Compositions: ceramic – Ceramic compositions – Carbide or oxycarbide containing
Patent
1990-11-30
1992-08-04
Dixon, Jr., William R.
Compositions: ceramic
Ceramic compositions
Carbide or oxycarbide containing
501 88, 501 90, 501 92, 501 96, 501 97, 126360R, 126366, 126367, C04B 3552, C04B 3556
Patent
active
051358930
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a sintered carbon-containing ceramic body to be used in contact with molten aluminum, zinc, copper, lead or like non-ferrous metals; a protecting tube made of such ceramic material for a thermocouple, a heater or the like which tube is used as immersed in molten metal; a transport pipe made of such ceramic material for transporting molten metal; an immersion-type heater for molten non-ferrous metal which is provided with the protecting ceramic tube as an outer tube; a furnace for melting non-ferrous metal which is provided with such heater; etc.
PRIOR ART AND ITS PROBLEMS
A furnace adapted to melt and hold aluminum or like non-ferrous metal needs a heater for heating molten metal, a thermocouple for measuring the temperature of molten metal, etc. Protecting tubes of ceramics are used for the protection of these devices. A protecting tube for a heater which is used as immersed in molten metal (hereinafter referred to as "heater tube"), for example, is required to have a function of transferring the heat generated by the heater inside the tube to the molten metal outside the tube. Therefore generally the internal surface of the heater tube has a relatively higher temperature than the outside surface thereof in contact with the molten metal. As a result, minute fissures are formed on the outside surface of the heater tube due to the difference of thermal stress between the internal and outside surfaces of the tube so that the interior of the tube becomes easily permeable, thereby tending to accelerate the progressive erosion of the heater tube in its entirety. Especially when molten aluminum is involved, its reducing ability is extremely high and the permeability of the tube and its susceptibility to erosion are increased with the rise of the temperature so that the tube becomes more easily eroded. Heretofore chiefly employed are heater tubes produced using silicon carbide interlinked with silicon nitride as an aggregate which has a poor wetting property on and a low chemical affinity with molten aluminum. This type of heater tube can be produced by feeding nitrogen while heating shaped bodies comprising a silicon carbide aggregate and silicon to react the nitrogen with the silicon through the pores in the shaped bodies for production of silicon nitride. However, having the pores, the obtained heater tube is not given a compact and uniform texture, and is increased in the wetting property against aluminum with the lapse of time, permitting the penetration and erosion of molten metal.
A method for production of a compactly and uniformly textured heater tube is conceivable which comprises shaping a mixture of a silicon carbide aggregate, silicon and carbon (or an organic substance as a carbon source) under a high pressure and reacting the silicon with the carbon (or the carbon source) with heating to produce silicon carbide. While the heater tube produced by this method has a slightly improved durability, no great improvement is achieved to make up for the disadvantage of the complicated method.
MEANS FOR SOLVING THE PROBLEMS
The present inventors conducted extensive research in view of such state of the art. In the course of the research, the inventors found the following. In conventional heater tubes, great importance has been attached to the erosion due to the penetration of molten metal, and consideration has been given mainly to measures for obviating the problem. Actually the durability of heater tubes, however, depends largely on the thermal spalling attributable to cracks or minute cracks formed due to the thermal stress in the early stage of use. The inventors continued the research to find out optimum materials for heater tubes, considering also the destruction by thermal stress. As a result, the inventors successfully produced heater tubes and like protecting tubes which are composed of an aggregate of silicon carbide and/or silicon nitride and specified amounts of flake graphite and boron carbide, and which are excellent in not only the
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Asada Hirokazu
Dohi Shoji
Hamada Kengo
Hayashi Yoshitaka
Imajo Mamoru
Dixon Jr. William R.
Marcheschi Michael
Osaka Gas Company Limited
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