OPTIFER, a further step in development of Low Activation Martensitic Steels. Results of Characterization Experiments

Within the framework of the development of low activation structural materials to be used in nuclear fusion reactors four martensitic Fe-9,5 Cr alloys were conceived with different contents of tungsten-tantalum and/or germanium as substitutions for Mo, Ni, Nb and Al. As a result of recent activation calculations, the maximum concentrations of all accompanying elements, which are not desirable under radiological aspects, were determined for the first time for these OPTIFER steels, and laid down in specifications for the manufacturers of the alloys, after double-vacuum melting, only the real alloys with some of these accompanying elements added are within the specifications. For the majority of alloys the gap between request in radiological terms and the metallurgical/analytical reality is still considerable. The behavior during transformation and heat treatment roughly corresponds to that of conventional martensitic 9-12%Cr steels. Progress has been conspicuous as regards the notch impact toughness behavior, both at upper shelf level and in ductile brittle transition (DBTT) the W(Ce) alloyed OPTIFER variant exhibits more favorable values than the conventional MANET-II steel from the fusion program, with better strength characteristics above 500 degree centigree. With only a moderate decrease in strength values (compared to MANET-II), the Ge (Ce) variant excels by a distinct improvement in notch impact toughness values and, theoretically, a stronger reduction in dose rate than the W(Ce) variant and comes close to the decay curve of pure iron. (Author) 21 refs