Extend refractory lining life with coreless furnace patching.
Most melt managers have the same problems today that they had 10-20 years ago - they must keep their molding lines supplied with quality metal while working to improve kilowatt-hour per ton melted and maximize the lining life vs. tons melted. In addition, managers often must be prepared to melt and pour 6 days/week - this includes having furnaces ready for production on Sunday night or Monday morning.
Based on proven experience in induction melt shops, this article covers iron melting issues commonly associated with the refractory life of a coreless induction furnace and the patching programs that have been implemented to increase productivity. While the procedures are not unique, they are successful when the necessary installation procedures and safety precautions are followed.
Workers in an induction-melting foundry were faced with a furnace lining problem. Lining campaigns at the foundry's three melting furnaces were lasting only 2 weeks, for a throughput of just 400 tons of metal even though there had been campaigns in the past of up to 9 weeks.
In order to understand the problem, it's important to understand melting department setup at the foundry. The operation employs two mainline and one medium-frequency coreless furnaces. The main line furnaces utilize a common power supply, and while they are capable of operating at 2500 kW, 1500 volts and 1050 amps, the average for the operation is 2400kW, 1450 volts and 950 amps. The main lines have a tap charger with a maximum of 10 taps, but workers mostly operate on tap nine. In addition, there are 31 capacitors that can be added manually or automatically. The medium-frequency furnace operates at 8000 kW (with a redline at 7000), 400 frequency (with a redline at 300) and 3000 volts.
A lining life of 2 weeks was unacceptable for the melt department, especially after the crew learned that the foundry was about to begin pouring 6 days/week. Realizing that something had to be done, workers studied why the linings would not last longer than 2 weeks or melt additional tonnage. It was determined that the quality of the foundry's carbon raiser was having a chemical reaction with the lining, causing it to deteriorate prematurely.
Even though the foundry eventually changed its carbon raiser, the major goal of the department was to further extend the life of the lining - instead of just solving the problem, they wanted to better the operation. Realizing that the toughest part of replacing the lining is to take out the material at the bottom of the furnace, which can be 12-14 in. thick, the melt staff, which knew of techniques to extend lining campaigns such as bottom patching, shaving and top patching, chose to try two procedures: a full patch and a top sleeve combined with a bottom patch. They had employed patching successfully in the past and believed it was the best solution to this problem.
By changing the carbon raiser and patching the lining, the foundry was able to boost melt efficiency to beyond what was experienced in the past, reducing man-hours spent in changing the lining and increasing tons melted per lining campaign.
Employees determine if a lining requires a full patch, a top sleeve and bottom patch, or a reline based on furnace power readings taken during the week and on weekends. Typically, the furnaces are operated at a line power factor of "one," the center point. In addition, a thicker lining will require more taps, or surges of energy, to melt.
On a typical Saturday, the furnace lining is measured for wear using a centering bar with a plumb bob attached to a string. The important thing, when measuring, is to look at how thin the lining has become. As standard procedure, foundries should determine when to patch based on degree of thickness and past furnace performance at that thickness. In this case, the maintenance staff determined the center of the furnace during coil installation and compared the original dimensions to the wear measurements. If there is less than 1 in. of lining loss, then a full refractory lining patch will be installed. If the lining wear is less than 2 in., a top sleeve and bottom refractory patch is installed, but if the wear is greater than 2 in., a new lining will be installed.
The furnaces are normally-drained on Friday and force-cooled. Operators use a garden hose to force-cool, aiming the flow of water at the side of the furnace, starting at the top of the furnace and, in a circular motion, proceeding downward until the water flow is at the furnace bottom. Throwing water on a hot lining can be dangerous - operators must remain cautious when adding water to the furnace. After watering, the furnace is ready for a blower or cooling fan.
The next morning, furnace measurements are taken and a small pneumatic chipping hammer is used to remove slag and rough the surface of the original lining. The goal of the chipping is to develop a rough surface that will allow the patch to mesh with the original lining without cracking the "hard face" of the lining.
After chipping and cleaning the old refractory material and slag, the area to be repaired is washed with a high-alumina plaster. Enough plaster is added to water to form a thick slurry compound - this serves as a bonding agent between the silica and the plastic. An 85% medium-grain aluminum plastic then is applied to the prepared surface.
A pneumatic rammer, a small trowel and a wooden mallet are used to apply the plastic, starting at the bottom of the furnace and continuing until the entire furnace has been patched. The plastic is installed by overlapping sections to prevent seams that could act as paths for the molten metal to penetrate the silica.
After ramming, the same plaster mixture is used to wash the entire furnace to seal any cracks. A standard method for measuring the patch should prevent over-patching that could result in insufficient melting on start up.
Finally, vents are scratched in the plastic to promote drying, and a torch is placed in the furnace to continue the dry out. The furnace is allowed to dry for a minimum of 8 hr. Without proper furnace drying, molten metal will "push" the moisture toward the coil, and the coreless furnace will detect a ground leak.
Top Sleeve and Bottom Patch
In the installation of a top sleeve and bottom patch, the worn area of the upper portion of the furnace is removed. When the operator took the original measurements, the amount of wear also was documented for the top and bottom walls of the furnace. With this information, it is determined how much top area must be removed.
When removing the top area in preparation for a sleeve, the operator determines how long a sleeve is needed to replace the worn section. A form that is 0.5 in. smaller in diameter than the original form is used so that the cut form can slide inside the original lining.
The maintenance crew cuts the form based on these measurements, and "wings" are welded onto the form to hold it in place during silica installation. After the form is in place, some high-aluminum plastic is placed at the bottom of the form and on the side walls of the furnace to prevent the new silica from falling out before and during vibration.
The same procedure used to install a new lining is used on the sleeve installation. The difference is the method for vibration and the importance of forking or deairing. Because the vibrator used is a paddle-type unit, forking becomes important. When the lining material is added to the furnace, air may become trapped, so a pronged ramming tool, or fork, is used to allow air to escape. Besides deairing, forking allows the silica to pack and mesh with the original lining.
Similar to full patching, after the top sleeve and the bottom patch are installed, a torch is placed in the furnace to induce drying.
During subsequent lining evaluations, workers noticed no additional wear in the original lining, which actually became a back up to the plastic used in bottom patching. There was some concern about some small "bbs" of iron that had formed on the original lining material, but when the patch and the lining were removed, there was no evidence of metal penetration past the original lining.
These procedures allowed the foundry to change its reline frequency from every 2 weeks to every 12-14 weeks, thus resulting in increased tonnage per lining campaign. Tonnage in the patched furnaces increased from 400 per lining campaign to 1200-1300 tons.
The other benefit of the procedure was reduced man-hours because the procedure did not take as long as the installation of new lining. Now, two workers spend 4 hr patching the furnace lining - a significant reduction from the six workers and 20 hr it used to take for a full lining installation.
Before attempting these procedures, foundrymen should review the following precautions:
* Examine the cost-effectiveness for your plant. There may be added costs for silica and plastic, but compare these costs against labor and furnace start-up savings;
* Determine how thin you want your lining to be after installation. For example, the original lining may be 4.5 in. thick, and you may decide to patch as long as there are 2.75 in. remaining. This is practical because the plastic becomes a working lining, and the silica becomes a back-up material; the plastic patch does not allow for severe metal penetration;
* Too much water when force cooling or patching could cause start-up problems;
* The linings must be measured carefully, with the operator making sure there is enough of the original lining remaining as back-up material;
* In the case of a Sunday start-up, the normal cold start (or anything other than the normal sinter used on a new lining) should be employed. While new silica material would have to develop a new hard face, the patched lining only requires slow heating to develop its expansion characteristics before it is ready for melting;
* The entire patching procedure should be documented, including everything that happens before, during and after the patch is installed. If a foundry has a lining failure or casting defects related to lining problems, it will be easier to adjust the procedure;
* The procedure should provide 810 hr to drain the furnaces and have them ready for the following week's production;
* Without force-cooling, it will be too hot for a repair crew to enter the furnace;
* Removal of the 85% aluminum plastic patch is going to take longer than removing the silica lining because the plastic is a tougher material;
* The floor of the furnace should not be patched because the ground wires would be covered. This is a potentially dangerous situation, because the furnace will not be able to detect a leak into the molten metal.
These procedures have been used successfully on furnaces that have a pushout system, but each foundry has to decide the cost-effectiveness of patching when balanced against the easier removal process. When used with a pushout system, extra cooling time should be given the furnace before attempting to "push" linings with a full patch.
This article was adapted from a presentation at the 1999 AFS Induction Melting, Holding and Pouring of Iron Conference. Proceedings are available from AFS Publications at 800/537-4237.
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|Comment:||Extend refractory lining life with coreless furnace patching.|
|Author:||Benion, Ernest L., Sr.|
|Date:||Jul 1, 1999|
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