Cementation, high-temperature alloys

In ancient India, a steel called wootz was made by placing very pure kon ore and wood or other carbonaceous material in a tightly sealed pot or cmcible heated to high temperature for a considerable time. Some of the carbon in the cmcible reduced the kon ore to metallic kon, which absorbed any excess carbon. The resulting kon—carbon alloy was an excellent grade of steel. In a similar way, pieces of low carbon wrought kon were placed in a pot along with a form of carbon and melted to make a fine steel. A variation of this method, in which bars that had been carburized by the cementation process were melted in a sealed pot to make steel of the best quaUty, became known as the cmcible process. 

The Tj-carbides are not specifically synthesized, but are of technical importance, occurring in alloy steels, stelUtes, or as embrittling phases in cemented carbides. Other complex carbides in the form of precipitates may form in multicomponent alloys or in high temperature reactor fuels by reaction between the fission products and the moderator graphite, ie, pyrographite-coated fuel kernels. 

Rhodium-platinum alloys containing up to 40% Rh are used in the form of wire or ribbon in electrical resistance windings for furnaces to operate continuously at temperatures up to 1 750°C. Such windings are usually completely embedded in a layer of high-grade alumina cement or flame-sprayed alumina to prevent volatilisation losses from the metal due to the free circulation of air over its surface. Furnaces of this type are widely employed for steel analysis, ash fusions and other high-temperature analytical procedures. 

A freshly prepared mixture of a fluid or soft binder with a solid filler is generally pliable and can be easily worked and shaped. As the binder solidifies, either because of drying, as when the binder is a cement, firing at high temperature, when the binder is clay, as in ceramics, or cooling down below the melting point, when the binder is bitumen or an alloy,... 

COBALT AND ITS COMPOUNDS Cobalt, Co, at wt 58.95, steel-gray metal, d 8.71 at 20°, mp 1495°, bp 2880° readily sol in nitric acid and less sol in dil hydrochloric or sulfuric acids. It is obtained from ore concentrates by roasting, followed by thermal reduction by A1 or other methods. The principal use of the metal is in alloys, especially Co steels for permanent supermagnets, Co-Cr high-speed tool steels, cemented carbides and high temperature-resisting alloys for jet engins, For a description of its alloys see Refs 1, 2, 3, 4, 5, 6 8. Cobalt Bomb is briefly discussed under Bombs in Vol 2,... 

Chromium carbides are mainly utilized in cemented carbide alloys with a nickel bonding phase. The.se alloys are notable for their good corrosion and scaling resistance combined with abrasion resistance and are therefore utilized in high temperature applications. 

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