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


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

Gräning, T.; Klimenkov, M.; Rieth, M.; Heintze, C.; Möslang, A.

Introduction:

Oxide dispersion strengthened (ODS) steels have proven to be a viable candidate as a structural material in a fusion power plant, due to their enhanced mechanical properties and resilience under irradiation and at high temperatures. A high interest in the development of austenitic ODS (AODS) steel became noticeable, which is shown by the increased amount of publications in that field in the recent years. That can be related to the inherently better corrosion resistance and superior creep properties compared to its ferritic counterpart. However, one of the major drawbacks of AODS steels was the more challenging mechanical alloying (MA) process and a lower powder production yield, caused by a more ductile and adhesive powder. This disadvantage was tackled by the addition of a process control agent (PCA) during the MA, but it was yet to be shown how the addition of a PCA affects the microstructure of the AODS.

Methods:

AODS with a carbon-containing PCA was mechanical alloyed, hot-extruded, and subsequently annealed at 700, 900, and 1100 °C for 2, 750, 1000, 1250 and 1500 hours to investigation the influence of carbon on the formation and stability of precipitates as well as on the grain size in comparison to available literature data. Scanning and transmission electron microscopy and atom probe tomography methods were applied in this systematic study to identify the possible growth of nano-precipitates. An Arrhenius-equation was used to determine the activation energy of the growth of precipitates.

Results and Discussion:

A growth of precipitates is barely detectable at temperatures equal or lower than 900 °C. The grain size remains stable. However, an unexpected increase of the grain and precipitates sizes and a decreased activation energy was measured at temperatures of 1100 °C for all annealing times. Recently published results of similar AODS steels have shown a stability of the microstructure up to 1250 °C. Due to this contradiction, we concluded that the growth of precipitates and the reduction of the grain boundary pinning force was supported by the diffusion of carbon and the formation of carbides.

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Publ.-Id: 27908