Cooling toughness

Microstructurc. Crystal size, porosity, and impurity phases play a major role in fixing the fracture characteristics and toughness of an abrasive grain. As an example, rapidly cooled fused aluminum oxide has a microcrystalline stmcture promoting toughness for heavy-duty grinding appHcations, whereas the same composition cooled slowly has a macrocrystalline stmcture more suitable for medium-duty grinding. 

If small specimens are prepared in which the austenite can be cooled to 250—500°C sufficiendy rapidly to avoid the above microconstituents, and transformed at temperatures in this range, the formation of a completely different phase, a bcc a-phase supersaturated with carbon and containing small cementite particles (bainite), which is both strong and tough, occurs. Bainite is rarely found in plain carbon steels, but it can be obtained in commercial practice by judicious alloying and is increasing in importance. 

Poly(phenylene sulfide) (PPS) is another semicrystalline polymer used in the composites industry. PPS-based composites are generally processed at 330°C and subsequently cooled rapidly in order to avoid excessive crystallisation and reduced toughness. The superior fire-retardant characteristics of PPS-based composites result in appHcations where fire resistance is an important design consideration. Laminated composites based on this material have shown poor resistance to transverse impact as a result of the poor adhesion of the fibers to the semicrystalline matrix. A PPS material more recently developed by Phillips Petroleum, AVTEL, has improved fiber—matrix interfacial properties, and promises, therefore, an enhanced resistance to transverse impact (see PoLYAffiRS containing sulfur). 

The yield strength and toughness of Dural differ enormously in these three conditions (slow-cooled, quenched, and quenched and aged) the last gives the highest yield and lowest toughness because the tiny particles obstruct dislocations very effectively. 

Romanoski, G.R. and Burchell, T.D., The effect of specimen geometry and size on the fracture toughness of nuclear graphites. In Proceeding of the IAEA Specialists meeting on the Status of Graphite Development for Gas Cooled Reactors, lAEA-TECHDOC-690, IAEA, Vienna, 1993, pp. 241 247. 

Cooling tower fan motors must be properly selected for long life and trouble free operation. In addition, it is important to match the motor s enclosure to the application. Motors in draw through applications that are mounted directly in the air stream, for example, experience tough duty and must be of the totally enclosed type. Motors are categorized as follows ... 

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