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Fracture Behavior of EUROFER´97 Steel After Thermal Ageing

Ferritic-martensitic reduced activation steel Eurofer 97 is candidate structural material for in-vessel components of proposed fusion reactors. The use of the steel is limited up to a temperature about 550 °C. On the other hand the efficiency enhancement of the fusion reactors to the level suitable for energy production is predetermined by an increase of temperature in reactor. The long term exposition of the steel at high temperatures leads to microstructural changes. The aim of the work was to investigate the influence of short and long term thermal ageing on fracture properties of Eurofer 97 steel. Short term thermal ageing of the steel was simulated by step-cooling treatment and long term thermal ageing was simulated by isothermal annealing treatment at 550 °C/5000 h. Charpy impact tests were applied to compare the fracture resistance of the sheet before and after thermal ageing. No evident changes in impact properties have been registered comparing the properties of the steel in as-received state and in state after step-cooling. Outstanding embrittlement of the thermally aged steel was observed after long term isothermal ageing. Introduction Ferritic-martensitic steels of (9-12)Cr-1Mo(W-Ni-V) type were developed for high temperature applications in nuclear power industry. These steels are nowadays under consideration as structural materials for perspective power sources: (i) fission reactors based on U a Th cycles [1], (ii) acceleration driven systems [2] and (iii) fusion reactors with magnetic plasma confining [3]. Several versions of RAFM (reduced activation ferritic martensitic) steel of (9-12)Cr-X type have been developed in Europe [4,5], Japan [6,7], USA [1] and China [8]. The most advanced version, the 9Cr-1W(V-Ta) Eurofer 97 steel, was selected as a structural material for DEMO fusion power plant prototype in-vessel and first wall components (outboard blanket) [4]. Operational embrittlement and subsequent brittle behaviour of the steels used in nuclear power industry is affected by synergy of two main influences: thermal and radiation damage. Radiation damage of the Eurofer 97 steel and elimination of its effect on mechanical properties is broadly studied as a preferred problem. There are also microstructural changes connected with operation temperatures (considered in the range 550 °C – 650 °C) such as carbidic reactions, grain coarsening and grain boundary embrittlement [9,10]. Detailed understanding of failure mechanisms of Eurofer 97 steel in as-received thermally unaffected state and state after thermal ageing is critical for its proposed future applications. The subgrain structure of Eurofer 97 steel in the as-received state composed of martensite laths of thickness ca. 500±200 nm. Also two main populations of carbides were identified in microstructure: Cr-rich carbides of the type M23C6, predominantly located on boundaries of prior austenite grains and martensite laths, and Ta and V-rich carbides of MX type located particularly inside of martensite laths [11]. Outstanding microstructural changes of Eurofer 97 steel after thermal ageing have been described, even at the temperatures lower than considered for application [10,12] (see Fig. 1). Coarsening of martensite laths, coarsening of carbides and segregation of P to grain boundaries was observed in particular already after 1000 h thermal ageing at 500 °C and