Response to the thermomechanical treatment of EUROFER97 steel: microstructure and mechanical properties

EUROFER97 steel is the European reference steel to manufacture structural parts in future nuclear fusion reactors. In this work, an extensive experimental campaign was carried out aiming to improve strength without reducing ductility. The main idea behind the present study is to achieve such result through grain refinement obtained by cold rolling and heat treatment. The combination of five cold reduction (CR) ratios (20%, 40%, 50%, 60% and 80%) and eight heat treatment temperatures from 400°C to 750°C, for 1 hour, have been examined. The treatment consisting of CR = 80% and heating at 650 °C induces a fully recrystallized structure with sub-micrometric grains (average grain size 600 ± 60 nm). This microstructure guarantees an increase of the yield stress and ultimate tensile strength of 18% and 5%, respectively, and a uniform elongation nearly double than that of standard EUROFER97 steel. Furthermore, the results obtained from high cycle fatigue tests evidenced a similar behavior.