Improvement of EUROFER's mechanical properties by optimized chemical compositions and thermo-mechanical treatments

Abstract

9%Cr reduced activation steels are the designated structural materials for future fusion reactors. The improvement of this class of alloys, especially for the extension of the operation limits, is the present scope of the EUROfusion materials project for advanced steels. Within this programme, new alloys are designed and fabricated to overcome some of the limitations of EUROFER97. In the present study, four 9%Cr alloys with some variations in the chemical compositions are compared to standard EUROFER97 batches. The main focus lies in the possible extension of the operation window to both higher and lower temperatures. The limits of the mechanical properties, which can be achieved through different heat and thermo-mechanical treatments, are explored within this work. The thermo-mechanical treatments used within this work consist of rolling within the austenite regime and rapid (water-) quenching to room temperature. Toughness from Charpy impact tests, creep to rupture lifetime and tensile strength are compared for the different treatments. EBSD and STEM investigations of the formed microstructures complete the presented work. Strong effects of the heat-treatments on the toughness and strength were observed. As expected, hardening the materials beyond the conventional treatment (980 °C/30 min + 750 °C/2 h) showed a major increase in tensile strength and decrease in toughness. Moderate variations in the alloy compositions open the possibility for extended temperature windows for these unconventional heat treatments (e.g. higher tempering temperature). Thermo-mechanical treatments are effective to improve hardening and other properties by modifications of the distribution of secondary phases. As expected, the different heat treatments also showed the well-known effects on the mechanical properties (e.g. hardness and strength). When compared, the effects of heat treatments on the mechanical properties are far beyond the influence of minor changes in the alloy compositions.