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Table of Contents
I. Introduction Pg 3
II. Making of Refractory Materials Pgs 3-6
III. Structure of Refractory Materials Pgs 6-8
IV. General Properties of Refractory Materials Pgs 8-10
V. Applications of Refractory Materials Pgs 11-12
VI. Conclusion Pgs 13
VII. Bibliography Pgs 14
First we will start with the definition of refractories and ceramics. Refractories and ceramics are non-metallic materials capable of maintaining physical and chemical stability at high temperatures. Refractories in modern practice are usually ceramic in nature, and are used in a wide variety of primary, secondary and tertiary industries. Wherever an industrial process involves heat in excess of 700 to 800 degrees Fahrenheit (roughly), one will find refractory material in place, either as a lining or forming the process vessel itself. Some common process vessels using refractories are; boiler combustion chambers, furnaces like the one in the foundry, incinerators, many emission control scrubbers, rotary kilns and so on. The list is by know means exhaustive. For example, Launch Pads 39A and 39B at the Kennedy Space Center are refractory lined. The shuttles themselves are lined with ceramic tiles to protect them from the heat of re-entry into earth’s atmosphere, these tiles are!
unique to the shuttle, but are non-metallic and heat resistant.
The Making of Refractory Materials
Chart # 1
The first step in processing ceramics is crushing of the raw materials. Crushing is usually done in a ball mill, either wet or dry. Wet crushing is more effective because it keeps the particles together and prevents the suspension of fine particles in air. The ground particles are then mixed with additives, the functions of which are one or more of the following:
1. Binder for the ceramic particles
2. Lubrication for mold release and to reduce internal friction between particles
3. Wetting agent to improve mixing
4. Plasticizer to make the mix more plastic and formable.
5. Various agents to control foaming and sintering.
6. De-flocculent to make ceramic-water suspension. De-flocculention changes the electrical charges on the clay particles so that they repel instead of attract each other.
Next, it’s time to begin the casting process. The shaping process for refractories are casting plastic forming and pressing. The most common casting process is slip casting. The slip is poured into a porous mold made usually of plaster of paris. Then inverted and the remaining suspension is poured out for making hollow object much like slush casting. The part is then trimmed the mold opened and the part removed.
The second process of shaping ceramics is plastic forming. We have various methods of plastic forming such as extrusion, injection molding and jiggering. Plastic forming tends to orient the layered structure of clays along the direction of material flow. This leads to anisotrophic behavior of the material, both in subsequent processing and in the final properties of the ceramic product. In extrusion, the clay mixture is forced through a die opening. The cross section of the extruded product is constant, and there are limitations to wall thickness for hollow extrusions.
Finally the third process in shaping ceramics is pressing. Dry pressing is used for relatively simple shapes. this process has high production rates and close control of tolerances. Dies are usually made of carbides or hardened steel. However the dies can be quite expensive as the must have a high wear resistance from the abrasive ceramic tiles. Wet pressing is used to make very complex shapes. Production rates are high but part size is limited, dimensional control is difficult because shrinking during drying, and tooling can be expensive. The third type of pressing is isostatic pressing mainly used to produce spark plug insulators, silicon nitride vanes for high temperature and so on. Isostatic pressing allows one to obtain uniform density distribution throughout a part. Finally we have hot pressing which combines pressure and temperature. The die life is short as a result of the temperature, and usually protective atmosphere’s are used along with graphite materials used in !
the punch and die materials. One example of a hot pressing part is the vane for a gas turbine engine in a jet airplane.
Finally, after the part has been cast in anyone of our methods above we begin drying and firing the part to give it strength. Drying is very critical as the part may want to warp and crack from variations in moisture and thickness within the part and the complexity of the shape. Control of atmospheric humidity and temperature is very important to avoid warping and cracking. Next, the part must
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Ceramic materials, Superhard materials, Nitrides, Carbides, Abrasives, Ceramic, Refractory, Silicon nitride, Cutting tool, Boron nitride, Sintering, Tungsten carbide
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