What is in this article?:
A sound refractory that has been properly installed, sintered and maintained, plus operator diligence to prevent a dangerous bridge from forming, are key to minimizing metal run-out. (The second of two parts.)
In the first part of this article (see FM&T August 2009), the authors explained that molten metal splash is the most common cause of melt deck injuries — and that it is caused by adding wet materials to the molten bath. They revealed how it can be minimized by diligent inspection and treatment of scrap. Also, they took a critical look at metal run-out, which ranks among the most severe accidents that can occur during melting and holding.
The physics of electrical induction requires the refractory lining between the induction coils and the bath to be as thin as possible. At the same time, it must be thick enough to fully protect the coils and prevent metal run-out during attacks by molten metal, chemical agents, and mechanical shocks.
Metal run-out ranks among the most severe accidents that can occur during melting and holding operations. Runouts occur when molten metal breaks through the furnace lining. If cooling, electrical, hydraulic or control lines become damaged, there is danger of a fire or water/molten metal explosion. Maintaining furnace lining integrity is key to preventing a run-out. Such integrity can be compromised by:
• Installation of the wrong refractory material for a particular application.
• Inadequate or improper installation of refractory material.
• Improper sintering of the refractory material.
• Failure to monitor normal lining wear and allowing the lining to become too thin.
• The sudden or cumulative effects of physical shocks or mechanical stress.
• The sudden or cumulative effects of excessive temperatures or thermal shocks.
• Slag or dross buildup.
Refractory lining material consists of a class of compounds called oxides. Refractory linings used in induction furnaces are commonly made of alumina, silica or magnesia, plus smaller amounts of binding materials.
Choosing the right refractory lining 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 lining is through close consultation with your refractory supplier, who has the most current information on specifications and performance characteristics.
Proper installation of the lining is as important as selection of the right material. If the material is inadequately compacted during installation, voids or areas of low density may form, creating a weak spot easily attacked by molten metal. If the crucible is created with a lining 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 predicted service life.
The refractory manufacturer’s procedures for installation, drying and sintering must be followed. If sufficient time is not allowed for refractory materials to bond, the lining will be more prone to molten metal and slag attack.
Coreless furnaces sometimes use preformed crucibles instead of rammed linings for nonferrous melting. Crucibles can be manufactured with a protective glaze to minimize oxidation of the crucible material and seal any small cracks that develop during routine foundry operation. The protective effects of the glaze last only as long as the coating remains undamaged. Should it become chipped or otherwise compromised during installation or subsequent operations, a small crack will begin to spread. Metal run-out may occur.
Monitoring Lining Wear
In induction furnaces, refractory linings and crucibles are subject to regular wear from the scraping of metal on the furnace walls, largely because of the induction stirring action caused by the furnace’s electromagnetic field. In theory, refractory wear should be uniform, but in practice this never occurs. The most intense wear occurs:
• At the slag/metal interface;
• Where sidewalls join the floor; and,
• At thin spots caused by poor lining installation.
The entire furnace should be visually inspected whenever it is emptied. Special attention should be paid to the high wear areas described above. Observations should be logged.
Although useful, visual inspections are not always possible. Nor can inspection alone reveal all potential wear problems. Some critical wear areas, such as the inductor of a channel furnace, remain covered with molten metal between relinings. Also, low density refractory areas can escape notice. These limitations make lining-wear monitoring programs essential. Follow your refractory manufacturer’s instructions for lining inspection and maintenance.
In situations where visual inspections of coreless furnaces are impossible, for example, when they are used for continuous holding and dispensing, operators should be alert for these warning signs.
• Attainment of maximum power at a lower than normal applied voltage.
• In a fixed-frequency power supply, an increase in the number of capacitors needed to be switched into the circuit to maintain unity power factor.
• In a variable-frequency power supply, running at a higher than normal frequency. Useful though they may be, changes in electrical characteristics must never be thought of or used as a substitute for physical inspection of the lining itself.
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. State-of-theart, automatic lining-wear detection systems that display the lining condition graphically are available also.
Regardless of the instrument a foundry uses to monitor lining wear, it is essential to develop and adhere to a standard procedure. Accurate data recording and plotting will assure maximum furnace utilization between relines while minimizing the risk of using a furnace with a dangerously thin lining.
Physical Shock and Mechanical Stress
The sudden or cumulative effects of physical shocks and mechanical stress can lead to failure of refractory lining. Most refractory materials tend to be brittle and weak in tension. Bulky charge material dropped into an empty furnace can easily cause the lining to crack upon impact. If a crack goes unnoticed, molten metal may penetrate, leading to a run-out with the possibility of a water/molten metal explosion.
Bulky material should be lowered into the furnace. If it must be “dump charged,” be sure there is adequate charge material beneath the charge to cushion its impact. The charge must be properly centered to avoid any contact with the sidewall. Remote controlled automated charging systems are engineered to put charge materials into the furnace without damaging its lining.
Mechanical stress caused by the difference in thermal expansion rates of the charge and refractory material can be avoided by assuring charge material does not become jammed within the furnace. Except for safety reasons, the melt must never be allowed to solidify in the furnace. In the event of a prolonged power failure, a loss of coolant or other prolonged furnace shutdown, the furnace must be emptied.