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11. REFRACTORY LINING Continued Inductive Stirring
In a coreless or channel induction furnace, the metal charge material is melted or heated by current generated by an electromagnetic field. When the metal becomes molten, this field also causes the bath to move in a “figure eight” pattern as illustrated in the drawing shown. This is called inductive stirring. It also serves the important purpose of mixing the metal, producing a more homogeneous alloy. The amount of stirring is determined by the size of the furnace, the power put into the metal, the frequency of the electromagnetic field and the type and amount of metal in the furnace. Inductive stirring circulates high-temperature metal away from the furnace walls, preventing overheating of the refractory surface, but also causes the refractory lining of the furnace to gradually wear away through the action of the moving metal on the furnace walls. This gradual wear requires that furnaces be relined periodically. It is vital that linings be replaced promptly when they wear down to their minimum thickness to prevent failure. ________________________________________________ Melt Automation Technology Helps The Operator To Prevent Accidental Superheating & Lining Damage
Screen from Meltminder® 200 melt control computer. Modern induction melting systems are often high powered and melt the charge very rapidly. This has spurred the development of computerized melting systems designed to provide the furnace operator with the ability to precisely control the melting process and reduce the risk of accidental superheating. Some of these systems operate on special computers, some are PC based and some are built into the melting equipment itself. Induction melting operations lend themselves to computerized control. A typical system takes the weight of the furnace charge, either from load cells or as entered by the operator; the desired pouring temperature; and then calculates the kilowatt hours needed to complete the melt. It then turns off the system or drops to holding power when the melt is complete. Thermocouple readings can be transmitted to the computer to further enhance accuracy. This precise melting control optimizes power usage by minimizing temperature overshoot, saves time by reducing frequent temperature checks and enhances safety by reducing the chance of accidental superheating of the bath which can cause excessive erosion, lining failure and the possibility of a furnace explosion. ________________________________________________ Electrical Monitoring Of Lining Wear
This computer control system calculates coil inductance to check average lining wear in a coreless furnace. A limited amount of information about the condition of the refractory material can be ascertained from changes in the furnace’s electrical characteristics. An important limitation of these measurements is that they reveal average conditions. Electrical measurement will not isolate a localized problem, such as a gouge or a void beneath the lining surface. One situation in which electrical measurements are very useful is in estimating wear in the induction loop of channel furnaces. Molten metal is always present in the furnace, making visual inspections between shut-downs impossible. This technique provides absolutely no information about the condition of the lining in the main bath. The main bath or upper case refractory can be subject to chemical attack at the slag line. The slag line can be at any level in the furnace depending on how it is operated. The lining must be checked visually and also the outside shell temperature must be checked. If the refractory lining is thin, this will show as a hot spot on the shell. Once detected, the furnace lining must be carefully inspected. If the lining is severely eroded, the furnace must be removed from service immediately. Normal shell temperatures may be as high as 500°F. If the shell temperature is above 500°F or if localized hot spots are more than 100°F above adjacent areas, the lining must be carefully inspected to determine why. Similar electrical measurements can be made of coreless furnace linings but, as noted, these measurements reveal average conditions. They will not disclose a localized problem so total reliance on this is not practical. Coreless furnaces are emptied with sufficient frequency to permit visual inspection and physical measurements, which are always more accurate. ________________________________________________ Pouring Cradle Provides Bottom Support For Crucible
Lifting a bilge type crucible with a conventional two-man ring shank provides no support to the bottom of the crucible. Should a crack in the crucible occur below the bottom ring support, the bottom of the crucible can drop and molten metal will spill and splash, possibly causing serious injury or death. To reduce this danger, a pouring cradle that provides bottom support for the crucible must be used. This not only reduces the chance of a crack allowing the bottom to drop from the crucible, it also increases crucible life by reducing stress on the sidewall. A crucible must never be subjected to physical damage or thermal shock and must never hold metal exceeding the crucible’s maximum rated temperature. It is important to check a crucible’s condition every time it is used and to replace it if it is worn, cracked or damaged. ________________________________________________ Push-Out Systems Minimize Refractory Dust During Lining Removal
Before automated lining removal systems were developed, removing a furnace lining was a labor-intensive, time-consuming process. Today, however, coreless induction furnaces equipped with lining push-out systems speed the lining removal process, lessen the risk of damage to the coil and reduce worker exposure to refractory dust. These systems can be supplied with new furnaces or retrofitted to existing furnaces. They consist of a hydraulic ram and a pusher-block in the bottom of the furnace. These work together to remove the bottom and side refractory material.
Make sure no one is standing directly in front of the furnace where they could be hit by lining material as it is pushed out. ________________________________________________ IMPORTANT: |
