Effects of heat treatment conditions on the microstructure and impact properties of EUROFER 97 ODS steel

Abstract

Probably the most important range of materials to consider for the blanket material in the tokamak design for fusion reactors such as ITER and DEMO is the high alloy Fe9Cr oxide dispersion strengthened (ODS) ferritic steels. These steels possess exceptional thermal conductivity and low thermal expansion while being strongly resistant to void swelling. Their main drawback is the high ductile-to-brittle transition temperature (DBTT), particularly in the ODS versions of the material. This paper describes attempts that are being made to reduce this DBTT in as yet unirradiated materials by a novel heat treatment procedure. The principle behind this approach is that low DBTT in the unirradiated materials will lead to relatively low DBTT even in He-containing material that has been irradiated with fusion blanket-type irradiations. New batches of high alloy Fe9Cr ODS (EUROFER) ferritic steel have been produced by a powder metallurgical route, and relatively homogeneous material has been produced by a hot isostatic pressing procedure. Mini-Charpy test specimens were made from materials that had been subjected to a matrix of heat treatments designed to show up variations in solution treatment (ST) temperature, cooling rate from the ST temperature and tempering treatment. The initial DBTT was in the range 150-200 • C. Extremely interesting results have been obtained. DBTT downward shifts of up to 200 • C have been observed by using a high 1300 • C ST temperature and a low cooling rate. The paper goes on to describe the microstructure of this material, and discusses the possible microstructural factors needed to produce these very high DBTT downward shifts. Low dissolved carbon and higher proportions of low-angle grain boundaries seem to provide the key to the understanding of the alloy behaviour.

Surface modification and deuterium retention in reduced-activation steels under low-energy deuterium plasma exposure. Part I: undamaged steels

“…Randomly distributed regions of large ferritic grains retained after the heat treatments as it was also mentioned in [20]. Transmission electron microscopy (TEM) analysis confirms the microstructural analysis suggested by SEM, showing ferritic matrix with carbides precipitated at grain boundaries (GBs) and within the grains in all the heattreated samples [23].…”

“…The mechanical and physical properties, the effect of alloying and the effect of heat treatments on Eurofer variants have been widely studied [4,[20][21][22][23]. The heat treatment during manufacturing was the following.…”

“…The mean grain size obtained by SEM is typically from one to ten µm and does not noticeably change with heat treatment up to 1000 K. The microstructure of Eurofer steel is ferritic/martensitic with precipitates both at grain boundaries and to a lesser extent within the grains. The elemental analysis suggests that the precipitates are M 23 C 6 [20,23] which can contain tantalum, tungsten, chromium, vanadium, etc. The density of precipitates on the grain boundaries increases after annealing up to 1000 K. We found some Ta-rich carbides, W-rich carbides and few V-rich carbides after annealing of Eurofer at 1000 K. This is in agreement with [20].…”