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Show FABRICABILITY AND DESIGN CONSIDERATIONS OF HEAT RESISTANT ALLOYS James Skarda Rolled Alloys Inc. Temperance, Michigan, USA Abstract The factors affecting environment resistance thermal fatigue, shock resistance and fabrica bility of heat resisting alloys as used in thermal processing equipment in 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 envi ronments, sigma formation, are reviewed with general guidelines for fabrication and welding techniques. FOR THE PU RPOSE 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) primari Iy single phase, fabricabi lity, avai labi Iity in a variety of product forms and reasonably priced for use in a temperature range of 12000F to 2300 oF. 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. Weill touch on two areas; design and fabrication considerations and alloys. FACTORS AFFECTING HEAT RESISTANT ALLOYS The two main applications as related to the heat treat industry are containers and conveyors. By an arbitrary definition a container is a method to contain an atmosphere prevent- 279 ing contamination, or to contain a generated atmosphere that is neutral to the work, or to develop enhanced properties of the work bei ng processed. Examples of this would be muffles, retorts and radiant tubes. The second application is conveyors which include fixtures, jigs, grids, trays or boxes that hold the work as it is carried throup,h the fu rnace. 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. Muffles, retorts and radiant tubes generally are not subject to the rapid fluctuations associated with conveying systems but are exposed to thermal aradients because of the temperature differences on the inside and outside surfaces. Other considerations are expansion rates and awareness of the fuel and thermal efficiencies. A rather oovious statement is to use only the mass (thickness) that can support the applied stress. This will certainly reduce material costs. However, even if this were not a factor, there are other valid considerations. An aspect is the fuel efficiency as related to the heat required to raise the temperature to operating levels. It can be formulated by the equation: Q = SM (T2 - T 1). This indicates the amount of energy Q requi red to rai se the temperature. The temperature of a container can be calculated by multiplying the specific heat times the mass (M), or weight, times the difference (T 2 - T 1). The AT is dictated by the process requirements and variations possible are quite limited if not fixed. Since variation in specific heats is insignificant, the real variant occurs in metal thickness. The thicker the wall, the more energy is required to raise the temperatu re to the desi red level. Another consideration is heat transfer. Fou rier l slaw : 9-.- = KA (T 2 - T 1) indicates t [ |
