Building refractory lining of induction furnaces with castables.
Autor: Evgeniy Demin, Demin SRM GmbH
Induction crucible furnaces have a lot of great advantages, the most important of which are the possibility of obtaining very
pure metals or alloys with precisely specified composition, the stability of the properties of the metal, low waste rate , high productivity,
high automation control , good HSE parameters. The requirements for the refractory lining of induction furnaces have some hidden traps,
sometimes even so small that in order to optimize the lining process and many developers of refractory materials or steelmakers,want to
neglect it. In fact, these neglected factors are so important and mutually connected so they need to be considered in combination and it is
a challenge.
Therefore, steelmakers prefer to leave all these issues to refractory suppliers, and those, in turn, try to approach the issue of
selecting the necessary material from the point of view of refractory materials technology and sometimes ignoring solutions that do not fit
into the circle of their knowledge. Such a rational approach to solving complex issues does not allow developing new solution, introducing
new technologies and new materials. This concerns more consumers of refractories rather than suppliers. The melting crucible is one of the
most critical components of the furnace, which critically determines its operational reliability :
- it must withstand high temperature stresses (the temperature gradient in the crucible wall reaches 200 K/cm)
- withstand the hydrostatic pressure of the melt column and the mechanical loads that occur during loading and upsetting of the charge
- the crucible must be chemically resistant to molten metal and slag and non-conductive at operating temperature
- the crucible should have the smallest possible wall thickness to obtain a high electrical efficiency
- the crucible material must have a low coefficient of linear (volumetric) expansion in order to prevent the occurrence of cracks in the crucible
under conditions of high temperature gradients in the wall and to reduce thermal stresses in the crucible.
Such high and, to a certain extent, contradictory requirements for the lining of induction furnaces have led to the fact that the
problem of refractory materials for furnaces of this type has not yet been finally resolved. The approach that we propose to solve
the quality-based selection of the lining of induction furnaces differs from the traditional choice of lining methods and lining
materials. If today the durability of acid-based crucibles can be considered satisfactory, then the durability of crucibles made of inert
and basic refractories is still low, especially when smelting high-quality and refractory alloys. The two main drawbacks of lining
induction furnaces for obtaining high-quality alloys - insufficient thermal stability and low chemical resistance can theoretically be
eliminated by using the purest aluminum oxide as a material. Traditionally, fused corundum is used as a chemically inert material,
which, as is well known, has significant drawbacks, primarily low thermal stability. In addition, the purer is the fused
corundum, the lower capability of diffusion sintering at temperatures below 1850C it has. Implementation of sintering additives to
create a strong contact surface of the fused corundum lining reduces the entire effect of chemical resistance to liquid metal and slag
and, along with this, reduces the already low thermal stability.
As is known, all sintering additives are quite strong electrolytes and, accordingly, as a result of migration inside the lining from the hot side to the cold
side, they can lead to undesirable consequences. The work carried out by us to improve the lining of induction furnaces showed the obvious advantage
of tabular alumina in comparison with fused corundum, corundum spinel and periclase spinel. The peculiarity and undoubted advantage of tabular
alumina lies in its structure, and the description of these advantages is not the purpose of this work. The research conducted by DIFK confirmed the
theoretical principles on which our research was based. In order to use advantages of tabular alumina, we developed a binder, which, first of all, is
a catalyst for diffusion (solid-phase) sintering and which brings unique properties that provide not only high resistance of the lining, but also increase
the purity and quality of melted in induction furnaces and alloy furnaces. In fact, this binder allows the lining of the induction furnace not to be
rammed, but casted. We are talking about the binder DESOL CF, developed by us specially about for these purposes. The casted lining can rather be
called non-cement concrete, with all the advantages of casted linings, but unlike other cement concretes, does not form cement stone throughout
the entire volume of the lining and does not contain chemically bound moisture. The amount of binder introduced when using the fillers specified in
this publication is 4.2 - 5.2% of the dry filler weight. The mass has excellent fluidity and evenly fills the entire space of the template when
using vibrocaster.
The heating time of the casting and turning the induction furnace into operation are much shorter than with dry ramming of the
crucible. In the process of research, it turned out that as a filler for bulk lining using our binder, the greatest effect, both economic and technical,
is achieved when using secondary tabular alumina products from refractory waste, in which high-alumina cement was used as a binder. This
material, which had been used at temperatures above 1700C, can even be called tabular calcium oxide stabilized alumina, since the calcium
aluminate most likely acts as a stabilizer and gives the refractory aggregate those unique properties that cannot be achieved by other methods.
In conclusion, I would like to note the fact that the use of the binder itself, without taking into account additional requirements for the grain
composition, as well as certain requirements for the composition of the binder itself, is the intellectual property of our company, the so-called
"Know-How". We provide instructions for the use of a binder in combination with one or another filler separately. These instructions indicate
the recommended grain and phase compositions, the recommended binder concentration and the recommended method of lining induction
furnaces, depending on the method of lining sintering. For all technical questions regarding this publication, please do not hesitate contact us
