Reducing furnace downtime is a key element in maintaining adequate productivity in foundry operations. One of the most common maintenance problems encountered in any given foundry is the repair and replacement of the refractory materials used to line the melting furnaces. The linings of aluminum melting furnaces are constantly exposed not only to high temperatures, but also other adverse conditions that may include impact and gouging when aluminum ingots are charged into the furnace.
Another main contributor to the deterioration of aluminum melting furnace linings is the formation of alumina or dross on the surface of the melt which can penetrate a refractory brick lining, thus requiring constant maintenance and repair. In many instances, a furnace may have to be repaired monthly and/or completely relined every one to three years in order to keep it operational.
A two-part refractory system (ThermbondR) has been developed by Stellar Materials, Inc., Delray Beach, FL, within the last several years and is showing promise as a durable, alumina-resistant furnace lining material. Below are two case studies that describe the application of this stable, abrasion-resistant refractory material and the benefits realized by two aluminum melting operations.
CASE STUDY No. 1AIRO DIE CASTING
Airo Die Casting, Inc., Loyalhanna, PA, was founded in 1975 as a producer of aluminum die cast compressor blades for the nuclear industry. The company has grown significantly and is currently serving a variety of specialized industries including telecommunications and transportation. The company's newly designed, 60,000 square foot manufacturing facility provides the ideal environment for a full range of service capabilities including tool design and construction, impregnation, pressure testing, chromating, assembly and powder paint.
Prior to redesigning the manufacturing facility, Airo Die Casting used gas-fired crucible furnaces at the individual die casting machines to produce the melt. Over the last several years, new equipment and methods have been implemented at the casting stations that outstripped the capacity of the local furnaces. By upgrading hydraulics and control systems on the casters, and through the use of reciprocators and automatic ladlers at each casting station, each machine is now capable of producing more parts than the capacity of the old furnaces.
To meet the need for more capacity, the company added a 40,000 pound gas-fired reverbatory furnace from Schaefer Furnaces, Inc., Dayton, Ohio. While finalizing the specifications for the new furnace, Schaefer engineers warned Airo about aluminum oxide that is inevitably generated in furnaces of this type at the surface of the molten metal where it contacts the air. Aluminum oxide has a tendency to penetrate and build up on conventional high-alumina refractories. As the aluminum oxide builds up, it gradually reduces the furnace's capacity and can even deform the walls of the furnace. Under normal circumstances, it is feasible that a furnace would have to be shut down every six months for major repairs costing the company thousands of dollars in materials, labor, and production time until the furnace is back in service.
To combat the problem of aluminum oxide build up, the furnace manufacturer recommended the use of the Thermbond family of engineered refractories instead of conventional refractory materials. This two-part system consists of a dry formulation and a liquid activator (Fig. 1).
These materials are supplied as pre-measured components that are added together to form a unique ionically bonded refractory. The key advantage of this material in aluminum foundry applications is that it is completely and naturally non-wetting to aluminum without the use of additives. This feature allows oxides to be easily removed during nightly cleaning operations without damaging the underlying refractory, ultimately resulting in a longer lining life. Other refractories typically use additives to achieve non-wetting characteristics, which eventually oxidize out of the products causing them to lose their effectiveness.
Another advantage of this Al2O3-resistant refractory is that its high early strength reduces installation time and allows furnaces to return to service faster. The refractory reaches a high compressive strength within an hour of casting and typically cures within a few hours after it is applied with no external heat required. The material bonds extremely well to itself and to other refractories so it can easily be repaired or veneered for extended service. In most applications, this refractory can be put into service immediately after bake-out.
Very soon after the new furnace was installed, the metal tenders noticed a major difference in maintenance requirements. Much less than the expected amount of aluminum oxide had adhered to the walls of the furnace and the oxide was removed with a minimum of effort (Fig. 2). Instead of spending several hours cleaning the furnace, it took less than 15 minutes. The easy removal of the aluminum oxide deposits indicated that the refractory was extremely durable since the main failure mode of the earlier refractory was breaking off along with the oxide.
After a little experience, a routine maintenance schedule was developed. Once per shift, a flux degassing wand is run around the walls of the furnace to soften up the oxide buildup. Then at the end of each shift, a steel "rake" is used scrape the walls. This minor amount of cleaning is sufficient to remove any build up of oxide and maintain the refractory lining at production standards.
For two years, the furnace has run 24 hours per day, six days a week and 12 hours on Sunday with only two shut downs (once related to damaged wiring and another for cold cleaning and inspection). During inspection, it was found there was no significant growth of oxide and no penetration of the Thermbond by alumina. The walls were made smooth again using Formula Patch and to date, the refractory material wall is still in excellent condition (Fig. 3). The walls are nearly as flat and smooth as when the furnace was first installed. Based on current experience, it is estimated that the furnace will provide another 24 months of continuous service before requiring another shut down for cold cleaning and/or replacement.
The cleanliness of the new furnace has also improved product quality. Airo has received no reports from customers concerning the presence of oxide inclusions in any of their products.
CASE STUDY No. 2MAGNODE CORPORATION
Magnode Corporation, Trenton, OH, is an independent aluminum extruder/fabricator. The firm's two extrusion presses enable it to produce extrusions from 0.015 lbs./ft. to 32 lbs./ft. and 1/2" to 17" in diameter. The company's 4500 ton extrusion press can produce extremely large parts while another 2400 ton press can extrude small and intricate aluminum components with extremely tight tolerances. Fabrication services include anodizing, welding, CNC machining, cutting, painting, bending, forming, bead blasting, and assembly.
Magnode's casting operations include a 95,000 pound gas-fired reverberatory melting furnace, a 65,000 pound gas-fired reverberatory holding furnace and a degassing well furnace (Schaefer) arranged in an in-line configuration. The company places a premium on continuous operation because if any one of the firm's furnaces is out of the service, the entire casting operation is down. Magnode uses primary aluminum feedstock, as well as its own high-quality scrap, to produce the billets used for the extrusion operations.
Prior to the installation of the alumina-resistance refractory (Thermbond), Magnode's foundry experienced frequent downtime. One major factor contributing to this downtime was the damage incurred by the conventional ceramic brick refractories while charging the furnace. Aluminum ingots and scrap pigs are often so large that they can gouge and tear the walls of the furnace as they are inserted. The charging hearth of Magnode's melting furnace is eight feet wide, yet much of the scrap generated by the firm is three to four feet long.
Other problems occurred when the furnace was cooled. Hot metal and oxides stuck to the refractory brick so tightly that the refractory itself frequently broke off during the cleaning process. It was necessary to shut down the furnace and perform emergency repairs on the refractory insulation before continuing safe operation.
The ceramic brick refractories also required extensive preventative maintenance. Approximately once per quarter, the furnaces were shut down for three or four days to perform necessary repair work. The cost of materials ranged from $5,000 to $10,000 dollars and labor costs were also high since the job required a crew of four or five persons. In addition, the furnaces had to be completely relined every three years. This required a two-week shut down at a cost of about $25,000 for materials and occupied a crew of five people.
Based on Schaefer's recommendation, Magnode engineers decided to install the new refractory on all three furnaces four years ago. The new material cost only slightly more than the conventional refractory used previously. Installation, including re-moval of the previous refractory, took one week or about half the time required to replace the conventional refractory.
The furnace maintenance staff noticed a difference immediately. The new material did not chip significantly when scrap was charged nor when oxide was scraped from the furnace walls. The buildup of oxide was considerably less than previously encountered and was removed easily during night shift cleaning operations. A mild flux was simply sprayed on the refractory walls and the walls were then scraped to remove excess metal and oxide.
When the normal quarterly preventive maintenance procedures were performed, it was found that the new refractory was in excellent condition. Because the regular damage to the conventional refractory had been eliminated, the furnace maintenance staff decided to the skip that repair step. Subsequently, the next two, scheduled repair shutdowns were also eliminated. One year after installing the refractory, only minor repairs were needed. The necessary repairs cost about half as much and took half the time as the quarterly maintenance procedures performed on the conventional refractory.
Four years after the initial installation, the furnaces have continued to operate without foundry downtime and without any significant maintenance. Annual repairs may now be completed over a weekend during a normally scheduled furnace shut down. The maintenance staff has estimated that the refractory lining will last another two years before requiring re-placement, providing at least twice the life of a conventional refractory.
The engineered refractory material has substantially reduced the cost of in-house foundry operations. The elimination of unscheduled downtime has allowed the foundry to operate its single furnace line continuously for 25 or 26 days per month. Another major benefit is the elimination of the quarterly maintenance, which saves the company considerable labor costs and about $25,000 worth of materials annually. The durability of the new refractory will save a similar amount by eliminating one furnace relining during its life cycle.
For more information contact Stellar Materials, Inc., 100 E. Linton Blvd. 500B, Delray Beach, FL 33483. Phone: 800-388-7555 Fax: 561-330-9355 Website: www.stellarmaterials.com. IH