Comparative analysis of the long-term strength of Russian ferritic-martensitic reactor steels

The paper presents the results of long-term high-temperature creep tests of Russian reactor steels with ferritic-martensitic structure (the duration of some measurements exceeded 8 years). In the current study, the structural-phase transformations, characteristics of creep and long-term strength at 650 °C, 670 °C, and 700 °C under 60–140 MPa in oxide-free and oxide containing steels were determined. The creep tests were performed on specially designed transverse micro-specimens prepared from fuel elements cladding used in the fast-neutron reactor. The creep velocity of the ferritic-martensitic reactor steels was established to be specified by resistance of lath martensite and ferrite structures to diffusion processes of return and recrystallization. The most heat-resistant oxide-free steel contains the largest amount of refractory elements and carbides. The best heat resistance was observed for the steel hardened with thermal-resistant yttrium-titanium nanooxides. The samples made of this steel demonstrated one order less creep velocity at 700 °C under 100 MPa and 100-fold time to fracture in comparison with the oxide-free reactor steels.