Degradation and reduction of small punch creep life of service-exposed Super304H steel

To ensure the safety and structural integrity of a power boiler in thermal power plants, residual life management of superheater tubes at elevated temperature is needed. Over the decades, small punch (SP) creep testing has been widely used as an effective method for measuring creep life and creep properties of the boiler tube materials. In this study, a series of SP creep tests were performed at 650 °C with virgin and service-exposed Super304H stainless steels. The service period was 54750 h and 68550 h, respectively. The residual creep rupture life of the 68550 h serviced Super304H material decreased significantly when it was compared with the virgin and 54750 h serviced materials. Coarsening of the M 23 C 6 precipitates along the grain boundaries made the adjacent region Cr-depleted, which could accelerate the formation of creep cavities at the grain boundaries. These microstructural degradations reduced the creep rupture life of the service-exposed materials. The Larson–Miller curve and the Monkman–Grant relationship were applied to predict the creep rupture life of service-exposed Super304H steels from the measured short creep rupture data.