Metal saturation and finning problem can be avoided: controlling metal saturation is a function of knowing refractory porosity, permeability.
Finning, on the other hand, is a metal-filled crack, or you might call it a casting of a crack. While saturation does not require a crack as a feeder, fins are often associated with saturation but seldom have any bearing on whether or not the saturation zone forms. In most cases, they are a consequence of the saturation zone's altered thermal expansion and strength characteristics, which promote cracking.
After investigating hundreds of refractory failures in channel and coreless induction furnaces over the years, it became obvious that a network of metal saturation virtually always developed in the powered area of a channel furnace. It occurs in the coreless furnaces when the glazing effects of silica do not completely seal the structure. Initially, this was attributed to porosity caused by the drying of the inductor refractory and, later, by the natural tendency of dry vibrated materials to have more interconnected pores. Further study, however, showed the problem to be considerably more complex than this. Controlling Metal Penetration
The principals controlling metal penetration into a refractory include: * pore size, * percent porosity,
fluid pressure (metal depth), * metal viscosity, including that of
second phases, * the wetting characteristics between
the alloy and the refractory material.
To these must be added the effects of vibration frequency and intensity. The presence of an electromagnetic field in the refractory also can change the temperature and, thus, the viscosity of the alloy on entry. This serves to break down the non wetting effects of surface tension and delay solidification by introducing energy.
All things being equal, the degree of electromagnetic coupling depends on the mass/electrical characteristics of the metal. Furnace frequency and power level will serve to delay solidification, reduce the effects of cooling and allow the alloy to "coast" deeper into the refractory.
Once solidification does occur, the latent heat of fusion required to remelt the metal network may provide an effective barrier and ensure that further penetration does not occur. But other factors are involved.
Metal network saturation takes place very soon after the furnace is charged or meltdown occurs. The network, once initiated, develops rapidly due to strong capillary action, quickly pulling the liquid into the refractory structure as water would enter a sponge.
Why does entry occur so readily in the powered zone of an induction furnace, such as around the channel in an inductor? The alloy itself does not readily enter such fine porosity. But examinations of samples from copper-melting furnaces and confirmed on refractory samples taken from iron-melting furnaces has shown the presence of a precursor that wets the surface of the refractory, enters it and makes it possible for the liquid metal to follow. The precursor that develops in a copper melting furnace is principally copper oxide, which readily makes a glass with the silica components of an alumina refractory. It wets and enters the pores easily with the help of the high fluid load and the special induction effects noted above.
The development of this precursor exactly at the interface between the liquid metal and the refractory surface is largely the result of the natural exodus of water vapor in the form of superheated steam from the refractory. Any refractory, whether it is a dry vibrated type, a castable or a wet ram, contains moisture that will be released gradually after the furnace is charged for as long as it takes the thermal profile to move back and establish stable isotherms.
Dry vibrated refractories, based on aluminum oxide without hydrates in the bonding system, have between 0.20.4% moisture adsorbed on the surface of the grain, in cracks and pores or included in natural hydrates. Those with hydrates in their composition may have well over 1% moisture that will not be released until the temperature of decomposition for the hydrate is attained in the working inductor.
Moisture released inside the entirely enclosed inductor moves as a gas (superheated steam) in all directions. Some escapes by way of joints or weep holes, while the balance may encounter the channel where immediate dissociation into hydrogen and oxygen occurs. The hydrogen atom, being the smallest component, moves readily through the melt and ignites at the surface with a blue flame. Oxygen, however, immediately reacts to form new oxides with the most available elements in the circulating liquid metal.
The result, immediately after charging, is the formation of a skin of oxides on the inside of the channel, creating the highly wetting precursor. This oxidation also accounts for the characteristic initial clogging mode that furnaces go through for anywhere from a few days to several weeks after start-up on a newly lined inductor.
In addition, the coupling of the electromagnetic field with the penetrating liquid phase volatilizes high-vapor pressure elements (i.e., manganese in cast iron) which move through the refractory structure, coating the surface grain and effectively acting as a wetting agent to further promote penetration. Other surface-reactive materials (or surfactants) that can be either metallic or oxides, include carbides, nitrides or plain carbon and can modify the internal surface structure of the refractory. These promote or, in the case of carbon, for example, retard penetration. Saturation Mechanism
Some effective procedures can be adopted if the mechanism involved in metal saturation is known. Most of these techniques are independent of each other and are selected to minimize their impact on furnace production or at least to confine special techniques to the start-up period on a onetime-only basis.
It is possible to slow or prevent metal saturation. Once the metal has entered the structure, there is no way to eliminate it without changing the refractory lining. Saturation is not necessarily harmful and, in fact, is found in virtually every lining. The negative effects of saturation increase with increased power, temperature and particularly with clogging, which causes the network to move deeper and to remelt easily. Often, clogging so superheats a penetrated area that it washes away, exposing fresh refractory to attack, or fuses the refractory ahead of itself, sealing off further penetration. Controlling Saturation
Because the accessibility of the internal structure of a refractory increases exponentially with increasing pore size and frequency, ensuring that pores are kept to a minimum is critical. While refractories are designed to a specification, they are, in fact, produced within a range characteristic of the materials selected and the size control imposed.
Inductors, however, are unforgiving machines and, if the porosity exceeds the critical size and frequency necessary to resist metal penetration, excessive saturation will occur and other actions taken to control it will be less effective. Porosity in refractories cannot be eliminated, but it must be kept to a minimum to offset other factors favoring the unusually heavy saturation found in inductors.
An effective inspection of new inductor refractories is worthwhile to screen out batches with a high incidence of porosity.
After evaluating refractories in many different ways involving grain size and distribution, density and the like, it is evident that none of the common tests relate directly to the problem of penetration. A gas permeability test (like the standard ASTM method) relates well to metal saturation caused by open structure and will identify refractory materials that are prone to this problem. The test can be performed by ceramic laboratories and universities at a reasonable cost, especially when compared to the expense presented by porous material in an inductor.
It is recommended that, in order to establish a benchmark for control purposes, samples be sent to a reliable outside ceramic laboratory for testing by the ASTM method. Mercury-type porosimeters utilize a very small sample and give results that are not necessarily representative of a coarse refractory structure. Boiling water porosity tests are helpful but not as a permeability test. They simply give an indication of total pore volume.
Once some basic benchmark permeability tests have been run on the refractory to be used, a relative test method can be set up wherein the samples are fired in-plant and subjected to a gas-flow permeability test using air or nitrogen. This is easy to set up, and although it will not give the same numbers as reported by a laboratory, it will be relatable and provide a sound basis for flagging products that could cause difficulty.
Specifically, how this test is conducted will depend upon the refractory product to be tested. It may be necessary to add a small amount of temporary binder to avoid having to fire at a high temperature. Nevertheless, the permeability of the structure will be readily detectable if it is out of line with "normal" batches that have been set up as a standard. Refractory Uniformity
Batch uniformity is not normally a problem, although it is wise to watch carefully for differences in color and texture that can indicate segregation has occurred in the refractory's processing.
When the material in the bags in which the refractory is delivered appear different in texture or appear porous, these should be set aside and returned for credit if there is sufficient material on hand to complete the reline. If not, at least three normal-appearing bags should be randomly selected and their contents thoroughly mixed with each bag that appears out of spec. This will dilute the size difference sufficiently to eliminate that as a major performance concern in most cases. This does not apply to foreign material in the refractory (such as wood, paper or metal), which is a totally different problem altogether.
Avoiding segregation during the installation sequence is extremely important in controlling metal saturation. With a dry vibrated product, referred to as DVC, there is a tendency for the coarse fraction to separate during the vibration sequence if a layer technique is used. This segregated layer must be intermixed with the refractory installed before another layer is applied, or metal saturation will occur following the coarse fraction in bands. Pouring refractory from a height of more than two or three feet can also contribute to segregation, and every effort should be made to prevent paper or plastic from getting into the refractory.
It is important to achieve maximum compacted refractory density and to uniformly pack it in place throughout the inductor. Care must be taken to avoid damaging forms or causing them to shift or collapse by the accumulated stress of high-density vibration. Controlled Firing
The early stages of firing have little to do with porosity and metal saturation, except where wet refractories are used and the moisture creates pressure by being driven through the structure too rapidly. With dry refractories, which is the most common material for both coreless and large channel furnaces today, the most important part of the initial firing is achieving the maximum temperature that will deliver the optimum rate of bonding without the interference of vapor or liquid. This must be done without melting out the form or, as it is sometimes called, the ramming frame. It is important not to go beyond this temperature but, rather, to hold it and allow initial bonding to occur. Although controlling the rate of heat-up at lower temperatures has some value in uniformity, differential stress control, etc., the rate of bonding goes up exponentially with temperature. Thus, the highest temperature one can expose the refractory to in a protected state with the forms intact is very important.
With inductors that still utilize burnout wooden forms, the final firing temperature before charging is no less important and should be selected to be well within the capabilities of the firing equipment. This temperature should not be above the initial priming alloy temperature by more than 200-300[deg.]F.
Where this type of burnout form is used, a compatible refractory primer, which is either sprayed on the interior of the channel after burnout or applied by the "rag and rope" technique, will help minimize initial penetration. This is particularly true where fluid second phases are present in the alloy as in the case of free-machining (leaded) brass. The same applies to coreless furnaces where removable or wooden burnout forms are used.
When a channel furnace is started by priming with liquid metal, the heel that is established should be no more than necessary to support the power for holding, plus about 15% for control latitude. It is important to avoid "kicking" from excessive pinch because of insufficient static pressure to offset electromagnetic forces.
Kicking creates a cavitation-like pulse that is potentially damaging to weakly sintered refractory and can initiate cracking and/or finning. Conversely, excessive metal head can initiate metal entry and establish a saturation network. Control and moderation are essential.
Hand in hand with pressure in determining initial entry is the viscosity of the startup alloy. This should be kept as low as is practical by using a minimum of superheat depending upon the large energy reservoir represented by latent heat of solidification, to give control and prevent freezing of the metal in the channel. As long as some induction power is applied, even if the circulation is minimal, there is little chance of freezing because hot metal is continually being pulled down from the heel in the uppercase.The only real danger in riding close to the melting point on an alloy in order to maintain high viscosity is in a long power delay in changing tap settings where the equipment does not have separate controls. Then, operator skill is important, and additional temperature is certainly justified.
After a furnace has been primed, it is advisable to plan to operate it with a low heel and a minimum of superheat for a period of between four and eight hours. This will allow the much more rapid and pervasive heat loading of the circulating liquid metal to sinter the refractory, allow dissipation of differential stress and allow the establishment of a more shallow penetration network than would result if higher temperatures were used. In light metal alloy and certain copper-base alloy applications, this is of particular importance, This is because proper conditioning helps resist the penetration of corrosive metal and slag components later in the campaign by plugging pores. With cast iron, it is also of value but more from the aspects of sintering and stress accommodation. Coated Meltout Forms
The slag precursor, composed of oxides that lead the metal saturation front into the refractory, is formed at the interface between the precursor and the meltout form. A coating applied to the form can act as a barrier to entry of the precursor into the refractory and is an effective way of helping to prevent the initiation of a penetration network. It is important that the coating not be glassy in composition or readily form glasses with the refractory since this will simply be creating a precursor of a different composition. It is best if the coating is composed of a single high-purity oxide or oxide complex compatible with both the metallurgical slag and the refractory involved. There should be no hydrates in the coating, as this adds to oxygen availability. Coatings that are fused on the form appear, at this stage, to be ideal for this purpose, although simpler approaches may be developed. Vapor Phase Control
As was described earlier, there is considerable outgassing of even a dry inductor refractory during the final firing. This usually occurs immediately after priming with liquid metal. Tests of inductors immediately after startup have shown that the environment within the inductor is saturated with live steam for an extended period. In my own experience, I have seen water dripping from inductors that have been in service with molten cast iron for several days.
The presence of water vapor in the refractory is a primary component in the development between a precursor and the liquid metal circulating in the channel. if this water vapor can be removed and the dew point thereby reduced, the oxidation potential and, hence, the refractory wetting characteristic of the precursor can be controlled, inhibiting or preventing saturation. In addition, the initial clogging that occurs can also be prevented by eliminating the flood of oxygen made available at the channel surface by the dissociation of water vapor.
Many foundries have available a mechanical vacuum system used with their molding systems, which are capable of reaching about one-half atmosphere of negative pressure. It is a simple procedure to apply this vacuum to the weep holes of an inductor to create a low, negative pressure to eliminate moisture in the new refractory. in certain metallurgical systems, such as pure copper, it also allows much more rapid deoxidation of the alloy. To further reinforce this, the meltout form should be preserved (kept from melting) after charging with liquid metal as long as possible.
In cast iron applications, a low-carbon steel form can be made, which dissolves slowly between 2350-2450F. This becomes an important factor in the startup procedure in order to obtain maximum benefit from the steel channel form and coating. It also allows the negative pressure to purge as much of the vapor from the refractory as possible. In cases where carbon is either an alloying element (as with cast iron) or used widely as a cover (as with copper-base alloys), some carbon control is justified while using a vacuum immediately after startup.
With cast iron, carbon additions should not be made during startup to avoid CO gas from entering the refractory structure and depositing carbon. This is unlikely to pose changes in current procedure since carbon normally would not be added at such low temperature where solution is slow or with the furnace on a low heel.
In the case of copper and its alloys, the carbon cover should be kept away from the area above the new inductor on multiple inductor units and added after final conditioning on single inductor units. Oxygen control can be provided by a combination of a cover (even a temporary one), the use of a lightly deoxidized copper such as DLP and the negative pressure applied to the inductor by the vacuum system.
None of the above procedures will control the problem of metal saturation by themselves, but taken as a combination, significant improvements can be made. Although some of these approaches may be in conflict with process parameters, they can be accommodated without sacrificing efficiency or production. With some alloys, there will be product improvement or at least early achievement of critical specifications, as in the case of oxygen control by use of vacuum. A dew point analyzer is also valuable in monitoring the effect of negative pressure on the case of the inductor. Negative pressure applied to coreless furnaces with cast bottoms is beneficial in reducing leakage to ground on start-up.
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|Title Annotation:||part 2 of 3|
|Author:||Stark, Ronald A.|
|Date:||Jul 1, 1991|
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