Mechanical and microstructural behavior of oxide dispersion strengthened 8Cr-2W and 8Cr-1W steels during creep deformation

Full text of publication follows: Oxide dispersion strengthened (ODS) steel is a promising candidate for fusion reactor material because of excellent mechanical properties. However, the ODS steel exhibits some defects, such as mechanical anisotropy and little elongation . To reveal details of these defects, we investigated correlations between mechanical and microstructural behavior of ODS ferritic steels during creep deformation at high temperature. The materials used in this study are two kinds of hot rolled ODS steels: Fe-8Cr-2W-0.2V-0.1Ta-0.2Ti-0.4Y2O3 (J1) and Fe-8Cr-1W-0.2Ti-0.4Y2O3 (J2). Creep tests was carried out on specimens sampling along both the rolling direction and the cross direction at 670, 700 and 730 deg. C. Microstructural analyses were made on the normalized and tempered condition by using OM, SEM, TEM and XRD. Creep ruptured and interrupted specimens were also investigated. Both J1 and J2 existed two phases, namely martensite and δ-ferrite which was elongated in the rolling direction. Y-Ti complex oxide particles were finely dispersed in martensite and δ- ferrite phases. Results of creep tests indicated that the time-to-rupture of specimens of J1 were much longer than J2, and the time-to-rupture of specimens sampling along the rolling direction were longer than cross direction. Accordingly, J1 sampling along hot rolling direction was the strongest, for instance, the time-to-rupture was 11400 h at 700 deg. C and 162 MPa. All specimens indicated that elongation was less than 1.3 % and the rupture occurred at steady state creep region from creep curves. Internal cracks were propagated in martensite phase along elongated δ-ferrite phase in the direction of hot rolling. On the other hand, δ-ferrite phases seemed to prevent combining cracks. These results suggest that elongated δ-ferrite and internal clacks in martensite strongly affect on the anisotropy and little elongation of creep. (authors)