycle again to re-establish a sealed hot face in mid-campaign.

A common startup problem with silica linings is heavy penetration of metal and slag in the lower one-third of the furnace including the taper and bottom. This frequently is caused by controlling temperature using thermocouples near the mid-coil (where temperatures are higher), ignoring the bottom (where temperatures can run several hundred degrees lower), and thus not be nearly as well-sintered. This, together with the higher ferro-static pressure of liquid iron, assures severe penetration in the lower one-third. Control thermocouples need to be placed low enough to assure that the critical bottom area, which needs to be tightly sealed, receives a proper sinter.

Reducing Metal Penetration

To reduce slag and metal penetration, the following actions are recommended:

1. Ensure uniform high-density installation.

2. Do not take short cuts or rush startup for the same reasons listed earlier with regard to C/S penetration.

3. Ensure that the top recommended temperature for final sinter is reached and held for the full sinter time recommended by the supplier.

4. Inspections of silica refractories must:

* test for boric acid and uniformity;

* develop a sizing specification with sand screens to test each lot. Retain results in a separate log noting where the product was used and performance;

* retain an "as received" sample of any refractory in use until that lining has been removed from service.

* keep records of the mode of refractory deterioration, location, rate and any unusual events or occurrences that might bear on performance such as power and water interruptions, use of city water, wet scrap explosions, carbon boil, bridging, and high miliamp readings.