Elephant Foot Errosion If

Reducing Elephant’s Foot Erosion in Coreless Induction Furnaces By understanding the origins of this refractory erosion, foundries can integrate practices into their lining, melting and maintenance to reduce the erosion and improve melt campaign life. David C. Williams and Ying H. Ko Allied Mineral Products, Inc., Columbus, Ohio

lephant’s the loose layer must be levWhat is “Elephant’s Foot” Erosion in a Coreless Induction Furnace? foot” eroeled before de-airing besion of silica gins to ensure an evenly refractory is the rapid lodistributed 4-in. loose layer. • It is erosion of the furnace lower calized erosion of the Foundries have found sidewall, taper and floor (at left). lower sidewall/taper secthat decreasing the loose • It occurs in the active power coil, tion and the floor of a layer thickness to 2-3 in. typically with low volumes of molten metal. coreless induction furhas proven to be more ben• It is more predominant when nace. It is observed when eficial despite the difficulty melting ductile-base iron. a coreless induction furin control due to the loca• It is more predominant in nace melts ductile base tion of the taper section. medium frequency and batch iron or gray or malleable Sintering of the Workmelting furnaces. iron. Its unusual wear pating Lining—The purpose • It is more likely to occur with tern takes on the appearof sintering any dry lower silicon-containing melts. ance of an elephant’s foot vibratable lining in a (Fig. 1) and occurs within coreless induction furthe active power coil, nace is to develop the leading to an accelerated Fig. 1. Elephant’s foot erosion is severe refractory erosion in the lower optimum ceramic “hot sidewall/taper section and the floor of a coreless induction furnace. erosion when the metal- Most refractory manufacturers recommend that any dry vibratable face.” With silica refracsaturated refractory is in- lining in a coreless induction furnace be removed and replaced when tories, this is represented 33% general wear has occurred (or 50% wear in any one location). ductively superheated. by the cristobalite phase, The development of which is the refractory elephant’s foot erosion (based on tion furnace, the most basic criteria is surface that is in contact with the field experience) is linked to eight the introduction of controlled loose laymolten metal. The intention is to dedifferent causes: ers of refractory. This becomes difficult velop a ceramic hot face that has • installation of the working lining; to accomplish on the first layers of minimal saturation (Fig. 3). • sintering of the working lining; sidewall refractory in the taper section Sintering refractories requires accu• chemical reactions of the initial once the melt-in steel form has been set rate temperature control using “K”-type charge materials; into place (Fig. 2). When introducing chromel/alumel thermocouples inside • design of the form’s taper section; the material in this area, the refractory the melt-in form, careful selection of • excessive, uncontrolled superheating must be introduced in 4-in. loose layers. the charge materials/starter blocks used of a low metal heel (including bridgNormally, the next installation step is to for the initial heat and careful monitoring of charge materials); de-air the loose refractory. However, ing of the induction power. If any one • altering the thermal gradiof these aspects is ignored, ent of the lower sidewall the area in the furnace that and floor; first will suffer the conseInstallation of a Working Lining • metal finning and localquences will be the floor Foundries may have: ized metal saturation; and taper section. • difficulty in controlling the • presence of nonferrous Chemical Reactions of 4 in. loose layer; metals in the charge. the Initial Charge Materi• difficulty in leveling around This article discusses how als—When melting ductile the form; elephant’s foot erosion debase iron, an accelerated re• difficulty in de-airing in velops from these eight action occurs between silica the taper section; causes and how foundries refractory and molten metal. • potential movement of can improve their practices Thermodynamically, there is the form; to reduce the problem. an equilibrium ratio of car• interference of the lower castable ring with the tapered bon (C) and silicon (Si) in Causes of Erosion wall thickness. molten iron at a given temInstallation of the Workperature (Fig. 4). ing Lining—When review- Fig. 2. This view of a coreless induction furnace with the melt-in In ductile base iron, the steel form set in place emphasizes the difficulty in installing the ing the installation proce- refractory in the taper section. Without an optimal installed ratio of C to Si greatly devidures for a dry vibratable density to the refractory, sintering can render a less dense hot ates from the required equilining for a coreless induc- face that is more prone to erosion. librium ratio. The molten 22

modern casting / July 2000

Fig. 3. Proper sintering is essential to minimize elephant’s foot erosion. Depending on the power and the frequency of the coil, the density of the charge makeup is important as well as the cleanliness. For all higher frequency coreless induction furnaces, it is important to densely pack the charge material since the power density is concentrated closer to the refractory hot face and melt-in form. It may require that a starter block be placed in the bottom and loose charge be packed around it.

empty furnace with the high-C, low-Si refractory in the severe tapered area ductile base iron will seek Si compenpig iron. When completed, the initial of the sidewall. When considering insation by reducing silica from the remelt in the bottom of the furnace conternal form vibration such as with a fractory into Si in order to satisfy the tains high C and very low Si. This partial pneumatic vibrator or a high-frethermodynamically required ratio. This melt will aggressively attack the silica quency vibrator, the foundry should can cause more profound elephant’s lining, especially in the floor and taper not expect the refractory to flow down foot erosion, which can encompass the section. To avoid this, a portion of the and around the severe taper angle entire sidewall. cast returns should be placed in the during the vibration sequence. Often, In many ferrous charges for coreless bottom prior to introducing the high-C the refractory is subjected to the force induction furnaces, a typical charge pig iron and carbon steel. of gravity and will simply compact in consists of cast foundry returns, pig Design of the Form’s Taper Seca vertical manner. Therefore, miniiron and carbon steel. This is evident in tion—The design of the form’s taper mizing the form’s taper angle often is Fig. 5. The sequence of the charge masection can contribute significantly to preferred. Without an optimal interials can play a key role in experiencthe installed density of the refractory in stalled density, sintering can render a ing or preventing the early chemical the taper section. Depending on the less dense hot face, which is more attack on the refractory in the taper geometry of the taper section, the inprone to erosion. section. When carbon steel charge is stallation of the refractory can be more Excessive, Uncontrolled Superheatadded to the bottom of an empty difficult. The severity of the taper angle ing of a Low Metal Heel—In a line coreless induction furnace, it is coninhibits a direct/straight de-airing techfrequency coreless induction furnace, ceivable that the initial molten metal nique. This difference in taper design is the penetration depth of the induction will have low C and Si levels. This will illustrated in Fig. 6. field creates a vigorous stirring effect in cause the melt to have increased If the Bosch vibration technique is the entire furnace cavity as well as the amounts of iron oxide (FeO) and manutilized, it is difficult to compact the taper area. This stirring is so violent ganese oxide (MnO). Both oxides will that the elephant’s foot eroaggressively attack the silica sion is more profound and in the floor and in the taper. includes a significant porPig iron often is an intetion of the sidewall as gral part of an iron charge. shown in Fig. 7. For gray pig iron, the nomiIn a medium- or high-frenal levels of C and Si in the quency coreless induction chemistry will cause little furnace, the stirring action reaction with the silica reis not as strong, although fractory. When considering the ability to inductively suthe high-C, low-Si pig iron perheat is intensified due used for ductile base iron to the increase in power. charges, this type of pig iron This has allowed will acquire more Si to satfoundrymen to melt faster isfy the C/Si equilibrium and to use the entire molstate, but it is necessary to ten metal bath (batch meltcreate the high-C/low-Si ing). However, if there is an chemistry required for a ducinterruption in the charge tile base melt. When blended in with treated duc- Fig. 4. The chemical stability of carbon and silicon in molten iron will sequence such as a bridghave an adverse effect on silica refractories. For gray iron, the tile cast returns, the aggres- carbon to silicon relationship gives an equilibrium isotherm similar ing condition or if a low sive nature is reduced. Many to actual practice. For ductile base iron, this ratio is much lower than molten metal heel is left in the furnace under high infoundries begin charging an actual practice, indicating a strong desire to acquire silicon. modern casting / July 2000

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Chemical Reactions of the Initial Ferrous Charge Materials • 2FeO + SiO2 → 2FeO • SiO2, Melting point: 2223F (1217C) • 2MnO + SiO2 → 2MnO • SiO2, Melting point: 2453F(1345C) • C + SiO2→ CO(g) + Si Carbon (C)/Silicon (Si) Contents of Various Ferrous Chemistries Gray iron: C 3-3.6%, Si 2-2.4% Ductile-base iron: C 3.4-3.9%, Si 0.7-1.8% Treated ductile iron: C 3.4-3.9%, Si 1.9-2.4%, Mg 0.025% (min.) Pig iron—high C, low Si grade: C 3.5-5%, Si 0.2-0.8% Carbon steel: C 0.1-0.5%, Si 0.2-0.8%, Mn 0.2-1% Fig. 5. The reactions of initial charge materials (as shown above) will influence the eroding condition in the taper section. For example, a charge with low carbon and high silicon will have increased oxides that aggressively attack the refractory.

penetration will be more profound in the taper section due to a reduced thermal gradient. The thermal gradient in this area is affected by the increased lining thickness as well as any structural components behind the lining. In medium-frequency/high-power coreless induction furnaces, this heavier saturation can lead to a melting of the dry vibratable refractory, causing a severe eroding condition. This initial metal finning can be alleviated by adhering to proper cool down and conservative cold restart procedures. Presence of Nonferrous Metals in the Charge—The presence of nonferrous metals in an iron charge is becoming more common due to the increased use of alternative iron units such as galvanized steel. Any galvanizing coating will contain a thin layer of zinc (Zn). Other coatings may contain a thin layer of tin or may be painted with lead-based paint. The typical melting points of these nonferrous metals are drastically lower than the melting points of any iron grade. When considering the depth of penetration of these metals into a dry vibratable sidewall or taper section (for any iron melting application), it may breach the entire working lining thickness. For example, Zn has a melting point of 780F (420C) and a vaporization point of 1700F (910C). The Zn within the iron melt can penetrate through the porosity of the dry vibratable lining as a vapor until the 1700F isotherm is reached. At this point in the cross-section, the Zn reverts from a gas to a liquid and will continue to permeate through the porosity of the refractory until the 780F isotherm is reached. A thin layer of Zn metal will solidify in a vertical orientation parallel to the coil grout’s surface. This early, severe metal penetration is difficult to prevent since it is a result of these low melting points buried deep within the dry vibratable refractory. In the taper section, where the cooling effect is reduced by the increased thickness of the dry vibratable working lining, the depth of penetration is worse. Any nonferrous metal penetration eventually can lead to iron penetration. The best way to minimize this penetration is to reduce the amount of coated charge material used.

duction power, the temperature of the lem, study the thermal conditions in the partial molten metal heel can increase taper including all types of structural dramatically, melting the refractory. support refractories behind the workThis usually corresponds to the refracing lining in the taper. A higher alutory in the floor and taper section. Caremina-containing castable should result ful control during the melting operain higher thermal conductivity, allowing for increased cooling from the coil tion, including continuous temperature measurement, is necessary. to reach the working lining. When conWhen a furnace is put into a holding sidering any change in the structural sequence and remains on high melt refractories, understand that there will power, superheating of the molten metal be a change in the thermal conditions will occur. With higher temperatures and consult the furnace and refractory and stirring, erosion will be accelermanufacturers prior to making any ated. The type and cleanliness of the change (see “Altering the Thermal Gracharge materials as well as careful vigidient” sidebar on p. 25). lance during the meltdown can help Metal Finning and Localized Metal avoid a bridging condition. Saturation—Metal finning in a coreless Altering the Thermal Gradient of induction furnace often is the result of a the Lower Sidewall and Floor—In any crack in the refractory formed by a thercoreless induction furnace, the coolmal shock condition or mechanical iming effect from the water-cooled coppact during charging. When the metal per coil creates a steep thermal gradifinning remains in the cross-section of ent across the refractory sidewall. In the dry vibratable refractory, the fin will the cross-sections of a sidewall (Fig. 8), be inductively heated and cause metal the insulating effect of a silica grout vs. penetration into the refractory in a localan alumina grout as well as the depth ized pattern around the fin. This metal of sintering can be seen. As the thermal gradient is less steep, the depth of sinter is deeper. This also would be true if this represented metal or slag saturation. Deeper penetration of metal or slag can be expected when the cooling effect from the water-cooled coil is reduced (such as when silica grout is used instead of an alumina grout). When considering the silica dry vibratable refractory in the sidewall and in the taper section, the difference in lining Fig. 6. Pictured above are two taper designs. The design of the taper section of the furnace form can contribute significantly to the thickness can result in heavier installed density of the refractory in the taper section. Without an penetration of metal or slag in optimal installed density, sintering can render a less dense hot face the taper. To combat this prob- that is more prone to erosion. 24

Solutions The key to reducing the elephant’s foot erosion problem in coreless induction furmodern casting / July 2000

Superheating a Low Metal Heel • It occurs from a rapid increase in metal temperature without the ability to measure it. • The higher the furnace frequency during superheating, the more concentrated the effect will be in the taper section of the refractory. • Continuous charging and careful selection of the charge is essential to reduce the erosion effect. • Careful control of the furnace power during the initial meltdown (until 50% of the charge is melted) is a good rule to follow to reduce the erosion effects. Fig. 7. If not carefully controlled, a furnace could rapidly superheat a small volume of molten metal and easily melt the refractory below the molten metal level. Extreme care must be adhered to when considering the holding power of a furnace vs. a small volume of metal.

naces is consistency in practices and procedures. Following is a summation of the guidelines to follow during furnace lining, melting and maintenance. Strict Adherence to Proper Installation and Sintering Procedures—Over the years, careful installation techniques have helped reduce the elephant wear by increasing the installed density, which allows the refractory to develop an acceptable ceramic hot face with minimal metal saturation. Use Quality Charge Materials and the Preferred Charging Sequence—To minimize the elephant’s foot erosion when melting ductile base iron, the charging practice must be modified. Instead of charging carbon steel, high-C pig iron or C first in the initial charge, cast iron returns should be charged first to reduce the Si-deficient molten iron. The carbon steel or the high-C pig iron is then charged when the molten bath has reached 50% capacity. Any remaining charge will consist of cast iron returns and the C additions. This practice reduces the elephant’s foot erosion significantly. The initial melting power is reduced when melting the cast iron returns first, instead of melting steel charge. No initial C addition is needed when cast iron returns are used as an initial charge. Also, melting carbon steel generates more FeO and MnO, which are more aggressive to silica refractory. Zircon Addition in Silica Refractory—It has been proven that the elephant’s foot erosion can be reduced when zircon is added to silica refractory. Zircon, as a non-wetting agent, modern casting / July 2000

Fig. 8. Pictured above is a comparison of the finite thermal profiles of two coreless induction furnaces through their sidewalls. The furnace on the top uses silica grout and the furnace on the bottom uses alumina grout. Deeper penetration of metal or slag can be expected when silica grout is used.

greatly reduces the wetting of molten iron to the silica refractory. This will minimize the reaction between the molten iron and silica refractory. Taper the Design of a Coreless Furnace’s Melt-in Form—The taper angle of a melt-in carbon steel form can impose some difficulty for proper installation of refractory in the taper area, especially for a large furnace. It is difficult to achieve a thorough de-airing and adequate vibration in the taper section. Modification of the taper angle has been attempted, but little success has been made. In order to have better access to the taper area for installation, the floor diameter should be enlarged. This reduces the taper angle and makes the taper section more accessible from the top of the furnace during installation. Utilize the Shave Repair Practice—In order to extend the furnace lining campaign, foundries have resorted to a shave repair procedure as a maintenance practice. The wear area in the taper section is

chipped away to clean up any penetrated metal and slag in the area. A clean refractory surface then is exposed. This must be prepared cautiously without causing an avalanche of loose material behind the semi-sinter refractory surface. The floor will be rammed first if it must be repaired. The floor should be over-rammed by 1 in. (25 mm) in order to level the floor and achieve a firm surface without any segregation of refractory. An undersized, short form with a height above the taper is placed and secured with charge inside the form. The installation is continued as the sidewall is installed using a Bosch vibrator. The sintering is similar to that of a new lining to ensure a proper sinter in the shaved repair area. The number of shave repairs can be limited due to the overall repair cost when compared to the benefit of extending service cam▼ paign per each refractory lining. For a free copy of this article circle No. 344 on the Reader Action Card.

Altering the Thermal Gradient Changing the thermal condition in the taper section of any coreless induction furnace may result in increased erosion or saturation of the working lining. Following are changes that alter the thermal gradient of the taper and the floor: • a change in the thermal conductivity of the backup support; • a change in the slip plane medium; • a change in the taper design of the

melt-in form; • an interruption in the cooling water circulation; • buildup on the surface of the refractory. Any decrease in the thermal conductivity will reflect an increase in the metal saturation depth. Eventually, it will lead to severe erosion when continually subjected to the ▼ induction field. 25