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Show Basic guidelines for design is to maintain minimum cross section, uniform cross section and allow for thermal expansion. Attempts should be made to eliminate fixed positions or rigidity. This is really not a reflection on the welding characteristics but a recognition of thermal expansion. An attempt at this is an articulated design which allows for freedom of metal movement. Often, applied stresses for a conveying system do not permit this, but when permissible it is quite effective. Uniform cross section is also an important consideration. Again, we relate to thermal gradients developed by differences of metal temperature. As Indicated previously, butt welding is the preferred method of joining because it maintains uniform cross section. If a joint is lap welded, the cross section will generally be doubled. Due to the low thermal conductivity of heat resisting alloys, this can result In a significant difference of temperature between the cross section area. The importance of this Is also illustrated by a typical cast alloy grid. The Inner sections of the crossing member are cored. This results in a hole at each Intersection which not only saves material but accommodates uniform cross section at each intersection. The above should not be considered intimidating. The suggestions are not really due to the difficulties of fabricating the alloys, but rather a reflection of the way it Is used. The harsher the service the more critical it is to develop good fabricating and welding techniques. APPLICATIONS Fabricated radiant tubes are used extensively in thermal processing equipment with proven cost-effective service life. On occasion, a tube will fail prematurely, not related to the engineering properties of the metal. Illustrated is an example of a tube which obviously shows melting of the base metal (Figure 8). Records indicated that the operating temperature never got beyond 18500 F, which Is probably true, and the past performance had been excellent. In this particular case, soot formation had accumulated on the surface of the tube, causing two problems. It became carburized, which can lower the melting point by perhaps 100°F. The burning of the soot due to oxidizing conditions probably resulted in an exothermic reaction raising the temperature above the melting point. It should be noted that any deposit on a container will prevent heat dissipation and will cause hot spots. The point here is that nickelchromium alloys do not melt at 1800° or 2000°F. If there is evidence of molten metal, the temperature at that point was 24000F or higher. Outstanding performance has often been observed. Illustrated is a muffle used in a continuous hardening operation (Figure 9). The muffle has been removed for straightening and put back into service. The picture shows the condition of the muffle after 10 years, operating at 1550 to 1610oF, two shifts per day, five days per week, idle at 14000 F, endothermic atmosphere. 7 |