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Maintaining Your Refractory Lining
Proper and well-maintained refractory linings are important for the safe operation of all metal melting furnaces. In induction furnaces, they are absolutely critical. The physics of electrical induction demand that the refractory lining between the induction coils and the bath be as thin as possible. At the same time it must be thick enough to fully protect the coils and prevent metal run-out in the face of attacks by molten metal, chemical agents and mechanical shocks. Assuring that the furnace lining remains within manufacturer-specified limits requires careful treatment of the lining during all furnace operations as well as comprehensive inspection and monitoring procedures. Without question, metal run-out ranks among the most severe accidents that can occur during melting and holding operations. Run-outs occur when molten metal breaks through the furnace lining. If cooling, electrical, hydraulic or control lines become damaged, there is an imminent danger of a fire or water/molten metal explosion. The integrity of the furnace lining can be compromised by: Any of these situations can lead to a metal run-out from an induction furnace. Therefore, careful attention to a furnace’s lining is absolutely critical to safe melting and holding. ________________________________________________ Choosing the Right Refractory Refractory material consists of several chemical compounds. The bulk of any lining material consists of a class of compounds called oxides. Refractory linings used in induction furnaces are commonly made of alumina, silica, magnesia, or zirconia plus smaller amounts of binding materials. Choosing the right refractory material for your specific melting or holding application is crucial. You must take into account the specific metal you will be melting, the temperatures you will be reaching, the length of your melt, how long you will be holding metal in the furnace, how much inductive stirring will take place, what additives or alloying agents you will be using and your furnace relining practices. The best way to select the right refractory is through close consultation with your refractory vendor. They will have the most current information on the specifications and performance characteristics of traditional and new refractory material. Your refractory vendor should be your source for instructions and training for the installation and sintering of the chosen refractory material. ________________________________________________ Proper Installation of a Furnace Lining
Proper installation is as important to the safe operation of the furnaces as the selection of the right material for your application. If the refractory material is inadequately compacted during installation, voids or areas of low density may create weak spots easily attacked by the molten metal. If the crucible is created with a form that is improperly centered, or one that has been somehow distorted during storage or shipment, lining thickness will be uneven. As a result, the lining may fail before the end of its expected service life.
It is especially critical that the refractory manufacturer’s procedures for drying and sintering be followed and never hurried. If sufficient time is not allowed for the refractory materials to bond, the lining will be more prone to attack by molten metal and slag. The sintering schedule must not be interrupted for any reason once it has begun.
Coreless furnaces sometime use preformed crucibles for nonferrous melting in place of rammed linings. One advantage of crucibles is that they can be manufactured with a protective glaze. In addition to minimizing oxidation of the refractory material, the glaze can seal-over any small cracks that develop during routine foundry operation. The protective effect of the glaze lasts only so long as the coating remains undamaged. Should it become chipped or otherwise compromised during installation or subsequent operations, a small crack will, rather than “self-heal,” begin to spread. Metal run-out may occur. ________________________________________________ Automated Sintering Control Systems
Computerized control of melting operations represents the most technologically advanced form of melt shop automation. The most advanced foundry melting automation systems provide fully programmable control of sintering, the ability to schedule and control furnace cold-start procedures and computerized control of the melting process. With furnace thermocouple feedback, computerized control of sintering can be more accurate and reliable than manual control. Automated control systems are designed to assist a fully-trained and experienced operator in running the furnace and power supply. They are not a substitute for the direct, careful and continuous attention that an operator must give to the furnace and power supply whenever they are operating. ________________________________________________ Monitoring Normal Lining Wear Induction furnace refractory linings and crucibles are subject to normal wear as a result of the scraping action of metal on the furnace walls. This is largely due to the inductive stirring action caused by the induction furnace’s electromagnetic field. In theory, refractory wear should be uniform; in practice this never occurs. The most intense wear occurs: The emptied furnace must be visually inspected. Special attention must be paid to high-wear areas described above. Observations must be accurately logged. Although useful, visual inspections are not always possible. Nor can visual inspection alone reveal all potential wear problems. Some critical wear areas, such as the inductor molten metal loop of a channel furnace or pressure pour, remain covered with molten metal between relinings. The presence of a low density refractory area can likewise escape notice during visual inspections. These limitations make lining-wear monitoring programs essential. Directly measuring the interior diameter of the furnace provides excellent information about the condition of the lining. A base-line plot must be made after each relining. Subsequent measurements will show a precise rate of lining wear or slag buildup. Determining the rate at which the refractory material erodes will make it possible to schedule relining before the refractory material becomes dangerously worn. Calipers are insufficient for this purpose and must not be used. Measurements must only be made using an accurately-positioned center post equipped with a radial measuring arm. Some warning signs of lining wear are: Useful though they may be, changes in electrical characteristics must never be used as a substitute for physical measurement and observation of the lining itself. A state-of-the-art automatic lining wear detection system which graphically displays the lining condition (i.e. Saveway or equivalent) can be used. Two commercially available instruments can be used to provide localized temperature readings. A magnetic contact thermometer attached to the steel shell of a channel furnace will indicate lining wear by revealing the position of a hot spot. Infrared thermometers make it possible to remotely measure temperature by looking at a furnace through the eyepiece of a device resembling a hand-held video camera. Regardless of the instrument used to monitor lining wear, it is essential to develop and adhere to a standard procedure. Consult your refractory vendor for information and training for how to monitor the condition of your lining ________________________________________________ Physical Shock & Mechanical Stress The sudden or cumulative effects of physical shocks and mechanical stress can lead to a failure of refractory material. Most refractory materials tend to be relatively brittle and very weak in tension. Bulky charge materials must always be lowered into the furnace. If it must be “dump charged,” be sure there is adequate material beneath the charge to cushion its impact. The charge must also be properly centered to avoid damaging contact with the sidewall. Mechanical stress caused by the different thermal expansion rates of the charge and refractory material can be avoided by assuring metal does not become jammed within the furnace. Except when it is done for safety reasons, dealing with a bridge for example, the melt must never be allowed to solidify in the furnace. In the event of a prolonged failure, a loss of coolant event, or other prolonged furnace shutdown, the furnace must be emptied. ________________________________________________ Excessive Temperatures & Thermal Shock Refractory manufacturers take furnace temperature extremes into account in formulating their products. For this reason it is important that refractory materials be used only in applications that match a product’s specified temperature ranges. Should actual furnace conditions heat or cool the lining beyond its specified range, the resulting thermal shock can damage the integrity of the lining. Cracking and spalling can be early warning signs of thermal shock and a potentially serious metal run-out. Thermal shock can also be caused by excessive heating or improper cooling. The best way to avoid overheating is to monitor the bath and take a temperature reading when the charge liquefies. Excessive overheating of the bath must be avoided. Careful monitoring is essential. Temperatures exceeding the refractory’s rating can soften its surface and cause rapid erosion, leading to catastrophic failure. The high heating rates of medium frequency coreless furnaces enable them to quickly overheat. Channel-type holding furnaces have low heating rates and thicker linings in the upper case. However, temperature control is also necessary because the inductor linings tend to be thinner. In all types of induction furnaces, kilowatt-hour counters, timing devices and computerized control systems can help prevent accidental overheating. When working with a cold holding furnace, be sure it is properly preheated to the refractory manufacturer’s specifications before filling it with molten metal. In the case of melting cold charge material, slowing the rate of the initial heat up until the lining is completely expanded will minimize the risk of thermal shock to a cold furnace. The gradual heating of the charge allows cracks in the refractory to seal over before molten metal can penetrate. When cooling a furnace following a melt campaign, follow the refractory manufacturer’s recommendations. Thermal damage to the refractory can also result from overfilling a coreless furnace. If the level of molten metal in the furnace is higher than the top cooling turn on the coil, the refractory material in the top of the furnace is not being cooled and is exposed to thermal stresses that could lead to its failure. Overfilling may also cause metal penetration between the working refractory lining and the top cap refractory material. Either situation may lead to metal run-out and possibly a water/molten metal explosion. Serious injury or death could result. ________________________________________________ Managing Slag or Dross
A foundry worker slags a furnace equipped with a backslagging mechanism designed to facilitate slag removal. Slag or dross is an unavoidable by-product of melting metal. Slag forms when rust, dirt and sand from the charge and refractory material erode from the furnace lining rise to the top of the bath. Dross is created when oxides form during the melting of nonferrous metals such as aluminum, zinc, Galvalume, etc. Chemical reactions between the slag or dross and the lining increase the rate at which the lining erodes. A highly abrasive material, slag or dross will erode refractory material near the top of the molten metal. It is not uncommon for this part of the furnace to be patched between scheduled relinings. In extreme circumstances, this erosion may expose the induction coils, creating the risk of a water/molten metal explosion. Careful monitoring of the refractory thickness is necessary so it can be patched or replaced before the coils are exposed. ________________________________________________ Slag Removal Automation The manual process of removing slag from very large furnaces is a time consuming and labor intensive operation. It also exposes furnace operators to high levels of radiant heat and physical exertion. Where overhead clearances permit, slag removal can be accomplished using a clamshell type slag scoop operated from an overhead or jib crane.
Clamshell type slag scoop in the closed position removing slag. ________________________________________________ IMPORTANT: |
