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.
|Fins of molten metal can penetrate worn or damaged refractory and come into contact with the coil. |
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.
|An explosion occurs when moisture contacts molten metal. |
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.
Excessive Temperatures and Thermal Shock
Refractory manufacturers take furnace temperature extremes into account in formulating their products. Therefore, it is important to use refractory materials that match the applications’ temperature ranges. If furnace operating conditions heat or cool the lining beyond its specified range, the resulting thermal shock can damage the integrity of the lining. Cracking and spalling are early warning signs of excessive thermal shock and a potentially serious metal run-out.
Thermal shock also can be caused by excessive heating or improper cooling. To avoid overheating, monitor the molten bath temperature by taking temperature readings when the charge liquefies. Avoid excessive superheating of the molten bath. Temperatures exceeding the refractory’s rating can soften its surface and cause rapid erosion, leading to catastrophic failure. The high heating rates of medium-frequency coreless furnaces enable them to quickly overheat. Channel-type holding furnaces have low heating rates and thicker linings in the upper case.
However, temperature control is necessary also because the inductor linings tend to be thinner. In all types of induction furnaces, kilowatt-hour counters and computerized monitoring and control systems can help to prevent accidental overheating.
When working with a cold holding furnace, be sure it is properly preheated to the refractory manufacturer’s specifications before filling it with molten metal. In the case of melting cold charge material, slowing the rate of the initial heat-up will minimize the risk of thermal shock to a cold furnace. The gradual heating of the charge allows cracks in the refractory to seal before molten metal begins to form. When cooling a furnace following a melt campaign, follow the refractory manufacturer’s recommendations.
Managing Slag or Dross
Slag and dross are unavoidable byproducts of melting metal. Slag forms when rust, dirt and sand from the charge material and refractory material, eroded from the furnace lining, separate from the melt and rise to the top of the bath. Dross is created when oxides form during the melting of nonferrous metals such as aluminum. Chemical reactions between the slag or dross and the melt increase the rate of lining erosion.
Highly abrasive materials, slag and dross erode lining near the level of the molten metal. It is not uncommon for this part of the furnace, above the molten metal line, to be patched between scheduled relining. In extreme circumstances, this erosion may expose the induction coil, creating the risk of a water/molten metal explosion. Refractory linings in this condition must be removed from service immediately.
Although unavoidable, the effects of slag attack can be minimized by limiting the amount of rusty scrap in the charge, shot blasting foundry returns, and avoiding excessively high temperatures. Dross formation can be controlled through careful regulation of stirring, metal level and temperature.
Ground and Leak Detection Systems
The ground detector is a primary safety device. Never operate the unit if the ground detection system is not fully operational. Many factors (lining condition, etc.) influence the operation and speed of operation of the ground leak detector. If a leak is suspected at any time, cease operation, clear the melt deck area of all personnel and empty the furnace.
The ground and leak detection system for use with most coreless induction furnaces and power supply units is crucial to safe melting and holding operations. The system, which includes both a ground detector module associated with the power supply and a ground leak probe, located in the furnace (except in removable crucible furnaces), is designed to provide important protection against electrical shock and warning of metal-to-coil penetration, a highly dangerous condition that could lead to a furnace eruption or explosion.
Key to this protection in furnaces with rammed linings or conductive crucibles is the ground leak probe in the bottom of the furnace. This probe is composed of an electrical ground connected to several wires extending through the refractory and in contact with the molten bath or conductive crucible. This system serves to electrically ground the molten bath.
In some small furnaces with nonconductive, nonremovable crucibles, where the bath cannot be practically grounded, the ground leak probe takes the form of a wire cage located between the crucible and coil. This wire cage serves to ground the bath if metal penetrates through the crucible.
Both of these probe configurations are designed to provide shock protection to melt deck workers by helping to ensure that there is no voltage potential in the molten bath. If molten metal were to touch the coil, the ground leak probe would conduct current from the coil to ground. This would be detected by the ground detector module and the power would be shut off to stop any coil arcing. This also would prevent high voltage from being carried by the molten metal or furnace charge. Such high voltage could cause serious or even fatal electrical shock if the operator were to come into conductive contact with the bath.
Coil cooling sections in the top and bottom of a steel shell furnace serve to maintain uniform refractory temperatures throughout the furnace to maximize lining life. In Inductotherm steel shell furnaces, these coils are electrically isolated from the active coil, principally to isolate the active coil from ground leakage to the top and bottom of the furnace. If a fin of metal reaches the cooling coil, the metal simply freezes, causing no damage to the coils. The ground and leak detection system also senses metal penetration to the cooling sections while maintaining AC isolation of these cooling sections from the active coil. The low DC voltage imposed on the cooling coils will only generate extremely low current, up to 150 milliamperes, when a ground fault is detected. Unlike systems that directly connect their cooling coil sections to the active coil to provide ground fault protection, this low current poses no risk to the coil. It avoids the danger of a high fault current melting a hole in the tubing used for the cooling coils.
|Inductotherm’s ARMS® (Automated Robotic Melt Shop) System enhances operators’ safety by handling temperature measurement, metal sampling, slagging, and other tasks on the melt deck. |
Your coreless furnace must not be operated without a fully functional ground and leak detection system, including a solidly grounded ground leak probe. The ground leak probe may not be required in removable crucible furnaces and heating applications. As a normal safety precaution, power to the furnace must always be turned off during slagging, sampling and temperature measuring.
The ground leak probes work in conjunction with the ground detector module mounted inside or external to the power supply. The electrical circuitry in the ground detector module continually monitors the electrical integrity of the systems. This module turns off power to the furnace if any improper ground is detected in the power supply, bus or induction coil. This is crucial to furnace safety. If the furnace refractory lining or crucible cracks or otherwise fails and a portion of the metal bath touches the energized furnace coil, the coil could arc and rupture. This could allow water to get into the bath, causing metal eruption or explosion. Both parts of the system, the ground leak probe and the ground detector, must function properly for safe melting operations.
To keep the ground leak probes working properly in a rammed lining furnace, care must be taken when installing the lining to ensure that the ground leak probe wires come into contact with the lining form. It is essential that the ground leak probe wires remain exposed, permitting contact with the furnace charge. If the wires are too short, extra lengths of 304 stainless steel wire must be welded to the existing wires to extend the wires into the charge material or into contact with a conductive crucible.
It is important to check your furnace’s ground leak probes daily, especially in rammed lining furnaces and furnaces with conductive crucibles. The probes can be covered during improper furnace relining, can burn off, can be isolated by slag or otherwise be prevented from providing a solid electrical ground. This check can be done with a ground leak probe testing device for verifying your furnace’s ground connection. Failure to ensure that the ground leak probe wires are providing a solid ground will result in the loss of protection for the operator and furnace provided by the ground and leak detector system.
Your melting system’s ground detector circuit also must be checked daily. In a typical system, this is done by pressing the test button on the detector, which briefly simulates an actual ground fault. Refer to your powersupply user’s manual for proper usage of the test button. Because of the crucial safety functions ground and leak detection systems have in coreless induction melting and holding furnaces, your furnace must not be operated without a fully functional ground and leak detection system.
In case of a ground fault trip, the melt deck around the furnace must be cleared of all personnel immediately. This is to reduce the risk of injury to personnel should there be an eruption of molten metal. After a reasonable period of time, if there are no indications of an imminent eruption (i.e., rumbling sounds, vibrations), qualified and trained maintenance personnel wearing the appropriate PPE for the assessed hazard may cautiously proceed to troubleshoot the cause of the ground fault trip. Furnace capacity must be taken into consideration when determining what is a reasonable period of time. If in doubt, keep all personnel safely away from the furnace until the metal charge has solidified.
An independent molten-metal leak detector can be used in certain applications to detect the presence of molten metal close to the coil. The system includes a series of mesh panels that are placed on the coil grout covering the inner diameter (I.D.) of the furnace coil. A similar method is used to extend molten-metal leak detection to include the bottom of the furnace. In the event that molten metal reaches the panel, an alarm will sound. An independent molten metal leak detector system is not a substitute for the ground and leak detector system.
Do not operate the equipment if the metal charge or molten bath in the furnace is not grounded through ground probe wires. Failure to ensure that ground probe wires are in contact with the charge or molten bath could lead to high voltage on the bath during operation. This could cause serious injury or death from electrical shock or molten metal eruption.
While it is impossible to remove all risk from melting metal, it is possible to make the melt shop an accident-free workplace. To accomplish this goal requires a partnership between foundry managers, the suppliers who equip the melt shop, and the equipment operators. It requires management to make safety a key corporate value and communicate that to the operators by selecting the safest available equipment, and by extending every possible effort to assure that workers are instructed in its proper use. Both operators and maintenance personnel must become familiar with the equipment and its operation by reading and understanding the equipment manuals. OSHA’s Personal Protective Equipment Subpart 1, Section 1910.132 (General Requirements) requires on the part of the employer that there be a workplace hazard assessment that identifies the proper PPE for each such employee, and further requires, Section 1910.132 (f), “The Employer shall provide training to each employee who is required by this section to use PPE.”
Melting, holding and pouring system operators and maintenance personnel must know how to safely operate and maintain their equipment. They must also be able to recognize the warning signs of a potentially dangerous situation and how to react to prevent or control uncommon problems such as bridging or run-out situations. Formal training and retraining of the personnel on the above is key to providing a safe working environment.
The use of automated equipment, such as remote charging, preheating/drying, computerized melt monitoring and control, increase the operator’s safety by distancing the operator from the furnace. The use of advanced automated furnace tending systems that use industrial robots for temperature measurement, metal sampling, slagging, etc., greatly enhances the operator’s safety by distancing the operator from the melt operations.
No matter how carefully equipment is manufactured, workers trained or procedures followed, the possibility of an accident is always present wherever metal is melted.
Emad Tabatabaei is the Director of Technology for Inductotherm Corp. Robert C. Turner, P.E., is the Chief Engineer with Inductotherm Corp.