Metal Temperature Estimation and Microstructure Evaluation of Long-Term Service-Exposed Super304H Steel Boiler Tubes

The reliability of service-life assessments for the boiler tubes in thermal power plants depends on the prediction of metal temperatures and the evaluation of microstructure changes. Three tubes—one as-received tube and two tubes service-exposed for 54,750 h and 68,550 h—were used to study metal temperature and microstructure characteristics. First, the average metal temperature was estimated (based on accelerated creep rupture test data for the tubes) using the Larson–Miller parameter. The average metal temperature of the tube experienced during its operational period was estimated to be 616 °C, which is 16 °C higher than the steam temperature of 600 °C. Next, the microstructure characteristics were investigated to evaluate precipitation behavior and creep damage. The results demonstrated that the prominent precipitates in the as-received tube sample included coarse primary Nb(C,N) precipitates at the grain boundaries and fine primary Nb(C,N) precipitates (maximum 200 nm) occasionally observed at dislocations. After long-term service, the size and shape of the Nb(C,N) precipitates were relatively stable. In contrast, string M23C6 precipitates and Cu-rich phase particles were identified at the grain boundaries and within the austenitic matrix, respectively. Increasing the aging time from 54,750 to 68,550 h caused strings of M23C6 precipitates to increase rapidly from 60 to 200 nm and the Cu-rich phase particles to increase from 12 to 32 nm. Finally, creep cavities were observed in both service-exposed tubes. However, creep damage characterized by creep cavities that link and grow into microcracks was observed only in the tube sample service-exposed for 68,550 h.