Strength Evaluation of Reaction‐Bonded Silicon Carbide Radiant Tubes after Long‐Term Exposure to Combustion and Endothermic Gas Ambients

The strengths of reaction‐bonded silicon carbide radiant tubes were examined after a 14‐month campaign at 800°–950°C in an industrial heat‐treating furnace. The inner tube of the single‐ended radiant tube (SER) burner system was exposed to a combustion gas environment, and the outer tube was exposed to both a combustion gas (inner surface) and an endothermic gas (outer surface) environment. Diametral compression testing of C‐rings and O‐rings was employed to assess the effect of exposure on the strengths where failure initiated from the outer and inner surfaces of the tubes, respectively. Comparisons of strengths obtained on exposed versus as‐received tubes revealed that exposure to the combustion ambient yields an apparent moderate strengthening, with a concurrent increase in strength variability. No significant difference in strength distribution was observed for tubes exposed to combustion versus endothermic gas ambients. Failure most often originated at silicon‐rich regions remaining from silicon infiltration of macroporosity in the slip cast tubes during reaction sintering. Specimens sectioned from the outer tube exhibited unexpectedly high strengths (based on predictions from Weibull statistics) compared to those sectioned from the smaller inner tube after the same exposure. These results indicate that the larger tubes have flaws with characteristics different from those present in smaller tubes, and underscore the need to use data from the same material, with the same flaw populations, in making predictions of the effect of component size on strength distributions and reliability in service.