Discerning the degradation and failure mechanism of T91 grade reheater tube through integrated experimental and simulation approach

The reheater tube is a critical component of boiler in thermal power plants. The component
contributes in determining the efficiency of the plant by regulating the temperature of steam
entering low pressure turbine. In the present investigation, the reheater tube experienced operational failure after 60,000 h of service in a coal-based thermal power plant. The component was
made with 9Cr-1Mo T91 grade of steel. The investigation encompassed bulk composition analysis,
microstructure examination, mechanical property evaluation, fracture surface analysis, and
thermodynamic- kinetic simulations. Composition and tensile strength of the alloy confirmed,
that the component met the specified standards. The microstructural analysis revealed the
presence of excessively coarse precipitates (up to ~ 4 µm). Multi-layered scale formation was
observed on both the inner and outer walls of the tube. Discontinuities at the interfaces between
the scale, and the virgin alloy disrupted heat transfer between the fire-side outer wall, and the
steam flowing through the reheater tube. Localized heat accumulation instigated overheating,
and subsequent microstructural degradation of the component. The signature of structural change
was endorsed by the coarsening of M23C6 precipitates and thick scale formation. The appearance
of high density of creep voids along the grain boundaries suggested high temperature exposure for
considerable time. Various theoretical approaches viz. Larson–Miller parametric approach and
thermodynamic and kinetic simulations were used to estimate the operational temperature.
Correlation of the predicted data with experimental findings indicated a temperature rise of up to
800 ◦C, which was beyond the recommended service temperature of the component. Long-term
overheating degraded the mechanical properties of the alloy and the component ultimately
failed pre-maturely during service