Thermal shock, refractories

Zirconia Refractories. The most common zirconia-containing refractories are made from zircon sand and are used mosdy for container glass-tank subpaver brick. Refractory blocks made from a composition of zircon and alumina, used to contain glass melts, are generally electromelted and then cast. These exhibit excellent corrosion resistance but are subject to thermal shock. Refractories made from pure Zr02 are extremely expensive and are... 

As a class, ceramics are hard, wear-resistant, brittle, prone to thermal shock, refractory, electrically and thermally insulative, intrinsically transparent. 

The modulus of elasticity (MOE) is related to the strength and can be used as a nondestmctive quaUty control test on high cost special refractory shapes such as sHde gate valves employed in the pouring of steel (qv). The sHde gate type must be selected to ensure chemical compatibiUty and it must be used in a way to reduce thermal shock. The performance of a properly selected and used sHde gate is direcdy related to its strength and therefore predicted by its MOE. 

The resistance against thermal spalling of fireclay and high alumina brick is indicated in Table 5. No standard test has been adopted for basic brick. Refractories composed of 100% magnesia exhibit poor thermal shock resistance, which is improved by addition of chrome ore. So-called direct bonded basic brick, composed of magnesia and chrome additions, exhibits good thermal shock resistance. 

Uses. Hot-pressed hBN is useful for high temperature electric or thermal insulation, vessels, etc, especially in inert or reducing atmospheres, and for special materials such as IITV semiconductors (qv). Its low thermal expansion makes it resistant to thermal shock. The powder can be used as a mold release agent or as thermal insulation. Boron nitride is also available in fiber form (19). BN deposited pyrolyticaHy on refractory substrates at 1200—1800°C has a turbostratic stmcture and low porosity it has greater chemical resistance and is impervious to helium. 

Because of high thermal conductivity and low thermal expansion, siUcon carbide is very resistant to thermal shock as compared to other refractory materials. 

This wear is caused primarily from high thermal and mechanical stress, chemical attack, attack by iron and slag, oxidation, and severe thermal shock. Thus the design of the hearth wall and the concepts employed ate just as important as the carbon or graphite materials chosen for the refractory material. Despite their benefits and properties, no carbon or graphite material can overcome the problems of an improper hearth wall design concept. 

Alumina—graphite refractories, almost all continuous casting ware, have come into much greater use as continuous casting has spread in steelmaking. These refractories are used in shrouds that conduct the molten metal from the ladle to the tundish, in the subentry tubes that take the metal from the tundish to the mold, in isostatically pressed stopper rods, and in shroud tubes for slab and bloom casters. The alumina—graphite compositions are used in these products because of the thermal-shock resistance and corrosion resistance they impart to the product. 

MiscelDneous. Other important properties are resistance to thermal shock, attack by slag, and, in the case of refractories (qv), thermal expansion. For whiteware, translucency, acceptance of glazes, etc, may be extremely important. These properties depend on the clay mineral composition, the method of manufacture and impurity content. 

The main characteristic of refractory oxides is their excellent resistance to oxidation. Their brittleness however makes them prone to thermal shock with the exception of silica which has a compensating low coefficient of thermal expansion. The chemical resistance of the major oxides deposited by CVD is rated in Table 17.7. 

Because of the larger surface area (compared with solid-ceramic refractories) the chemical resistance of fibers is relatively poor. Their acid resistance is good, but they have less alkali resistance than solid materials because of the absence of resistant aggregates. Also, because they have less bulk, fibers have lower gas-velocity resistance. Besides the advantage of lower weight, since they will not hold heat, fibers are more quicHy cooled and present no thermal-shock structural problem. 

Beryllium oxide shows excellent thermal conductivity, resistance to thermal shock, and high electrical resistance. Also, it is unreactive to most chemicals. Because of these properties the compound has several applications. It is used to make refractory crucible materials and precision resistor cores as a reflector in nuclear power reactors in microwave energy windows and as an additive to glass, ceramics and plastics. 

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