Effect of vanadium and niobium on creep strength in 10% chromium R steel analyzed by STEM-EDS

The effect of vanadium and niobium on creep strengthening was studied in 10% chromium steels. Eleven kinds of samples which varied the additive amounts of vanadium and niobium were prepared. From the creep deformation behavior of them, the threshold stresses were estimated. The distribution maps of precipitates were obtained by STEM-EDS. In a steel with added 0.06% vanadium where no particles were observed in the lath, creep strength was slightly increased, indicating that it was not strengthened by a particle hardening effect in the lath. Lump-shaped precipitates and film-shaped precipitates including chromium and vanadium, which were observed on lath boundaries in the steel with added vanadium, are expected to inhibit the lath boundaries from migrating. This inhibition is the mechanism of the improvement in the steel with added vanadium. In the case of steels with added niobium, creep strength was found to be increased by dispersion hardening due to fine precipitates in the lath. The threshold stress was quantitatively estimated depending on the particle spacing. The estimated threshold stress corresponded to the one obtained by the creep deformation behavior. These results revealed that vanadium and niobium each have a role in improving creep strength. In steels with added both vanadium and niobium, the effect on creep strength was expressed by the sum of the effects due to each element, which were the retardation of the lath boundary migrations and the pinning of the dislocations in the lath.