# Grain Refinement and Improved Mechanical Properties of EUROFER97 by Thermo-Mechanical Treatments
###### Abstract  
EUROFER97 steel plates for nuclear fusion applications are usually manufactured by hot rolling and subsequent heat treatments: (1) austenitization at 980 \({}^{\circ}\)C for 30 min, (2) rapid cooling and (3) tempering at 760 \({}^{\circ}\)C for 90 min. An extended experimental campaign was carried out with the scope of improving the strength of the steel without a loss of ductility. Forty groups of samples were prepared by combining cold rolling with five cold reduction ratios (20, 40, 50, 60 and 80%) and heat treatments at eight different temperatures in the range 400-750 \({}^{\circ}\)C (steps of 50 \({}^{\circ}\)C). This work reports preliminary results regarding the microstructure and mechanical properties of all the cold-rolled samples and the effects of heat treatments on the samples deformed with the greater CR ratio (80%). The strength of deformed samples decreased as heat treatment temperature increased and the change was more pronounced in the samples cold-rolled with greater CR ratios. After heat treatments at temperature up to 600 \({}^{\circ}\)C yield stress (YS) and ultimate tensile strength (UTS) of samples deformed with CR ratio of 80% were significantly larger than those of standard EUROFER97 but ductility was lower. On the contrary, the treatment at 650 \({}^{\circ}\)C produced a fully recrystallized structure with sub-micrometric grains which guarantees higher strength and comparable ductility. The work demonstrated that EUROFER97 steel can be strengthened without compromising its ductility; the most effective process parameters will be identified by completing the analyses on all the prepared samples. EUROFER97; RAFM steels; thermo-mechanical treatments; microstructure; dislocation density; recrystallization +
Footnote †: journal: _Sci._  
0000-0002-0001-9980-0002-  
In particular, Mo was replaced with W and Nb with Ta and V. Likewise, the concentration of Ni, Cu, Al, Co and other impurity elements, with high long-term radioactivity, was kept as low as possible [12; 13; 14]. EUROFER97 steel is the reference RAFM steel in Europe for test blanket modules (TBMs) in the ITER reactor [14; 15; 16], as well as a possible candidate material for the first wall in the DEMO reactor and other highly stressed sections as a blanket, vessel and divertor [17; 18; 19; 20]. It is produced through a hot-rolling process followed by three heat treatment steps [21]: (1) austenitization at 980 \({}^{\circ}\)C for 30 min, (2) rapid cooling in air and (3) tempering at 760 \({}^{\circ}\)C for 90 min; henceforth, this treatment condition will be referred to as \"standard\" in comparison to the variants tested during the present activity. EUROFER97 exhibits good weldability and the original microstructural and mechanical characteristics in molten and heat affected zones can be in large part recovered after suitable post-welding heat treatments [22]. Under irradiation EUROFER97 steel has good mechanical performances in the temperature range between 350 \({}^{\circ}\)C and 550 \({}^{\circ}\)C [23; 24; 25] and several studies have been performed aimed to extend the operating temperature range. A possible solution to raise the maximum operating temperature, above 550 \({}^{\circ}\)C, is a variant of EUROFER97 steel reinforced by dispersion of oxide (ODS) [26]. EUROFER97-ODS is produced by mechanical alloying of yttrium oxide powders (Y\({}_{2}\)O\({}_{3}\) at 0.3 wt.%) [27; 28]. The presence of nano-oxide, finely dispersed in the microstructure, inhibits grain boundary sliding and tends to retard grain growth at high temperature. In this way, the improved thermal stability leads the operating temperature to increase up to about 600-650 \({}^{\circ}\)C [29; 30; 31; 32; 33]. Moreover, the dispersion of oxide nanoparticles strengthens the steel [34] and reduces the irradiation swelling because it acts as a trap for the defects induced by neutron radiation.