The design of the WCBB (Water Cooled Breeding Blanket) of the first DEMO reactor foresees a minimum irradiation temperature in the range of 280–300 °C. It is well known that the RAFM (Reduced Activation Ferritic Martensitic) steels suffer a marked DBTT (Ductile to Brittle Transition Temperature) increase under irradiation. This DBTT shift depends both on the DPA (Displacement per Atom) damage and on the Helium production. While the former kind of embrittlement goes to saturation for DPA higher than 20 the latter does not saturate with the dose and results therefore the less manageable issue. Several experimental evidences concerning ODS (Oxide Dispersion Strengthened Steels) RAFM (Reduced Activation Ferritic Martensitic) steels showed that it is possible to improve the resistance to irradiation damage by intra-granular precipitation of Y-Ti oxides and by simultaneously decreasing the grain size. In any case, the multiplication of the grain boundaries is expected to increase the dilution of He on grain surface, delaying the formation of He bubbles on grain boundaries and, therefore, the susceptibility to the He embrittlement. Hereafter we show that it is possible to reduce the grain size by an order of magnitude at the tempering stage, by combining cold working and tempering stages, acting on the tempered martensite. The microstructural observations have been carried out by means of both Scanning and Transmission Electron Microscopy (SEM and TEM). Also the positive effect of grain size reduction on the toughness of the material will be taken into account; the DBTT curves resulting from impact tests on KLST specimens show a peculiar nature, characterized by a smoother transition to the brittle regime, compared to the usual abrupt transitions achieved with the “standard” normalization & tempering treatments on RAFM steels. The outcomes of the microstructural observations, as well as the mechanical characterization (tensile, hardness and impact tests) will be discussed in this paper.
Achievement of Ultrafine Grain structure by means of recrystallization treatments
Cristalli C.;Pilloni L.
2022-01-01
Abstract
The design of the WCBB (Water Cooled Breeding Blanket) of the first DEMO reactor foresees a minimum irradiation temperature in the range of 280–300 °C. It is well known that the RAFM (Reduced Activation Ferritic Martensitic) steels suffer a marked DBTT (Ductile to Brittle Transition Temperature) increase under irradiation. This DBTT shift depends both on the DPA (Displacement per Atom) damage and on the Helium production. While the former kind of embrittlement goes to saturation for DPA higher than 20 the latter does not saturate with the dose and results therefore the less manageable issue. Several experimental evidences concerning ODS (Oxide Dispersion Strengthened Steels) RAFM (Reduced Activation Ferritic Martensitic) steels showed that it is possible to improve the resistance to irradiation damage by intra-granular precipitation of Y-Ti oxides and by simultaneously decreasing the grain size. In any case, the multiplication of the grain boundaries is expected to increase the dilution of He on grain surface, delaying the formation of He bubbles on grain boundaries and, therefore, the susceptibility to the He embrittlement. Hereafter we show that it is possible to reduce the grain size by an order of magnitude at the tempering stage, by combining cold working and tempering stages, acting on the tempered martensite. The microstructural observations have been carried out by means of both Scanning and Transmission Electron Microscopy (SEM and TEM). Also the positive effect of grain size reduction on the toughness of the material will be taken into account; the DBTT curves resulting from impact tests on KLST specimens show a peculiar nature, characterized by a smoother transition to the brittle regime, compared to the usual abrupt transitions achieved with the “standard” normalization & tempering treatments on RAFM steels. The outcomes of the microstructural observations, as well as the mechanical characterization (tensile, hardness and impact tests) will be discussed in this paper.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
