The paper presents the results of testing the microstructure and mechanical properties of austenitic Super 304H steel after approximately 31,000 h of operation at a temperature of 570 °C. The microstructure analysis showed that the utilization of the tested steel contributed to the precipitation of numerous M23C6 carbides and individual sigma phase particles at grain boundaries, while dispersive ε_Cu and MX particles was observed inside the grains. At the grain boundaries, the precipitates sometimes formed a continuous mesh. The presence of numerous secondary phases resulted in higher than standard strength properties while maintaining the required plastic properties.