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

다국어 초록 (Multilingual Abstract)

The reliability of service-life assessments for the boiler tubes in thermal power plants depends on the prediction of metaltemperatures and the evaluation of microstructure changes. Three tubes—one as-received tube and two tubes service-exposedfor 54,750 h and 68,550 h—were used to study metal temperature and microstructure characteristics. First, the average metaltemperature was estimated (based on accelerated creep rupture test data for the tubes) using the Larson–Miller parameter. Theaverage metal temperature of the tube experienced during its operational period was estimated to be 616 °C, which is 16 °Chigher than the steam temperature of 600 °C. Next, the microstructure characteristics were investigated to evaluate precipitationbehavior and creep damage. The results demonstrated that the prominent precipitates in the as-received tube sampleincluded coarse primary Nb(C,N) precipitates at the grain boundaries and fine primary Nb(C,N) precipitates (maximum200 nm) occasionally observed at dislocations. After long-term service, the size and shape of the Nb(C,N) precipitates wererelatively stable. In contrast, string M23C6precipitates and Cu-rich phase particles were identified at the grain boundariesand within the austenitic matrix, respectively. Increasing the aging time from 54,750 to 68,550 h caused strings of M23C6precipitates 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 linkand grow into microcracks was observed only in the tube sample service-exposed for 68,550 h.

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