Microstructure Refinement and mechanical properties improvement of EUROFER97 steel

EUROFER97 steel for nuclear fusion applications are usually manufactured by hot rolling and subsequent heat treatments: austenitization at 980 °C for 30 minutes, rapid cooling and tempering at 760 °C for 90 minutes. The aim of this work is to refine the microstructure of traditional EUROFER97 steel by means of a thermo-mechanical treatment, in order to improve its mechanical properties without losing ductility. Starting from the traditional treatment conditions, the steel has undergone a cold rolling process and an annealing treatment, with the aim of studying its effects on the microstructure and mechanical properties. The experimentation was conducted on 40 samples, combining 5 different cold reduction ratios (CR: 20%, 40%, 50%, 60%, 80%) and 8 different heat treatment temperatures in the range of 400 ° C to 750 ° C (ΔT: 50 ° C) with a soaking time of 1 hour. For each treatment condition, Vickers hardness measurements, X-ray diffraction and microstructural characterization by optical and scanning electron microscopy were performed. Furthermore, the preliminary results of tensile tests on the samples deformed with the highest reduction ratio (CR: 80%) are reported. Following primary recrystallization, a microstructure with equiaxed grains of sub-micrometric size is observed. In particular, for the sample with CR: 80%, after heat treatment at 650 ° C a high microstructural refinement is achieved which guarantees the strengthening with ductility comparable to the traditional condition of steel. The work has shown the possibility of strengthening EUROFER97 steel without compromising its ductility.