Long-term stability of the microstructure of austenitic ODS steel rods produced with a carbon-containing process control agent

Austenitic oxide dispersion strengthened (ODS) steel rods containing a carbon-rich process control agent were annealed at 700, 900 and 1100 °C for up to 1500 h. The addition of a process control agent was necessary during the mechanical alloying process to achieve a reproducible production process. In this study, the impact of the added carbon-rich process control agent on the microstructure was determined. For that reason, an extruded austenitic ODS steel rod, which designated operation temperature in future power plants is about 700 °C and beyond, was annealed and examined. The investigation revealed, that the microstructure remains stable up to 900 °C. However, a long-term annealing at 1100 °C increased the grain and precipitate size significantly, which resulted in a decline of the mechanical properties. The results indicate, that the introduced carbon and the formation of carbides led to a reduction of the temperature threshold for coarsening of nano-sized oxide precipitate. The direct impact of carbon on the formation of nano-sized precipitates and their behavior at elevated temperature was shown by calculations of the activation energy of precipitate growth.