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

Abstract

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