Character evaluation of strength in dispersion strengthened ferritic steel

Nakashima, Hideharu*; not registered

It has been examined about mechanisms of strength anisotropy and ductility dip of ODS ferritic steel induded with YO particles at high temperatures. The results are summarized as follows. (1)Above 650C, 0.2% proof stress measured by tensile test and axial cleep rupture strength agree with the threshold stress for dislocation motion evaluated by the theory of the dispersion strengthening. But the bi-axial creep rupture stress is smaller than the threshold stress. It is presumed that grain boundaries in transverse section of cladding tube slide by the hoop stress below the threshold stress. It is considered that the strength anisotropy comes from an elongated grains formed by fabrication. (2)The ductility decreases in two different temperature ranges which are around 600C and 1100C respectively. The ductility-minimum temperature in the lower temperature range is independent of the strain rate in a high strain rate range, whereas that temperature is dependent on the strain rate in a low strain rate range. The reason why the strain rate dependence of the ductility-minimum temperature is different between high and low strin rate ranges has not been clarified. But it is suggested that the dip in the lower temperature range should be related to the grain boundary sliding and the grain-boundary precipitation. In the higher ductility-dip temperature range, this alloy has an - duplex structure. Therefore, it is considered that the ductility dip arises from the concentration of the deformation in phase.