Rethinking Repair & Relining Procedures
Through detailed evaluation and analysis of its operations, one iron casting facility resolved issues related to its coreless induction furnaces.
Robert Wandel, Motor Castings Co., Milwaukee, and Ernest Benion Sr., Consultant, Milwaukee
(Click here to see the story as it appears in the August issue of Modern Casting.)
For most of its 95 years in operation, Motor Castings Co., Milwaukee, relied on cupolas in its melting operation. In the last 20 years, though, the job shop transitioned to two 15-ton, medium frequency, coreless induction furnaces to melt and pour compacted graphite, ductile and gray iron.
Still, over the years, Motor Castings’ in-house methodology for furnace maintenance remained based on repairing cupolas and not specifically tailored to coreless induction furnaces. A contractor was used to facilitate efficient bricking, relining and repairs. Although Motor Castings assumed the contractor used proper relining techniques, the question remained: “How do we know we are getting maximum life out of the linings?”
The answer had been either, “This is how we’ve always done it,” or “This is what I was trained to do.” Motor Castings realized it lacked the internal knowledge necessary for coreless furnace repairs, which led to its pursuit of a culture change. This article looks at the case study to see how the Wisconsin metalcaster arrived at its solutions.
Three Process Improvements
Historically, Motor Castings’ melt department drained the furnaces at the end of each operating day and inserted 10,000 lbs. of cold charges (pig iron and steel scrap) for the following day’s production. The furnaces normally are empty by 5 p.m., put into a low-kilowatt hold cycle and switched to full power between 8 and 10 p.m. A computerized system puts the furnace on hold at a temperature of 1,600F (871C). The melt department supports a green sand molding line and two chemically bonded lines, one with flasks and one without. The iron is transferred in pouring trucks to barrel-style and truck ladles.
Over time, the schedule changed to the point where the furnaces did not sit for long periods, which reduced the cooling time for the refractory. The company then switched to a low-kilowatt hold power that never went above 300 kW. The furnaces operated with demand controllers that helped prevent full melting power during specified hours. Motor Castings secured a special production rider from its electric supplier that allowed full melting from 10 a.m. to 12 p.m. and from 8 to 10 p.m.
In 2007, Motor Castings had its furnaces on an automatic, eight-week reline schedule with weekly repairs, but company officials wanted to extend the lining’s life. The melt department began with daily visual inspections of the linings after the furnaces were drained to look for excessive wear or premature refractory loss. During this period, no obvious issues were discovered. Motor Castings personnel also took a closer look at the linings during weekend repairs to determine what, if anything, could be improved. After several weeks, the company developed a list of four issues:
1. Hand-rammed spouts were knocked out on every reline.
2. Furnace linings needed to be accurately measured.
3. Furnace bottoms were wearing out prematurely.
4. Iron was getting behind the furnace lining.
Motor Castings’ melt department first changed its procedure for furnace measurements. Previously, only the bottoms of the furnaces were measured to determine wear, and measurements of the side walls were not taken. Employees, who had been filling out the correlating form incorrectly, were required to submit a fully completed form.
Unfortunately, during the coil installation, no center point of the inside of the furnace relative to the coil was established. To find a baseline, Motor Castings’ management directed operators to measure the lining on a new installation and used that to help determine wear. Realizing employees needed to be retrained on the proper measurements procedures, melt department managers demonstrated the necessity of removing slag and any foreign objects from the sidewalls before taking the measurements. Employees measured in four quadrants, from top to bottom, using 12, 3, 6 and 9 o’clock positions. The depth also was measured. Motor Castings was concerned the furnaces may have been superheated, although they operated with a computerized system to prevent it. The measurements showed the furnace bottoms required relining before the sidewalls experienced significant wear. The company contacted its refractory supplier for recommendations to improve lining life.
With help from Sharp Refractories Inc., Oak Creek, Wis., the Motor Castings team determined the mechanical deterioration of the furnace bottoms resulted from the daily drainings and the impact of the cold charges. Sharp’s Greg Payleitner recommended a different bottom material to better handle cold charges, which eliminated the need for premature relines.
Two Furnaces, One Big Problem
The final challenge for Motor Castings also was the most significant. Every time employees knocked out either furnace, a significant amount of metal was stuck to the brick behind the lining (Fig. 2). Management began reviewing the weekend repair work, because penetration was in the general area of the weekend repairs. Employees used a chipping hammer to remove the slag from refractory, which was concerning because the refractory material also could have been chipped away. In addition, cracks may have been created that would not seal during the sinter cycle.
To remedy the problem, melt department employees began using scalers instead of chipping hammers to remove the slag (Figs. 3-4). Employees were instructed to chip downward on the slag and not inward toward the coil. After these changes, however, the penetration remained present.
Motor Castings solicited technical support from Sharp Refractories representatives and other individuals familiar with similar lining issues. Several suggestions came from those discussions:
1. Conduct daily spout maintenance.
2. Look into different material as a top cap. (A low sintering alumina material was used because of the difficulty in getting sufficient heat to the top of the linings during sintering.)
3. Change top cap procedures.
4. Review sinter cycle.
The Motor Castings team believed the spouts were maintained appropriately because the furnaces were drained daily so the spout could be repaired. However, they were open to other material options. The Motor Castings team adjusted the sinter cycle to ensure a high level of molten iron and instructed operators to turn off dust collection during sintering to generate as much heat as possible around the top of the furnace to ensure proper sintering. Motor Castings also experimented with different top cap material, including silica, alumina and aluminum castable. Although a number of different materials were tried, the melt department determined this was not the source of the problem.
The team continued to monitor the operation and stayed in contact with suppliers. However, it could not seem to resolve the problem of metal penetration. The melt department would go for weeks and see no penetration, but it then would reappear.
Frustrated after several years of trying different things, the Motor Casting team again met with Sharp Refractories. During the changes to the installations and weekend repair practices, Motor Castings went from relying on its installer for repairs, bricking, grouting and lining installation to doing such operations itself. The company, however, needed help to resolve the problem and asked the installer to take a second look.
Sharp Refractories’ Payleitner first reviewed how Motor Castings handled weekend repairs. One weekend, after cleaning slag from around the furnace, the installer’s crew began removing lining material underneath the spout, assuming it was the root of the problem. (See Fig. 5.)
As with previous cases, metal was found behind the lining. The supplier determined metal penetration occurred at the seam between the spout and the lining. With Payleitner’s help, the company began changing procedures for installing new linings and weekend repairs. The Motor Castings team tried to seal between the lining and the spout with refractory paper or a brick in that location during lining installation. It added more silica in the spout and added mica paper between the brick and the lining material to prevent metal penetration. Still, these repairs led to similarly inconsistent metal penetration.
Finally, during weekend repairs, observers took a closer look at the lining and spout separation. A technical expert determined that a monolithic plane needed to be established from the tip of the spout into the furnace. The company wanted to eliminate the seam’s visibility between the spout and lining. After the first tap of a new lining, the furnace operator would ram a layer of plastic material from the tip of the spout to the inside of the furnace to eliminate the seam as an avenue for metal penetration.
This method offered some improvement but was difficult because of molten metal in the furnace. Then, on the daily drain off, the operator did a better job of ramming in the plastic that was covering the area. This appeared to solve the problem. Apparently, as long as the operators properly rammed the plastic, penetration could be prevented.
In addition to what had been learned, the company experienced several benefits that were direct results of the investigation:
- Elimination of the dangers of metal penetration behind the linings with the risk of penetrating to the coil (a major safety issue).
- A 15% increase in melted tonnage per lining campaign.
- Improved refractory material.
- Improved furnace repair practices.
- Employee education on proper furnace repairs.
The resolution to Motor Castings’ problems could not have been accomplished without the help of experts outside of the organization. This situation also highlighted the importance of documentation and evaluation of the information gathered during the process. The analytical skills of the employees improved because they were heavily involved in the entire process. The answers to the problems were not new or something unknown to the industry. However, this case study reinforces the need for continual training and development of internal staff.
This article is based on paper 14-025 presented at the 2014 AFS Metalcasting Congress.