OCR Text |
Show Paper #18 Alloy Selection and Design Considerations for High Temperature Service James Skarda August, 1989 Abstract The factors affecting environment resistance thermal fatigue, shock resistance and fabricability of heat resisting alloys as used in thermal processing equipment and heat treating equipment. A review of metal properties as. related to design considerations and fuel efficiencies in muffles, retorts, fixtures and radiant tubes. The surface characteristics of oxidation, green rot and metal dusting as affected by environments, sigma formation, are reviewed with general guidelines for fabrication and welding techniques. For the purpose of this paper, we will concentrate on the family of alloys, primarily nickel and chromium combinations, that serve industry In thermal processing eqUipment, in particular. To differentiate these alloys from the more exotic and special conditions of space and turbine blade materials, the general parameters are: Long-time use (10,000 hours) primarily single phase, fabricability, availability in a variety of product forms and reasonably priced for use in a temperature range of 12000F to 2300oF. It should be noted that the temperature is related to metal temperatures, not processing or operating temperatures. There are several alloys that can perform satisfactorily in this temperature range and selection becomes a matter of detailing enVironments, cycles and heating rates. We'll touch on two areas; design and fabrication considerations and alloys. FACTORS AFFECTING HEAT RESISTANT ALLOYS The two main applications as related to the high temperature Industry are containers and structurals/conveyors. By an arbitrary definition a container is a method to contain an atmosphere preventing contamination, or to contain a generated atmosphere that is neutral to the work, or to develop enhanced properties of the work being processed. Examples of this would be muffles, retorts and radiant tubes. The second application would include fixtures, tube hangers/separators, jigs, grids, trays or boxes. Each has its own design criteria. Common to both is to use the least amount of material that is necessary to resist the operating stresses. The difference is the frequency and rate of heating and cooling. 1 |