Induction crucible furnaces as melting devices have great advantages, the most important of which are the ability to obtain very pure metals and alloys of a precisely specified composition, the stability of the resulting metal’s properties, low waste of metal and alloying elements, high productivity, the possibility of full automation, good working conditions for maintenance personnel, and low environmental impact.
The characteristics of the lining of induction furnaces are determined by several factors, which are sometimes so subtle that many specialists, whether developers and manufacturers of refractory materials or metallurgists, tend to neglect these factors in order to optimize the lining process. In fact, each of these factors is so important and all of these factors are so interconnected that they must be considered in combination, and this is sometimes quite difficult. Therefore, metallurgists prefer to leave all complex issues to the developers of refractory linings, who in turn try to approach the selection of the required material from refractory material technology and sometimes bypass solutions that do not fit within their knowledge scope. Such a rational approach to solving complex problems does not allow for the development of new directions, the introduction of new technologies, and new materials. This affects less the manufacturers of refractory materials and more the consumers of refractory products.
The crucible is one of the most critical components of the furnace and largely determines its operational reliability. High demands are placed on the crucible:
– It must withstand high temperature loads (the temperature gradient in the crucible wall reaches 200 K/cm). – It must withstand the hydrostatic pressure of the molten column and the mechanical loads that occur during charging and tamping. – The crucible must be chemically resistant to the melt and slag and non-conductive at operating temperature. – The crucible should have the thinnest possible wall to achieve a high electrical efficiency. – The crucible material must have a low linear (volumetric) expansion coefficient to prevent cracking under high temperature gradients in the wall and to reduce thermal stresses in the crucible.
Such high and partly contradictory requirements for the lining of induction furnaces have led to the problem of refractory materials for such furnaces not yet being definitively solved.
Our proposed approach to solving the quality selection of induction furnace linings differs significantly from the traditional selection methods and lining materials.
While today the service life of acidic crucibles can be considered satisfactory, the service life of crucibles made of inert and basic refractory materials is still short, especially when melting high-quality and refractory alloys.
The two main disadvantages of the lining of induction furnaces for producing high-quality alloys – insufficient thermal stability and low chemical resistance – can theoretically be eliminated by using the purest alumina as a material.
Traditionally, fused alumina is used as a chemically inert material, which is known to have significant drawbacks, primarily low thermal stability. The purer the fused alumina, the less it is suitable for diffusion sintering at temperatures below 1850°C.
The use of sintering additives to create a solid working surface of the fused alumina lining reduces the overall effect of chemical resistance to liquid metal and slag, thereby reducing the already low thermal stability.
All sintering additives are known to be quite strong electrolytes and can therefore migrate within the lining from the hot to the cold side, leading to undesirable consequences.
Our work to improve the lining of induction furnaces showed the obvious advantage of tabular alumina over fused alumina, alumina spinel, and periclase spinel.
The peculiarity and undeniable advantage of tabular alumina lie in its structure, and describing these advantages is not the purpose of this work. Research conducted by Almatis together with the DIFK confirmed the theoretical foundations on which our research was based.
To fully exploit the advantages of tabular alumina, we proposed a binder that is primarily a catalyst for diffusion (solid-phase) sintering and gives the lining of the induction furnace those unique properties that not only ensure high quality and lining durability but also increase the purity and quality of the alloys melted in induction furnaces.
In fact, this binder allows the induction furnace lining to be cast rather than rammed.
We are talking about the binder DESOL CF, which we specifically developed for this purpose.
The cast mass can rather be described as cement-free concrete, with all the advantages of cast linings, but unlike cement concretes, this mass does not form cement stone throughout the entire volume of the lining and contains no chemically bound moisture.
The amount of binder added when using aggregates specified above is 4.2 – 5.2% of the dry weight of the aggregates. The mass has excellent flowability and, in the presence of a vibrator, evenly fills the entire space of the mold.
The heating time of the lining and the commissioning speed of the induction furnace are significantly shorter than with dry crucible packing.
Research has shown that using tabular alumina-based concrete products, where alumina cement was used as a binder, achieves the greatest economic and technical effect for mass linings with our binder. This material, used at temperatures above 1700°C, can even be referred to as alumina stabilized with tabular calcium oxide, as the calcium aluminate form in this compound most likely acts as a stabilizer and imparts those unique properties to the refractory aggregate that cannot be achieved by other methods.
Finally, I would like to note that the use of the binder itself, without considering additional requirements for grain composition, as well as certain requirements for the binder’s composition itself, is proprietary to our company, the so-called “know-how.”
We provide instructions for using a binder in combination with one or another filler separately. These instructions specify the recommended grain and phase composition, the recommended binder concentration, and the recommended method for lining induction furnaces depending on the sintering method.
For all technical questions regarding this publication, please contact us in writing.