This paper discusses the results of failure examinations of Grade T91 superheater tubes that failed due
to creep damage (long-term overheating). Failed and unfailed portions of the superheater tubes were
examined using standard metallographic techniques. Measurements of wall thickness, outside diameter,
inside diameter, ID scale thickness, creep void density, and hardness were used to estimate the
remaining life of the superheater tubes using different models and methods. Models derived by
extrapolation 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 to
be 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 to
elevated temperatures. Tube metal microstructures showing different degrees of long-term thermal
degradation were examined using Scanning Electron Microscopy (SEM). SEM images are provided in
this paper.