Abstract
The effect of V addition on the mechanical properties of 9Cr-1Mo-V-Nb-N steel, especially creep rupture strength (CRS), has been examined and the factors that control the CRS have been discussed.
Though the specified V addition is 0.18 to 0.25 mass% in ASME SA387 Grade 91 steel, the increase in V addition from 0.15 to 0.32 mass% resulted in the decrease in high temperature proof strength (PS) and CRS. The δ ferrite is not observed even in the 0.32 mass% V containing steel. The δ ferrite is not the cause of decrease of CRS. The change of CRS should be explained by precipitation strengthening ; not only by the inter-precipitate distance but also by the size of precipitate and the coherency strain around precipitate.
The increase in V addition causes the sparse precipitation of VN. This could be the reason of the decrease in CRS by increased addition of V. The increased V addition also causes the increase in size of VN and the decrease in coherency strain around VN. Supposing the local climb model for creep deformation, these are also the factors that affect the CRS : their effects are estimated to be almost same in magnitude and to cancel each other.
