Effect of thermal ageing on the impact fracture behaviour of Eurofer’97 steel Hynek Hadraba * , Ivo Dlouhy Institute of Physics of Materials, Academy of Sciences of the Czech Republic, Zizkova 22, 616 62 Brno, Czech Republic article info PACS: 62.20.Mk abstract 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 thermal ageing on fracture properties of Eurofer’97 steel. Thermal ageing of the steel was simulated by step cooling treatment. Charpy impact tests were performed before and after thermal age- ing. No evident changes in impact properties have been registered when comparing the properties of the steel in as-received state and in state after step cooling. Ó 2009 Elsevier B.V. All rights reserved. 1. 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 235 Ua 232 Th cycles [1], (ii) acceleration driven systems [2] and (iii) fusion reactors with magnetic plasma confin- ing [3]. Several versions of RAFM (reduced activation ferritic–mar- tensitic) 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 ver- sion, the 9Cr–1 W(V–Ta) Eurofer’97 steel, was selected as a struc- tural 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 550–650 °C) such as carbi- dic reactions, grain coarsening and grain boundary embrittlement [9,10]. Detailed understanding of failure mechanism of Eurofer’97 steel in as-received thermally unaffected state and state after ther- mal 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. 200 nm. Also two main populations of carbides were identified in microstruc- ture: Cr-rich carbides of the type M 23 C 6 , 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]. 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 600 °C. It seems that segregation of P on grain boundary was connected with coars- ening of carbides. Carbide coarsening caused Cr depletion of grain boundaries and allowed subsequent P segregation. The steel was also after 10 000 h of thermal ageing partially recrystallized to equiaxed ferrite grains of diameter ca. 1 lm decorated by coarse (up to 350 nm) M 23 C 6 carbides. The temperature dependence of impact energy for 14 mm sheet of Eurofer’97 in the as-received state including scatter band of data collected from literature [10,13–15] is given in Fig. 1. Data repre- senting the influence of long-term thermal ageing on impact en- ergy of 14 mm Eurofer’97 steel published in literature [10,12] are also compiled in Fig. 1. It is evident, that relatively short-term ther- mal ageing 5000 h at 500 °C and 600 °C led to t DBTT shift ca. +10 °C and +15 °C, respectively (t DBTT was À65 °C for 14 mm sheet in as- received state). The long-term thermal ageing at 10 000 h at 500 °C and 600 °C led to t DBTT shift ca. +20 °C and +25 °C, respec- tively. The slight t DBTT shift after short-term thermal ageing at 5000 h and the evident t DBTT shift after long-term thermal ageing at 10 000 h were explained just by partial recrystallization of martensite matrix to the ferrite grains [10,12]. The effect of P seg- regation observed already after short-term (1000 h) ageing was not described in literature, whereas this mechanism could lead to anomalous fracture behaviour. Grain boundary impurities 0022-3115/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2008.12.319 * Corresponding author. Tel.: +420 532 290 369; fax: +420 541 218 657. E-mail address: hadraba@ipm.cz (H. Hadraba). Journal of Nuclear Materials 386–388 (2009) 564–568 Contents lists available at ScienceDirect Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat