Evaluation of creep properties of reduced activation ferritic steels

Reduced activation ferritic steels (RAFs) are among the leading candidates for structural materials of fusion reactors. These structural materials are required to have both good fracture toughness and creep properties. In order to attain these properties, Ta is added to RAFs. Ta is not typically present in general heat-resistant steels, however, and Ta behavior in steels has not been clarified in detail. In this study, the basic creep properties of RAFs such as F82H pre-IEA heat, F82H IEA heat, F82H mod3 and JLF-1 HFIR heat were examined with an emphasis on the effects of Ta. Possible effects could include a large prior austenite grain (PAG) size, F82H pre-IEA heat, which has the same major composition as F82H IEA heat but with finer PAGs, or a lower creep strength. In the low-stress range, there was no significant difference in the creep strengths among all of the RAFs. This could be caused by differences on the stress dependence of the minimum creep strain rate, as it did not change at every stress level for the F82H IEA heat but did increase in the low-stress range for the F82H mod3 and JLF-1 HFIR heat. This difference in the F82H IEA heat behavior could be explained by the presence of Ta(C, N), as the XRD analyses on the extraction residue suggested there was no Ta(C, N) formed in the F82H IEA heat.