Induction melters examine furnace and pouring technology, refractories.

With advancements paving the way for induction melting, this conference outlined ways to optimize furnace performance, refractory life and automated pouring technology.

If you are operating under the thought that iron melting technology is standing still, the recent developments shared at the AFS Melting Methods & Materials Div.'s Induction Melting, Holding and Pouring of Iron Conference held January 14-16 in Orlando, Florida, is certain to change your perspective.

The conference drew a parallel to the nearby Disney exhibit - General Electric's "Hall of Progress" - which traces the continual improvements in the standard of living provided by the harnessing and application of electricity for a family over the past three generations. Throughout the advancements overtime the common theme remained, "Now is the best time of your life." In the world of induction melting technology, the conference expressed that "now is indeed the best time of the induction melter's life," and also showed that this sentiment will be repeated as melting rapidly progresses into the future.

More than 30 presentations by industry experts to more than 170 attendees covered topics that ran the gamut of induction melting, from furnace designs to refractories to charge materials to operations and maintenance of melting, holding and pouring vessels. This article serves to recap the technology of furnace design and use with particular emphasis on refractory issues. Other areas covered by the conference will be examined in subsequent issues.

Coreless Furnace Technology

Inductotherm Corp.'s Hans Heine and John Mortimer and Ajax Magnethermic's Dave Lazor presented an overview of technical advancements in coreless induction furnaces. They also suggested actions that can be taken to capture these advancements in current coreless induction furnace melt shops to improve the quality, productivity and cost of the metal melted. Heine shared his observation that "technical advancements in the foundry and related equipment have improved dramatically, making the foundry smaller, more automated and a much better, safer place to work." Further, he said: "Batch melting with launder transfer to a holder or dual systems with two furnaces appears to be gaining acceptance as the preferred primary melting method."

The batch melting method, said Mortimer, is where the furnace is poured empty after each melt and successive melts are started using unheated or preheated charge materials. He added: "The widespread adoption of batch melting as an operating process is itself propelling the creation of even more powerful induction power supplies, versatile melt control technology, high-power density furnaces, automated charging systems and automated pouring systems."

Batch melting in a coreless induction furnace offers foundries many advantages over older methods that required a molten heel to remain in the furnace. Significant is the ability to use smaller furnaces for comparable productivity, faster melting, easier and more reliable chemistry adjustments, less oxidation of the melt, less manpower and much improved charging safety.

Coreless induction batch melting practices became economically feasible through the development, use and improvements of solid-state medium frequency induction power supplies. The application of the computer with solid-state units is compatible and, for this reason, the computerization of induction furnaces with solid-state power units has made great strides over the past years, As Mortimer stated, "Industrial engineers used to focus on the man-machine interface when it came to control technology. Today, the focus is on the man-computer interface, and the computers control the machines."

Adapting Line Frequency

Lazor also presented a paper on upgrading the line frequency of coreless furnaces to solid-state medium-frequency power supplies, which have provided a high degree of flexibility and reliability, he said. Line frequency power systems were the preferred method for harnessing electrical power high enough for large coreless induction furnaces as in the 1960s and 70s. While many are still in operation today, he said, fixed line frequency systems result in limitations that result in reduced refractory life, some gas pickup in the molten metal and tapping volume restrictions to maximize power utilization.

"Any existing line frequency furnace now in use can be upgraded with a new solid-state medium frequency power supply," he said. In his presentation, which described the technology and method of conversion, he identified the major considerations of the upgrade decision-making process as: maximum utilization of existing equipment; design of the furnace, coil and yoke; and possible upgrading of charging capabilities to keep pace with increased productivity [ILLUSTRATION FOR FIGURE 1 OMITTED]. All of these tasks should be conducted in such a manner as to minimize production down-time during the upgrade.

Lazor echoed earlier comments regarding the benefits derived from solid-state medium frequency conversion stating, "These systems offer a great deal of flexibility, are simple to operate and have almost infinite power control capabilities and better power utilization. They have increased power density capabilities that result in higher production capacities per furnace."

Channel Furnace Technology

A panel with Jim Bailey, Ajax Magnethermic Corp., Jeff Brickey and Ron Parrett, Waupaca Foundry, Inc., Walt Matthews, Decatur Castings, H. Rogers Ozment, Martin Foundry, and George Walker, Walker Co., presented operational issues pertaining to channel induction furnace melting of iron. Channel induction air-cooled furnace melting has been employed in nonferrous applications since the beginning of the 20th century. However, its application for iron melting at higher operating temperatures began in the 1950s and brought greater challenges. High-powered melting furnaces create an electrical field called flux that encircles the coil and loop. Brickey stated, "if these areas of stray flux are not cooled properly, the inductor case, furnace vessel, or even bolts will increase in temperature to a point that they will melt."

Walker stated in his presentation that these challenges were addressed by "the birth of water-cooled coils, bushings and inductor cases" as well as inductor designs to optimize "the relationship of metal flow to channel temperature" and the development of better refractory products "to withstand higher temperatures, erosion and saturation." Although water sources may vary, it is critical to control its quality by complying with furnace manufacturer's specifications. Prevention of scale buildup helps assure the designed flow and heat dissipation in the electrical components associated with high-power utilization and improves refractory performance as well.

According to Bailey, channel furnace preventative maintenance is critical to successful operations. As he stated, "Channel furnaces do not lend themselves to power shutdowns or furnace draining while maintenance is being performed. There is no worse situation than having to terminate a perfectly functioning channel furnace campaign for items such as a water leak, hydraulic leak or an electrical problem." Bailey described the need, method and consequences of preventive maintenance pertaining to the electric, hydraulic and water systems of channel induction furnaces. The message to be carried is that preventive maintenance "should be a part of the total systems approach because world-class manufacturing requires world-class maintenance."

Matthews shared his experiences concerning the operation and control of channel furnaces for gray and ductile iron during his tenure at T.B. Woods. He stressed the importance of properly installing a new lining as crucial to the overall success of furnace operations, stating: "There are no shortcuts to the sintering process." T.B. Woods' sintering and start-up procedure, as well as its process controls, record keeping and visual inspection was discussed in detail. Particular emphasis was devoted to training, operator participation and management involvement. The methods used at T.B. Woods doubled refractory life, cutting costs in half. As Matthews stated, "It certainly gives you something to think about."

The operation and maintenance of a 4500 kW single-loop inductor at Waupaca Foundry's Marinette, Wisconsin facility was discussed in detail by Brickey and Parrett. When melting ductile base iron at power levels at less than 3000 kW, Waupaca experienced plugging or clogging of the inductor channels. With 4500 kW inductors, Waupaca has not encountered any "plugging" since the system's installation in 1994.

Loss of power can be devastating, particularly if the melting furnaces have just been charged, the metal temperature is low and the outage is extended. Unfortunately, this was a real-life situation at Martin Foundry. Ozment described in detail the events that lasted for 11.5 hr during which iron solidified over the top of the furnace. New transformers were installed and the furnace was rewired and finally placed back in operation. Clear thinking and quick action brought this situation to a successful conclusion. The furnaces were operated 4.5 and 5 months after this incident to their normal scheduled reline time. Ozment said: "Fins and metal penetration were no more than what we normally see at the end of a year's campaign."

Automatic Pouring Technology

The application of automated metal pouring methods is needed to keep pace with today's high-production molding machines. Ray Gilli, ABB Metallurgy, Inc., Bill Pflug and Emad Tabatabaei, both from Inductotherm Corp., addressed the subject of induction autopour units.

According to Pflug, "The success of all automated pouring systems depends on metering metal, which is directly related to some shape or size of orifice." While Tabatabaei recognized that there are several varieties of autopour systems on the market, they all have four basic components: a heated or unheated metal holding vessel that may be pressurized, a molten metal metering device (valve), a valve actuator and a controller [ILLUSTRATION FOR FIGURE 2 OMITTED].

Tabatabaei discussed the sensors and controls used in dispensing molten metal into sand molds by using a stopper rod and nozzle. He stated that laser and vision systems are widely used in measuring the molten metal level and that safety, relative costs and availability of components must be considered in the selection process.

Pflug discussed the application and design of pressurized autopour units in ductile iron processing. He stated, "The ultimate success - or failure - of an automated ductile iron pouring furnace hinges on three basic criteria: furnace construction, treatment practices and furnace maintenance." Increasing the metal depth to diameter ratio and employing an inert gas are two effective measures to combat magnesium fading of ductile iron. Clean ductile base iron should be a standard objective. He also advised striving to keep slag-forming elements like calcium, aluminum, sulfur and oxygen at low levels and avoiding over-treatment of the metal with magnesium. He discussed the importance of various maintenance techniques stating: "The best furnace design and treatment practice does not take the place of a simple, regimented maintenance program."

Gilli described a successful automatic pouring vessel using a coreless inductor at Aarrowcast, Inc. This foundry produces ductile iron utilizing the in-mold process which "substantially reduces maintenance on the autopour unit." The application of automatic pouring in this operation greatly enhanced the consistency of treatment results and was reflected in improved casting yield, reduction of scrap and working conditions. Additional information on this installation will be provided in the August issue of modern casting.

REFRACTORIES

A key component to any successful induction melt facility is having good, consistent refractory practices. Often, in any molten metal containment application, the refractories are subjected to severe temperatures and mechanical abuse. Ultimately, the wear of the refractory controls the amount of tonnage throughput and the need to shut down the vessel for reline. The refractory presentations helped to emphasize the practical refractory theory as well as new refractory developments in the coreless induction, channel induction and the automatic pouring sectors.

Recently, the increased use of slip plane materials has been reemphasized for various reasons: a method to monitor refractory wear; an impervious vapor barrier; and a layer to facilitate rapid lining removal systems. Michael Hermann, Safeway USA, presented 6 years of operational experience of a unique method of measuring lining wear in coreless induction furnaces and in an inductor of a vertical channel induction furnace. This would offer another means to measure refractory wear besides actual mechanical measurement.

The use of mica as a slip plane medium in a coreless induction furnace was discussed by Michael Chaisson, Cogebi, Inc. Chaisson described the benefits of mica as a slip plane material as well as its ability to be a vapor barrier to moisture and carbon monoxide that is permeating through the porous working lining. With foreign black deposits collecting underneath the coil grout and between the turns of the copper coil, there is a need for a mica slip plane to retard or prevent the gases from reaching the coils.

Chaisson's presentation offered proven concepts that could be true for any generic mica slip plane product. Table 1 compares the permeability of mica, ceramic paper and woven glass. based on the comparison, Chaisson asserts that if a low-permeability slip plane material is used, it can perform an important function of mitigating the conductive gases to the induction coils.

The refractory presentations for coreless induction melting seemed to draw considerable interest from the audience. Tim Green, Allied Mineral Products, Inc., emphasized the key concerns for improving the service life of the working lining of a coreless furnace. Green described the most common failure modes in coreless refractory linings: hot-face erosion, mechanical erosion, buildup, bridging, finning, spalling and saturation. He further explained the various contributing factors to each failure mode. For the best results, Green suggested that foundries: select the correct refractory for the melt system; install and sinter the refractory properly; conduct melting practices to the lining design specifications; control the melt chemistry; and minimize mechanical abuse. Documentation was stressed in the interest of depending on accurate record keeping rather than memory and guessing.

New refractory installation technology always is needed to improve furnace downtime and the consistency of refractory installation. Ayton Grady, Gradmatic Equipment, Inc., developed a method of installing silica dry vibratable refractory into large coreless furnaces used for melting iron. Within the past 5 years, Grady has been successful in expanding the application of the automated installation system for dry vibratables in coreless induction furnaces for both ferrous and nonferrous metals. Grady's presentation highlighted not only the consistency benefits of the installation, but also the health/safety issues regarding reduced exposure to respirable silica during the installation.

Channel Furnace Refractories

Four of the 6 presentations on channel induction furnaces reiterated the importance of careful monitoring of the refractory during installation, sinter and under normal melt operation.

Following these, John Curtis, Vesuvius Foundry Group, presented the different refractory applications for vertical channel furnaces. Curtis emphasized the change in the refractory criteria when considering an inductor vs. an uppercase. The different refractory systems were compared for all applications, such as dry vibratables, wet rams, firebrick, and conventional and low-moisture/low-cement castables. Another valuable engineering tool that Curtis presented was the use of thermal profiles of sectioned uppercase refractory walls. A comparison of a dry vibratable uppercase to a low-moisture/low-cement castable uppercase was reviewed to share the benefits of the thermal calculations as well as to demonstrate the thermal modeling that is necessary prior to lining the furnace.

Charging Practices' Impact

When considering the prolonged refractory campaign in a coreless or channel induction furnace, both Ken Copi, Exolon Co.-ESK, and Gene Muratore, Rio Tinto Iron and Titanium America, made the audience aware of the effects of charging and the presence of iron oxide. Copi mentioned the effects of surface oxidation, higher power levels, alloy recoveries, charging higher carbon-containing metallics first, optimizing the charging density and the possible use of an inert gas shield (or cover). Longer lining life, less total slag volume, fewer slag defects, reduced kWhr/ton for a higher melt rate and higher/consistent magnesium recoveries are all attainable. Muratore emphasized the importance of the everchanging alternatives for ferrous charge materials. Most have been rejected due to contamination from other elements, excessive slag generation or safety issues. Muratore commented on the quality of today's steel scrap, foundry returns, pig iron, carbon/graphite and ferroalloys. He also discussed the sequence of the charge going into the furnace. The proper selection of charge materials affects the production rate and refractory life, as well as safety on the melt deck. A balance must be struck between cost and price. Lower priced charge materials offer lower levels of chemical control. Closer control of the chemical analysis and contaminants will lead to a successful melt operation.

Silicon Carbide 

Silicon carbide historically has been added to iron in order to increase the amount of carbon and silicon in the melt. Recently, it has been added successfully to the melt and has reduced the iron oxide and manganese oxide contents in the slag. Rick Falkowski and Thomas Richter, North American Refractories Co., presented the advantages of using silicon carbide in refractories. Richter discussed the intrinsic properties of silicon carbide, such as volume stability/thermal expansion, high thermal conductivity, oxidation resistance, abrasion resistance and non-wetting characteristics.

Richter also mentioned that higher thermal conductivity in conjunction with lower thermal expansion is necessary for excellent thermal shock resistance. This sets silicon carbide-containing mixtures ahead of most conventional alumina, alumino-silicate or alumino-magnesia spinel mixtures. When considering the mechanism by which silicon carbide suppresses oxidation of the carbon, as described by A. Yamaguchi in a 1984 paper:

SiC(s) + 2CO(g) [approaches] Si[O.sub.2](S) + 3C(s)

Further reactions can occur for the given temperature, but all seem to have some benefit towards the refractory matrix. Richter commented on the improved non-wetting nature of the silicon carbide-containing refractory. When tested simultaneously against +90% alumina brick, high alumina-chrome brick and resin-bonded alumina-silicon carbide brick in a slag corrosion test, tile silicon carbide appeared to have the best results. In addition, when three different low-cement castables were tested, similar results were obtained.

Refractories for Auto Pouring

David Williams, Allied Mineral Products, Inc., discussed the different refractory applications for pressure-pour channel furnaces and autopour vessels. The key refractory requirements for automatic pouring were porosity of the refractory matrix, the "hot strength" of the refractory, the "non-wetting" nature of the refractory and the thermal conductivity of the refractory. In each case, Williams described how the requirements affect the overall performance of the refractory.

Other topics Williams discussed included the problems surrounding the detachable spouts and the severe clogging conditions that exist in most pressure pour furnaces. Attacking the clogging condition by identifying the source of the insoluble constituents and developing a maintenance program to periodically clean the openings was discussed. Williams commented that mechanical and thermal solutions can be accomplished with proper assistance for the refractory supplier and the furnace manufacturer. A review of real-life problems with pressure pouring such as loss of pressure and any changes in various melt conditions was discussed.

When considering automatic pouring vessels, it is always necessary to be aware of the thermal loss of the vessels. Unlike channel furnaces, the pouring vessel will have relatively thin refractory walls so backup insulation is limited. Foseco's Brian Alquist and Texas Foundries' John Mullan discussed the benefits of insulating linings over the conventional rammed-in-place monolithic linings. From a successful field trial at Texas Foundries, Alquist reported that cleaner metal with improved alloy recovery was achieved. More consistent pouring and less deviation in pouring temperatures helped to reduce casting defects. Lower heat loss due to improved insulation in the side walls led to improved lining life in the ladle refractory. based on the observed field trial data, the following projections from using insulating linings were surmised: lower electrical energy consumption, reduction in natural gas energy costs, improved alloy recovery, refractory costs offset by other savings, reduced number of furnace relines, extended fade times, less pinhole scrap, improved productivity in pouring, reduction in labor, and improved ergonomics/working environment.

An interesting concept for veneering castable refractories in pouring furnaces was presented by Bill Fletcher and Bror Pokorny, Premier-LaFarge Svenska Hoganas. In the past, castable linings in pressure pour furnaces were removed from service once the lining had exceeded the recommended wear limit or when it was convenient to shut the furnace down. Pokorny presented a case study in which a 5-ton pressure pour furnace casting gray iron was "veneered" (gunned) with an ultra-low cement castable and vibratable refractory with a chrome oxide addition that increased performance life 1 year. This new procedure incorporated the use of a sprayed-on insulating castable layer against a safety insulation brick. Then, about 7.5 in. of ultra low castable was sprayed onto the previously sprayed insulating layer. This was all done while carefully studying the thermal conditions of the wall. After the 2-year campaign was completed, the refractory hot face was veneered, the uppercase form was set into place and a self-flowing castable was applied directly to the top of the old castable. This allowed the foundry to reline the furnace at a minimal cost.

This article was adapted from presentations at the AFS Induction Melting, Holding, and Pouring of Iron Conference. Conference proceedings are available from AFS Publications at 800/537-4237.