Microstructural evolution during creep test in 9Cr-2W-V-Ta steels and 9Cr-1 Mo-V-Nb steels

In order to clarify the mechanism of remarkable creep strength of the 9Cr-2W-0.2V-Ta steels, high-Cr heat-resistant steels with Mo replaced by W and Nb by Ta for reducing induced radioactivity, the microstructural evolution during creep tests has been studied and compared with that in conventional 9Cr-1Mo-0.2V-Nb steels. Particular attention was called to quantitative analysis of solute W and Mo contents evaluated with the EDX spectra measured with the thin foil in the FE-TEM analysis. It has been supposed that the martensitic microstructure and fine MX-type precipitates have no significant effect on the difference in creep properties between the W-containing steel and the Mo-containing steel. Moreover the creep strength of the W-containing steel can not be attributed to the larger solid solution hardening by W. Mo distribution may encourage creep strain localization in solid solution, which results in the deterioration of creep strength of Mo-containing steels. Comparing M23C6 and Laves phase, the latter is assumed to be the main governing factor.