High Temperature Mechanical Properties and Microstructure in 9Cr or 12Cr Oxide Dispersion Strengthened Steels

Oxide Dispersion Strengthened (ODS) ferritic steel, a candidate material for fast reactor fuel cladding, has low thermal expansion, good thermal conductivity, and excellent resistance to irradiation damage and high temperature strength. The origin of the excellent high-temperature strength lies in the dispersion of fine oxides. In this study, creep tests at 700 degrees C or 750 degrees C, which are close to the operating temperatures of fast reactors, and high-temperature tensile tests at 900 degrees C to 1350 degrees C, which simulate accident conditions, were conducted on 9Cr ODS ferritic steels, Mll and MP23, and 12Cr ODS ferritic steel, F14, to confirm the growth behavior of oxides. In the M11 and F14 creep test samples, there was little oxide growth or decrease in number density from the initial state, indicating that dispersion strengthening by oxides was effective during deformation. After creep deformation of F14, the development of dislocation substructures such as dislocation walls and subgrain boundaries was hardly observed, and mobile dislocations were homogeneously distributed in the grains. The dislocation density increased with increasing stress during the creep test. In the high -temperature ring tensile tests of MP23 and F14, the strength of both steels decreased at higher temperatures. In MP23, elongation decreased with increasing test temperature from 900 degrees C to 1100 degrees C, but increased at 1200 degrees C, decreased drastically at 1250 degrees C, and increased again at 1300 degrees C. In F14, elongation decreased with increasing temperature. It was inferred that the formation of the delta-ferrite phase was responsible for this complex change in mechanical properties of MP23 from 1200 to 1300 degrees C.