Creep Life Assessment of Grade T91 Superheater Tubes Using Creep Void Density and Hardness Measurements – Case Studies

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Long-term exposure of superheater tubes to normal operating conditions results in gradual changes in
the alloy's microstructure and properties eventually leading to creep cracking and failure. The method
used most frequently to predict the remaining life of these tubes is based on curves showing the variation
of the Larson-Miller parameter (LMP) with stress needed for creep rupture. This method works well for
Grades such as T11 and T22 alloy steels since the tube metal temperature can be reasonably well
estimated based on the ID scale thickness.

Product Number: 51323-19366-SG

Author: Ewa Labuda, Robert D. Bartholomew, and Emory H. Hull

Publication Date: 2023

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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.