Long-term creep deformation characteristics of advanced ferritic steels for USC power plants

Creep rupture and deformation behavior of T122-type steels with different matrix phases such as α  ′ (martensite) and α′+δα′+δ (martensite and delta-ferrite) at different stress levels has been studied using creep testing and a detailed FE-STEM microscopy. Long-term creep rupture strength of the dual phase steel is found to be lower than that expected from the short-term creep testing. Fine grain microstructure enhances the creep deformation of the dual phase steel at lower stress region, but is not the major factor on the degradation. Decrease in fine MX and unequal distribution of MX in the ferrite matrix are found to be the major causes of the strength degradation in the dual phase steel with the higher Cr content. It is thus concluded that the heterogeneous creep deformation is much more pronounced at lower stress level in the dual phase steel, which is due to inhomogeneous microstructure consisting of α′+δα′+δ phase matrix and the relevant heterogeneous distribution of fine precipitates such as MX and M23C6 in the δ  -ferrite matrix and near the interface between δ/γδ/γ.