Rammable Conductive Lining for Induction Heating Systems
Rammable conductive lining (RCL), a new patented concept in materials technology for induction heating systems, is an in-situ rammable, conductive carbon/ceramic lining material (Fig. 1) that can couple with the induction field in the unfired state. This capability allows it to be fired in-situ without emission of noxious fumes, without the need to pre-condition the lining, without thermal shock issues and without degradation of the lining during use through oxidation (Fig. 2).
This behavior is unique among unfired carbon and ceramic products as the resistivity of such systems is traditionally much too high for coupling with induction fields at normal frequencies. Through a focused program of materials development, Morganite Crucible overcame these limitations to create a complex functional materials technology that can couple with standard frequency induction fields in the unfired state (Fig. 3).
The original focus of the material/system development was the metal melting/ foundry sector, but it quickly became apparent that the material and concept would have practical uses in other fields. In addition to its in-situ rammable form, it can also be provided as preformed shapes for use in applications where lining contact and erosive conditions require a higher density lining.
This new concept and the unique abilities of RCL allow faster turnaround when relining an induction system. In addition, since the electrical characteristics of the unfired RCL are similar to those of the fired material, areas of the lining that are showing signs of wear can be repaired, thus extending greatly the time between full relines. Because RCL is placed in-situ, it offers the induction system designer the ability to optimize coil design without the constraints of lining shape or geometry. This means that designers for molten metal handling are no longer constrained to an existing preformed crucible shape. Because RCL works by heating up the lining to indirectly heat the charge, it also gives the designer the opportunity to exploit induction furnace/heating technology for metals and materials that are poor susceptors, for example, aluminum.
The primary objective of this technology was to provide the customer with a rammable induction lining that can be installed directly onto the inside surface of a refractory lined induction coil. Eddy currents, which are induced into the electrically conductive material by the action of the induction field, provide enough heat to both simultaneously cure and fire the material without the need for the protective glazes normally found on preformed carbon-base crucibles. Glazes on crucibles are used to prevent the burn-out of carbon-base materials in air at elevated temperatures. RCL contains an internal oxidation protection mechanism in the formulation that prevents burn-out of the critical carbon-base components of the material without the need for external protection.
Curing and firing of carbon-base materials is normally accompanied by the emission of noxious hydrocarbon fumes generated by the decomposition of carbonaceous binders, such as tar, pitch, phenolic resin, sugars, etc. RCL is formulated using a unique water-based binder system, which provides sufficient electrical conductivity to maintain the eddy currents that heat the lining, while only generating water as the temperature rises rapidly and the material hardens.
The binder system cures at a temperature around 200°C (390°F). Electrical properties, and hence heat-up characteristics, are maintained relatively constant throughout the wide temperature range experienced in changing from the unfired to the fired state due to a high degree of connectivity maintained between conductive particles. This is achieved through the use of structural materials that are designed to bridge the gaps between particles in the lining formulation. These materials are the underlying reason that RCL can heat up even in the unfired state, a unique ability for a carbon/ceramic material.
The rate of lining heat up is dependant on the electrical properties of the lining, which is linked with the density, and also depends on the frequency and power of the induction coil. Lining density is heavily influenced by the method of installation. The more tightly packed the lining material, the higher will be the conductivity (to enhance coupling with the induction field) and the higher will be the density (to enhance resistance to erosion and corrosion during use in contact applications). For severe contact applications, where erosion and corrosion is a significant life limiting factor for the lining, RCL can be supplied in an isostatically pressed form to maximize density.
With well-compacted RCL, heat-up rates are rapid, with some practical demonstrations having reached 1600°C (2910°F) in under 30 minutes. Such quick temperature rises would destroy most ceramic materials through thermal shock, but the structure of RCL is specifically engineered to optimize thermal shock resistance, allowing it to survive even the most rapid of heat up rates from the as-installed, cold state.
Unlike other water-based refractory materials, no controlled, careful heat-up schedule is required for RCL. Most water-based refractories need to be heated up slowly through the 100°C (212°F) region as water is released very rapidly at this point, leading to explosive spalling. RCL contains a unique patented mechanism to assist in the release of this free water through this normally sensitive temperature range, and, thus, can be heated up rapidly without concern for spalling of the lining. It is possible to have the charge material already positioned inside the freshly placed RCL lining from the start, such that no subsequent charging procedure is needed on the first melt (or heat treatment). However, because water will be evolved during the first heat up of RCL, this procedure should be avoided for materials that are sensitive to moisture.
Thanks to its unique capabilities, RCL is opening up many new applications as a new material for inductively heated systems. In metal melting, some metals are poor susceptors in the solid state (for example, aluminum and silicon) and are normally not processed using inductive heating. RCL allows these metals (and any other poor susceptor including nonmetallics) to be heated indirectly using an inductive source for melting or other heat treatment activities (Fig. 4).
Under such conditions, the penetration depth of the eddy currents, which generate the heat to indirectly melt the metal, is dependent on both the supply frequency and conductivity of the rammed lining. At low frequencies, the thinner the rammed lining wall, the more conductive the mix needs to be, otherwise the lining will not couple. The wall thickness of the rammed lining should be greater than the penetration depth of the eddy current. Typically, the penetration depth of the eddy currents is about 75% of the rammed lining wall thickness. Table 1 shows typical rammed lining thicknesses required for a range of resistivities at normal operating frequencies (2 and 10 kHz).
The resistivity of a newly rammed RCL lining as installed is typically 0.01 Wcm. Therefore, the rammed wall thickness should ideally be greater than 12 cm (~4.75 in.) at 2 kHz for optimum performance. However, practical trials have demonstrated that metal melting is possible using thinner rammed linings at lower frequencies. The rammable lining is used in this case to both contain the molten metal (or other material) and protect the induction coils, when rammed onto a suitable refractory/insulating lining.
The company has worked with several major induction heating-equipment manufacturers to develop practical systems to exploit the benefits of the material. Several prototypes have been successfully demonstrated in both metal melting and incineration applications (Fig. 5). Potential applications currently under development include wire heat treatment, where RCL allows controlled heat treatment up to 1400°C (2550°F), much higher than traditional metal-lined systems; in air preheaters; reclamation; steel billet heaters and other applications where localized temperature control in high temperature operations is a benefit.
RCL is a new, unique materials technology that is introducing induction heating technology into new applications. The key design/application benefits of RCL are in:
- High temperature induction melting and heat treatment of metals and materials (e.g., glasses) that have poor coupling characteristics with induction fields
- Large-capacity and irregularly shaped induction furnaces, where preformed conductive crucibles are not available
The secret of RCL's abilities is a creative blend of new materials technologies, and, as such, RCL represents a generic set of technologies, which can be reformulated to match specific application conditions as appropriate. When discussing new applications with potential partners, it is important that we have as much detail about the application conditions as possible so the most carefully matched product formulation possible can be designed. This product, the materials technology, the design concept and the exploitation in related applications are all the subjects of recent patent grants and applications.