Correlation between creep properties and microstructure of reduced activation ferritic/martensitic steels

Reduced activation ferritic steels (RAFs), JLF-1 (9Cr-2W-V,Ta steel) had been studied as one of the reference materials in the IEA RAF R & D activity, because of its superior resistance to high-fluence neutron irradiation up to 100 dpa. Recently, JLS-series (9Cr-xW-V,Ta steels; x=2.5, 3.0 and 3.5) have been produced as options for RAFs for use at higher temperatures in advanced blanket systems. In this series, improved high temperature mechanical properties were intended by increased tungsten contents. The creep properties improved with the tungsten contents. A microstructural examination after creep rapture test revealed differences with a increase of tungsten content from 2.0 (JLF-1) to 3.0 wt% (JLS-2). According to the examination of tested specimens (100 MPa, 700°C), more intragranular fine precipitates and more intergranular precipitates formed preferentially on prior austenite boundaries and martensite lath boundaries were found in JLS-2 than in JLF-1, martensite lath widths became narrower and dislocation density increased with the increase of tungsten content. Such microstructural features are likely to improve the creep properties by inhibiting grain evolution, microstructural recovery and strengthening grain boundaries at elevated temperatures.