Optimizing induction furnace refractories.

Reduction of oxygen in the lining of channel induction furnaces will help to eliminate saturation and allow longer refractory life.

All aggregate refractories installed in induction furnaces display some degree of porosity. Because the component grains of these refractories do not fit together exactly or bond over their entire surfaces, this porosity is interconnected, forming a web of tiny passageways that make the refractory permeable to gas or liquid.

One of the major consequences of permeability is refractory saturation, which occurs when liquid metal penetrates the refractory face. Once inside the refractory, the liquid works through the network of passageways toward the outer shell of the furnace, leaving the grains in the refractory intact as it moves around them. This deep damage to the refractory dramatically shortens its life, while jeopardizing the integrity of the shell.

The effect of metal saturation is like pouring a 3-D metal casting of the interconnected network porosity of the lining. The refractory essentially acts as a mold and core structure, around which the metal flows. If a piece of saturated lining is cut on a fine diamond saw, the saturation resembles the structure of a sponge surrounding each refractory grain.

Though saturation is common in all induction furnaces, it is particularly prevalent in channel induction furnaces. While there are many physical, chemical and mechanical factors that contribute to it, the electromagnetic flux of the channel furnace is the root cause of saturation.

Electromagnetic Flux

Unlike the coreless furnace, in which the electromagnetic flux is distributed over most of the crucible wall, the intensity of the flux in the channel furnace is uniquely concentrated because of the relatively small volume of the secondary channel.

Saturation networks that develop in the area of the channel display very different characteristics than those encountered with the saturation of an open-structure refractory such as a porous insulating backup brick. The web of the saturation network in an electromagnetically powered area is typically much finer than that which could be developed under static, unpowered conditions, even with considerable fluid static pressure.

As saturation moves toward the shell, it continues to conduct the electromagnetic flux, but without the benefit of liquid metal circulation to cool it. The result can be the superheating of the lining, possibly leading to "leaker" type runouts.

If furnace power is reduced to holding level after a channel refractory becomes saturated, the oxides at the point of metal entry will begin to clog behind the saturating metal. This cuts off the metal feed from the heat of the melt and it begins to solidify, separating and beading up internally from surface tension.

The loss of continuity means the metal is less conductive and will not pull as much power, keeping it cool enough that it will not resume its course toward shell when full power is reapplied. In this way, foundries can sometimes control saturation by adjusting furnace temperature at the right time.

Refractory Atmosphere Effect

The main accomplice of the flux in causing saturation is the moisture inherent in most refractories. Although the flux allows for concentrated network formation around a channel, the initial entry of the clean alloy would be almost impossible by itself because of the non-wetting or "beading" effect of liquid metal surface tension. Wetting must be enhanced for the metal to enter the refractory.

Enhancement is provided by gas given off from the refractory when it is charged for a melt. The gas is primarily superheated steam generated from the deeper drying out of the lining and decomposition of refractory hydrates at melt temperature. A fused aluminum oxide refractory grain, for example, will typically have 0.1-0.4% moisture mechanically trapped in cracks and pores or stored chemically in natural hydrates.

Some types of groins, such as magnesium oxide, are themselves hydratable. The amount of hydrate varies by sizing, relative humidity and the presence of any other bond hydrates they may be mixed with. Wet rammed refractories or castables present even larger quantities of moisture. Thus, the refractory is a substantial source of water vapor.

The resulting superheated steam will dissociate into hydrogen and oxygen on contact with the liquid metal. This dissociation leaves behind oxygen at the metal/refractory interface. The oxygen immediately combines to form oxides, a process that continues for six to twelve weeks.

The accumulation of these oxides causes a general oxidation of all the reactive components of the alloy in the channel, creating both reactive and stable oxides congruently. A coating of slag begins to form on the interior channel surface consisting of amorphous, glassy material and higher melting point oxides, such as aluminum, calcium, magnesium, zinc and mineral complexes like spinel or calcium hexaluminate.

The slag begins to penetrate the refractory, while simultaneously destroying the integrity of the surface tension of the metal behind it. As the slag precursor diffuses into the refractory pore structure (leaving the stable oxides behind in the form of clogging material), the metal follows the same path, creating a network that is actually a casting of the interconnected pore space available.

Once entry has been made, the forces of capillarity and fluidity, combined with the electromagnetic flux, ensure that even the tiniest pore space is filled. As long as the metallic feed remains and fluidity is provided by the inductive power, the saturation network continues to penetrate deeper into the lining, resisted only by the steep thermal gradient. In some cases, it will solidify when power is reduced to holding level and then remelt again at full power. The deeper it goes, the hotter it gets when full melting power is reapplied because it is no longer effectively cooled by circulating channel metal.

Preventive Furnace Operation

Since the electromagnetic flux is responsible for the furnace's melt/superheating operations, little can be done with it to control saturation. However, much can be done to manipulate the furnace atmosphere and reduce the moisture present in the refractories.

In taking measures to control the atmosphere, great care is needed because very little moisture is necessary to reach a saturation dewpoint and initiate oxidation of the liquid channel alloy in an inductor. Also, the declining thermal gradient of any refractory toward the shell and bushing creates a virtual "time release capsule" for moisture sourced oxygen.

The first step in reducing the moisture is to adequately dry the refractories by prefiring them. Additionally, after startup, the furnace can be placed on hold to "season" it--a final curing to remove remaining moisture. This must be done in a way that will make sure that the whole refractory cures and not just the face.

To further retard oxide formation, the drilling of "weep holes" into the shell of the furnace allows the superheated steam somewhere to go besides into the liquid metal. Also, the application of a low vacuum at two to six points on an inductor (depending on its size) should help reduce the availability of oxygen. This will jointly reduce the volume of clogging material that can build in the channel and the slag precursor, which causes the particularly deep and damaging saturation network. Though the metal network will not be eliminated, the potential for deep penetration/superheating will be greatly reduced and longer campaign life can be achieved.

The objective of the application of the vacuum is not to create a sharp reduction in pressure because that will cause as many problems as it corrects. Rather, the vacuum depletes the water vapor backup in the refractory pore environment. The resulting imbalance promotes diffusion of the vapor within the refractory toward the inductor case and away from the channel area in its effort to reestablish balance.

The level of vacuum required is already available in most foundry systems used to operate molding lines, or in a simple mechanical pump. Negative pressure should be 2-5 psi and the vacuum should be left on the unit for at least the first 15 weeks of a campaign, except for automatic pouring furnaces, where the vacuum should be kept at under 3 psi and applied only when the furnace is not pressurized. There should be a fine ceramic filter disc or porous insulation material in front of each evacuation point to avoid clogging the lines and removing refractory fines.

This use of a vacuum has not been widely field tested as of yet, though it shows considerable promise for increasing the lifespan of induction furnace refractories by preventing the abnormal conditions that cause the damage. As with any foundry procedure, care must be taken to ensure safety and optimal performance. Those considering the use of a vacuum should seek and follow the advice of the furnace builder for hookup details.