Steels, Chrome Silicon

The input to muffles or radiant tubes is limited by the strength, durability, and conductivity of their wall materials. The great temperature difference across a muffle or tube wall not only reduces its useful life but also causes the products of combustion to exit at a very high temperature, raising the fuel bill. For both reasons, muffle and tube walls are made as thin as practical, using a material that has both high thermal conductivity and resistance to heat. Alloy steels and silicon carbide are the most suitable materials for muffles and radiant tubes. Silicon carbide radiant tubes can withstand higher temperatures and are more resistant to oxidation than nickel-chrome alloy steel tubes, but the latter are less brittle and cheaper. 

Media Small pieces of material used inside a mill jar to increase mixing energy and efficiency. Commonly used materials include alumina, stabilized zirconia, flint pebbles, steel, chrome-plated steel, boron carbide, silicon carbide, and plastic-coated versions of these. Common shapes include spherical, cylindrical, radius-end cylinders, and long rods. Sizes range from 1 mm or less to over 5 cm. 

Absorption trains of early ammonia oxidation processes to nitric acid were constructed of chemical stoneware or acid-proof brick, which restricted acid production to near ambient atmospheric pressure because of the low strength of the structural materials. The discovery that Duriron (silicon-iron) or high chrome stainless steels could tolerate these corrosive conditions well allowed the adoption of pressure absorption. This measure markedly decreased the size of the absorbers required and reduced nitrogen oxide stack losses. Pressure operation was easiest to achieve by compression of the feed gases at the front end of the process. In this way improved acid production is obtained at comparable capital costs per unit of product by operation at atmospheric pressure. 

TABLE 11.1.11 Comparison of Characteristics between Ceramic Material (Silicon Nitride) and High-Carbon Chrome Bearing Steel (SUJ2) and Advantages of Ceramic Bearings... 

Low alloy steels, also called weathering steels, contain small amounts of copper, chrome, nickel, phosphorus, silicon, and magnesium (< 1 %, typically). Their resistance to atmospheric corrosion generally exceeds that of carbon steel. Indeed, when exposed to environments that are not too strongly polluted a dark brown patina forms over some years that slows down the corrosion rate. On buildings this natural layer can thus replace a paint coating. For this reason, alloyed steels find numerous applications in architecture. 

Common metals are suitable as materials of construction. These include the following metals and metal alloys chrome-molybdenum steel, IVpe 300 stainless steel, brass, lead. Monel, K-Monel, and nickel. Piping and appurtenances for undiluted diborane must be designed by experienced engineers and safety and fire protection specialists. Sa-ran, polyethylene, Kel-F, Teflon, graphite, and high-vacuum silicone grease are satisfactory for use with diborane. 

The metals initially considered as possible adherends included 750-T5 aluminum, iron-silicon-bronze, 440C stainless steel, ductile Niresist, cast-iron, SAE No. 52100 steel, and chrome-plated steel. Since the thermal expansion coefficients of most resinous adherends and adhesives are several times that of these metals, the most advantageous metal adhesive match would utilize those metals with the highest coefficient. This would indicate the use of the aluminum, bronze, or stainless steel. The elimination of these materials, in favor of 52100 steel, due to factors outside the scope of this report, heightened the problem of thermal stressing. The use of Teflon further emphasized the thermal stress problem. 

As silicone elastomers flow easily under pressure, the forces involved in moulding can be lower than other elastomer types. Many prototype or short production-run moulds can be made of relatively soft materials, though for normal production situations, hardened steel is preferred. Highly polished cavity surfaces may well prove adequate, but chrome plating is often specified. 

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