Changes in creep property and precipitates due to aging of T91 steel after long-term service

The creep deformation behavior of virgin and long-term service-aged (77000 h) T91 steel of superheater tubes in a power plant was assessed via creep tests along with microstructure examination. A series of miniature tensile creep tests were conducted at 590 °C under applied stresses ranging from 120.0 to 224.1 MPa for both material types. The creep behavior was examined in terms of variations in the microstructural properties correlated with creep rupture life and ductility. X-ray diffraction, optical microscopy, field-emission scanning electron microscopy, hardness measurements, and image analyses were used to analyze the shape and morphology of the existing δ-ferrite, tempered martensite microstructure, and precipitates in both the virgin and long-term service-aged materials. The presence of larger residual δ-ferrite in the virgin microstructure was one of the causes of creep rupture strength reduction in the virgin material. On the contrary, the synergistic effect of the tempered martensite lath microstructure and additional precipitation strengthening of fine precipitates had beneficial effects translating to retention of the creep rupture strength of the long-term service-aged material.