Compositions: ceramic – Ceramic compositions – Refractory
Patent
1997-05-22
1999-07-20
Group, Karl
Compositions: ceramic
Ceramic compositions
Refractory
501 983, 501 984, 501 986, 501 964, 501100, 264647, C04B 35581, C04B 35599
Patent
active
059255854
DESCRIPTION:
BRIEF SUMMARY
The invention relates to novel refractory materials consisting of grains bound in a matrix of aluminium nitride or of sialon, containing titanium nitride and, optionally, particles of graphite and/or boron nitride which are dispersed within it, as well as to a process for manufacturing them.
There is a need in iron and steel metallurgy, as well as in aluminium metallurgy, for refractory materials of increasing performance and reliability: what is required in fact is to improve, simultaneously, the corrosion resistance, the hot strength and the heat shock resistance.
The main applications in question are: the feed streams of cast iron or steel. Particular examples of such components are slide-valve shut-off plates, feed-stream shroud tubes, submerged nozzles and stopper rods; mechanical or gas-injection kind, in the molten metal; and for devices for regulating metal feed streams, as well as impact tiles for ladles and tundishes; belt and crucible; nozzles, bots and flow-offs.
The wide variety of stresses which these materials experience often results from the equipment being operated in a non-continuous manner: there is the heat shock at start-up and then at the end of a cycle, and during a cycle the refractory components come successfully into contact with molten metal and then molten slag. Finally, between two cycles, the refractory components remaining at a relatively high temperature are subjected to oxidation by the ambient air.
Purely mechanical stresses are always present: thermal shocks and stresses resulting from handling operations, confinement stresses created by a metallic outer jacket and, finally, in the case of stream-regulating systems, stresses associated with the actual function of the refractory components, that is to say shut-off effects and movements.
Finally, it will be noted that in all cases the refractory components in question are subjected to the erosive action of molten metal.
The list of properties desired for these refractory materials therefore comprises the following: effects of erosion by the flow of metal or slag; special alloys; powders; to limit the extent to which they infiltrate into grain boundaries, cracks and pores, but also to reduce the risks of the skins catching as the product cools down; some steels;
Despite a complex and corrosive environment, the refractory components in question need to be very reliable since any accidental failure may have catastrophic consequences, both for the plant and for the operators.
Materials based on a silicon carbide granulate, bound in a sialon matrix, are widely used as bricks for lining blast furnaces. In this application the material is intended to withstand a continuous flow over its surface of cast iron for more than 10 or 15 years. However, sialon is slightly soluble in iron and therefore a binding matrix has been sought which is more inert with respect to the metal.
Refractory materials are known from EP-A-0,480,831 which are formed by an alumina-based granulate bound in a binder formed from sialon, these materials being used for the manufacture of plates and nozzles of slide-valve shut-off devices for steel-making ladles and tundishes.
The lifetime of these components very rapidly decreases when they are exposed to very corrosive steels, such as ultra-low-carbon steels treated with calcium carbide (CaSi), that is to say those containing a high dissolved calcium content (>50 ppm).
Finally, materials are known from EP-A-0,482,981 and EP-A-0,482,984 which are based on various refractory granulates, respectively bound in an aluminium nitride or sialon matrix and containing dispersed materials of graphite and/or of boron nitride. These materials are useful for manufacturing slide-valve plates, but above all for manufacturing feed-stream shroud tubes, submerged nozzles and stopper rods. The additions of BN and graphite make it possible to achieve the excellent heat shock resistance required by these applications. However, these materials are attacked, in their binding matrix, when they are used for long periods in contact wit
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Bitouzet Jean-Philippe Marie
Schoennahl Jacques Paul Raymond
Group Karl
Savoie Refractaires
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