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Long-term exposure of superheater tubes to normal operating conditions results in gradual changes inthe alloy's microstructure and properties eventually leading to creep cracking and failure. The methodused most frequently to predict the remaining life of these tubes is based on curves showing the variationof the Larson-Miller parameter (LMP) with stress needed for creep rupture. This method works well forGrades such as T11 and T22 alloy steels since the tube metal temperature can be reasonably wellestimated based on the ID scale thickness.
This paper discusses the results of failure examinations of Grade T91 superheater tubes that failed dueto creep damage (long-term overheating). Failed and unfailed portions of the superheater tubes wereexamined using standard metallographic techniques. Measurements of wall thickness, outside diameter,inside diameter, ID scale thickness, creep void density, and hardness were used to estimate theremaining life of the superheater tubes using different models and methods. Models derived byextrapolation of short-term data were found to be unreliable. Based on the results presented in this paper,the most useful method for estimating the remaining life of Grade T91 superheater tubes was found tobe based on the creep void density and hardness. This method does not require temperature estimation,and it is solely based on the condition of the tube metal microstructure affected by long-term exposure toelevated temperatures. Tube metal microstructures showing different degrees of long-term thermaldegradation were examined using Scanning Electron Microscopy (SEM). SEM images are provided inthis paper.
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