Crucibles for Induction Melting Guidelines for Selection and Use

The proper crucible for melting of ferrous alloys is often viewed as a insignificant decision when, in reality, considerable effort should be given to this important decision. Crucible performance can have dramatic effects on foundry costs and the competitive ability of the shop. The melting crucible can affect the overall quality of the castings directly through scrap rates and it can also affect a shop’s labor costs necessary for frequently changed crucibles. There are many variables that influence crucible performance. This article addresses many commonly overlooked variables that can have a dramatic impact on crucible performance and overall life. The selection of the proper crucible forms the basis for extending crucible life and performance. This is easily accomplished by viewing the selection process first in three different ways and selecting the crucible that will best fulfill all conditions. It is important to understand that all of these considerations are interdependent and therefore, by changing one property or characteristic, you will likely affect another. Selecting a crucible is always a compromise and therefore, it is often necessary to experiment in order to find the most cost-effective solution for your particular shop. Physical Considerations Proper fit of the crucible in your furnace is essential. The overall dimensions of the crucible should be such that there is sufficient room for ramming the backup material behind the crucible. If the area is too small (less than 1 inch), the backup ram may not be sufficiently dense, possibly causing the crucible to be inadequately supported, the thermal profile to be uneven, or most importantly, the induction coil to be poorly protected. When selecting a crucible, it is also important to consider wall and bottom thicknesses. Crucibles are often purchased with only the capacity in mind, yet thicker walls and bottoms can significantly increase crucible life and can reduce labor costs since change overs will be less frequent. A supplier with custom shape capabilities can modify an existing design to accommodate this option. Chemical Considerations Chemical considerations are often entirely overlooked when selecting a crucible for induction melting; yet this can be the most important consideration. Alumina crucibles, for example, are often used because of the ability to perform under a wide variety of melt chemistries; however, there can be considerable price and performance differences among this class of crucibles. For best performance, chemical composition of the crucible must be compatible with the chemical composition of the melt. This is visualized by the childhood experiment of mixing vinegar (an acid) with baking soda (a base). The products chemically react and consume each other. Crucibles can also be defined as either acidic or basic as well. An acidic crucible in contact with a basic melt or slag will cause a chemical reaction that results in fast erosion of the crucible and possible inclusions in the melt.  Table 1  Condition Recommended Material Highly Acid Zircon Silica  Neutral Alumina Spinel  Basic Magnesite Table 1 is a general guideline for the selection process. It is best, however, to solicit help from your crucible supplier when evaluating chemistry considerations. Thermal Considerations Thermal considerations are important because of the limitations of many materials. Consideration must be given to the rate at which metal is melted, the number of melts per day, and the maximum temperature at which melts are performed. Although a ceramic is hard and brittle at room temperature, a crucible becomes plastic at operating temperatures because of the formation of liquid phases. The more plastic a material becomes, the more susceptible it is to erosion. If operating temperatures are high enough, an upgrade to a more refractory, less plastic material may be required. In addition, once these liquid phases develop, they become glassy (amorphous) phases when they cool. Glassy phase ceramic is usually prone to thermal shock. Therefore, cycling can have a detrimental effect on crucible life. Repeated heating and cooling leads to crack formation and eventual failure of the crucible. The Final Decision The previously mentioned considerations should not be taken independently. A material with excellent thermal considerations will likely sacrifice desirable chemical and physical properties. When selecting the proper crucible, seek the best combination of physical, chemical and thermal properties rather than the best of each category. Some operational considerations should also be addressed. For example, vacuum melting requires crucibles with certain chemical properties. Other atmospheric conditions may require additional consideration. Your crucible supplier should be your most valuable information source.

Inspection and Grouting The melting crucible (liner) is a ceramic material and can be easily damaged. Examine the crucible prior to installation to assure that it hasn’t sustained damage from shipment or storage. This can be done by visually inspecting for cracks, and by tapping the crucible to assure that a bell-like ring is present. If the crucible has cracks or structural flaws, it should not be installed; a minor crack can lead to catastrophic failure. Once the liner has been inspected, the coil chamber should be considered. The bottom brick (base) of the furnace must be inspected to determine if a replacement is needed. If a replacement is necessary, a high-strength brick, as recommended by the furnace manufacturer, should be used. Any voids in the base should be sealed with a high temperature refractory cement (or grout) and dried slowly to assure that excessive cracking, shrinkage, and blistering does not occur.

The furnace coil must also be examined for voids in the grouted surface. Any voids should be sealed with an air-setting, high-temperature grout as specified by the furnace manufacturer. Grout must be applied to about a 1/4" thickness to the coil surface to form a nearly symmetrical cylinder. All grouting must be thoroughly dried prior to continuing installation in order to avoid problems from blistering and cracking of the grout layer. Next, all leak detector leads should be inspected to be sure that they are intact, at proper spacing, and of the correct length (as recommended by the furnace manufacturer’s specifications). A thin fiber lining can then be placed into the cylinder to allow for vertical movement in the lining. The fiber liner must fit the contours of the coil grouting and should be as cylindrical as possible, from the top of the chamber to the bottom. Crucible and Refractory Installation All refractory backing material must be selected to suit your operation. For best results the backing material should be kept dry, have a minimum fusion at use temperature, have a low linear thermal expansion at use temperature, and be of proper grain sizing to obtain a good ramming density. Your refractory supplier can provide assistance with backup ram selection. Backup refractory must be rammed to a dense consistency to assure proper support of the crucible. The first layer of backing material (base-layer) should be approximately 3" of loose dry material in the bottom of the cylinder chamber. This should be rammed to a firm pack with a ramming foot (See Figure 1). Successive layers should be added using 1-2" of additional ram for each layer and then rammed to a firm pack. The rammed density of each layer should be as uniform and level as possible. Between the individual layers of ram, scratches should be introduced to promote knitting of layers and avoid laminations in the base pack. Once the base has been completed (to a height slightly higher than required for seating), the pack should be leveled with a straight edged instrument and excess packing material should be removed. After the backing base is leveled, the crucible is then lowered into the chamber, centered in the coil, and checked for a level seat (See Figure 2). A spacing arrangement can be used to assure the crucible remains centered during installation. Any arrangement involving clamps or grooves should be avoided as this can cause damage to the upper crucible. Next, the crucible must be firmly seated, using a twisting motion, and checked for uniform leveling again. Any voids between the crucible and its base can result in cracking from insufficient support. A substantial weight should then be carefully placed into the crucible to prevent shifting during ramming. Spacers at the top of the crucible will also help to inhibit motion from side to side. Once a good seat is accomplished, the wall backing is rammed much like the base. All exposed base material should first be scratched to avoid laminations between the base and wall backup. Layer by layer, the refractory backing should be rammed with the same fork and foot tools used to densify the base. Each successive level should be about 1-2" of loose material rammed to consistent densities (See Figures 3 and 4). Ultimately, the height of backup material should be taller than necessary and should be scraped back to expose a dense top surface prior to top-cap forming (See Figure 5). Installation of Cap and Spout The top-cap on the crucible should begin about 2" from the rim. In order to avoid excessive shrinkage during curing, make sure capping material (castable, cement or refractory plastic) is not excessively wet. Work the initial layer of cap material into the dry backup to avoid laminations (See Figure 6). Use very little water to smooth down the cap and spout areas (See Figures 7 and 8). Excessive moisture can lead to cracking, spalling, and blistering during the cure out stage. Several holes should be carved into the top-cap and spout to allow for moisture escape during the first heat-up and curing (See Figures 9 & 10). Once the top-cap has been formed, a heat source should be applied to speed moisture removal. Curing of Unit, Start-up and Use After the crucible and top-cap are free of all moisture, the curing/sintering of the unit can begin. Curing procedures vary substantially from operation to operation, but it is important, no matter what your curing procedure is, to follow it as closely and consistently as possible for all installations. One possible method is the weighted charge start-up (See Figure 11). Care should be taken at each cold start-up to heat the chamber with the charge’s radiation prior to achieving a molten condition. This will decrease the likelihood of a thermal shock failure to your liner. Using a weighted charge of 2/3 or more of metal in the crucible (mainly large material such as ingots, carefully placed to avoid wedging), power should be applied slowly to gently warm up the chamber. A heating rate between 300°F and 500°F per hour is recommended for the first 1-1.5 hours. As the charge slowly approaches melt temperature, the top-cap, crucible, and backup material will release all remaining moisture. If power is applied too rapidly, uneven heating of the crucible could result in potentially catastrophic failures such as thermal shock and hazardous steam explosions. Your furnace manufacturer or refractory supplier should be contacted for further information regarding start-up of your equipment or materials selection. How to Maximize the Life of Your Crucibles The most common reasons for early crucible failure can be reduced drastically with proper material selection and installation of your melting crucible. However, many operational techniques influence crucible life as well. Below are some guidelines that can help maximize the life of your crucible. Thermal Cycling and Shock Limit the amount of cycling as much as possible and try to raise and lower refractory temperatures as gradually as possible. If there is flexibility in your scheduling, arrange for consecutive heats to maintain a hot crucible. Allowing the crucible to cool to room temperature between heats is detrimental to crucible performance. Also, unlike metals, refractory ceramics are brittle and can crack when rapidly heated and cooled. Always remember to preheat your crucibles when possible. The use of a crucible cover to slow the rate of cooling can help to extend life. Mechanical Abuse Treat your crucible with care. Ceramics are brittle materials, and once damaged, will most likely degrade rapidly and fail. Try to minimize mechanical abuse when handling crucibles and, in particular, when cleaning and charging crucibles. Always be careful to place the charge into the crucible as carefully as possible. Dropped charges can damage the crucible and cause cracking. Be careful not to wedge the charge. Because the metal expands more rapidly than the crucible when heated, a wedged charge can cause a crucible to crack. Although cleaning is necessary, avoid striking the crucible in a way that can cause a crack to form. Moisture Store crucibles in a dry place. Moisture can be absorbed into the crucible if it is allowed to contact water or if it is stored in a humid environment. If crucibles get wet, dry them slowly by placing in a dryer or oven. Gas lances can be used, but care must be taken to heat evenly and slowly. NEVER add wet material to a molten charge. Other Recommended Practices Minimize the use of fluxes. Fluxes can be incompatible with refractories and cause aggressive corrosion. Be sure your metal supplier provides a consistently "in-spec" product. Variations in metal chemistry have been known to adversely affect crucible performance. NEVER allow molten metal to solidify in the crucible. Because metal expands more rapidly than the crucible when heated, a heel can cause a crucible to crack. When molten metal is on hold, it is imperative the power be no higher than required to maintain bath temperature. Some operations shut the unit off rather than retain on hold. Also, turn the power off when the furnace is in the tilted position; uneven temperature may crack the crucible. A good rule of thumb is to minimize the time the furnace power is on. During this time, erosion of the crucible is most severe due to the metal flow within the crucible. Using a tall crucible with a lip cut out, and capping up to the top of the crucible rather than capping down to the top, will help avoid metal penetration between the crucible and capping material. Never melt metal in the top cap. Conclusion Proper selection, installation and use of your melting crucible can have a significant impact on crucible performance and overall costs. Being aware of the many variables that can affect crucible performance is the first step in making improvements in practice and extending the life of the crucibles. Maintaining a closer working relationship with your crucible supplier will help assure that you are using the most cost-effective products available for your operation.

Reprinted from INCAST(R), International news magazine of the Investment Casting Institute, May, 1995, pp. 8-12. 8350 N. Central Expressway, Suite M 1110, Dallas, TX 75206-1602. Phone 214-368-8869, fax 214-368-8852.