The use of dry vibratable refractories as metal contact linings in coreless induction furnaces is well known. Dry vibratables have been successful because of their ease of installation and short turnaround time.

One weakness that is typical of most ceramics, including dry vibratables, is their brittle nature of failure. Brittle failure is particularly problematic during low temperature metal alloy production of zinc, galvalume, and aluminum alloys. Imparting greater fracture toughness and resistance to mechanical abuse is of great value in these circumstances because it extends refractory lining life.

The use of dry vibratables in iron or copper inductors is commonplace. A move to apply them to zinc or aluminum inductors has been a difficult challenge. Among the concerns regarding their application to these metals and their alloys are the potential for saturation by low viscosity molten alloys, providing adequate hot face strength at low temperatures, having sufficient non-wetting properties, and their not being a traditional application in the marketplace.

Each of these concerns is resolvable through proper product design, for example, optimum particle sizing, minimum porosity, raw materials selection, and the addition of non-wetting agents. Because of traditional failure modes in these applications, the benefits of dry vibratable technology are clearly part of the solution. Now, coupled with new technology, dry vibratables may solve these problems.


Background
The application of dry refractory materials addresses common failure modes associated with melting low temperature alloys, such as zinc and aluminum, by induction heating and melting.

To address these failure mechanisms we proposed a dry vibratable refractory having good resistance to crack propagation. The refractory composition includes specially selected metallic fibers, the compositions of which are suitable for use in these metal containment applications. This “fiber reinforced dry vibratable” technology will be referred to as FRDV technology.

Compared with conventional refractory linings, such as castables and ramming materials, dry refractories provide better resistance to crack propagation because of unique bonding systems that allow these linings to respond to varying thermal conditions. Nevertheless, once sintered, the refractory lining will exhibit brittle behavior that can progressively increase. Ultimately, this cracking can lead to lining failure.

It is advantageous, then, to develop a dry refractory that possesses increased fracture toughness and greater resistance to crack propagation. Such a material would exhibit less brittle behavior when mechanically and thermally stressed, offering longer life.

The addition of metallic fibers to advanced ceramics or refractory castables is commonplace; up until now, the addition of such fibers to dry vibratables was considered impossible because of concerns about inductive coupling with the metallic fiber (drawing power from the furnace) or contamination of the alloy being melted. These are legitimate concerns.

The influence of frequency on penetrating depth and power density of metallic fibers at constant field strength were determined. Calculations indicate that the fiber inductively couples only slightly in a 2.6 MW furnace, with approximately 0.001 percent of the power drawn off by the fiber at 100 Hz (assuming 95 percent operating efficiency of the furnace). At 500 Hz, less than 0.003 percent of the power is drawn by the fibers. This indicates the power required to affect the metallic fiber is high, and the fiber is therefore able to reinforce the ceramic matrix under these conditions.

The second issue to address is that of chemical compatibility. Iron is not a desirable element to pick up in either aluminum or zinc alloys. SHG zinc typically requires ultra-low levels of iron, the maximum typically being 0.003 percent.

All the case studies described here monitor chemistry very closely. Customer results indicate negligible to undetectable chemistry changes to their metal in each case. The essence is that there is limited exposure to the metallic needles because they are discontinuous and dispersed in a refractory matrix. In addition, the grade and alloy of the metallic fiber can be optimized to reduce exposure to specific undesirable elements.


Summary
We have discussed several successful case studies in aluminum, zinc, and galvalume whereby an atypical approach was taken to improve the fracture toughness and impact resistance of dry vibratable refractories in induction melting. With the patent-pending FRDV technology, greater resistance to crack propagation in mechanical and thermally stressful operations is possible. The nature of the FRDV technology permits modification of the refractory system (aggregate and metal fiber) to optimize results for any given furnace and alloy configuration.
For additional information contact Allied Mineral Products, Inc., Columbus, OH. Tel. 614-876-0244; Fax 614-876-0981