Microstructural degradation and creep damage of T91 steel: a case study of fish-mouth rupture in heat recovery steam generator superheater tubes

This study offers a multi-scale investigation into the creep rupture of a Grade T91 Cr-Mo steel superheater tube from a Heat Recovery Steam Generator (HRSG) that experienced a catastrophic fish-mouth fracture after prolonged service. Unlike conventional case studies that rely on a single analytical technique, this research combines optical microscopy, scanning electron microscopy (SEM/EDS), atomic force microscopy (AFM), X-ray diffraction (XRD), and hardness profiling on the same failed component. This method allows a direct link between nanoscale features and large-scale fracture behaviour. Findings reveal progressive carbide spheroidization, boundary segregation of (Cr, Mo)-rich precipitates, and intergranular void formation, along with oxide scale thickening that led to increased localised overheating. These observations are consistent with established tertiary-creep degradation mechanisms reported for T91 steels. AFM analysis was used to characterise surface topography and grain-boundary relief associated with creep damage, complementing conventional microscopy techniques. The combined hardness reduction and phase analysis confirms tertiary creep rupture as the primary failure mode, aggravated by internal oxidation and localised wall thinning. Key features of this failure include a pronounced fish-mouth rupture geometry, of sulphur and chlorine detected on the steam-side surface, and the comparative chemistry of inner and outer oxide scales. This comprehensive approach not only clarifies the combined effects of creep and oxidation in T91 failures but also underscores the diagnostic value of integrating nanoscale AFM with traditional metallurgical techniques for service-life prediction and failure prevention in high-temperature power plant components.