Microstructural degradation of Gr.91 steel during creep under low stress

Microstructural changes during creep at 600 °C under 70 MPa were investigated in the case of interrupted Gr.91 steel samples by taking into account the dislocation structure and Z-phase formation. The creep life monotonically increased with a decrease in the applied stress at each temperature considered in the study. However, the long-term creep life was shorter than that determined from the short-term creep data at 600 °C and 650 °C, meaning premature failure. The subgrain size gradually increased during creep up to 70,000 h, after which rapid subgrain coarsening occurred. Preferential recovery of the subgrain structure occurred around the prior-austenite grain boundary (PAGB) after 50,000 h and 70,000 h. After creep rupture, subgrain recovery was observed over the entire area of each sample. Z-phase formation was clearly visible for 30,000 h after creep. The number density of the MX particles gradually decreased after 30,000 h because of Z-phase formation. After creep rupture, the number density of the MX particles was almost the same as that of the Z-phase particles. During creep, the V content of the Z-phase gradually increased but the Nb content decreased. Changes in the chemical composition of the Z-phase occurred after a longer time in Gr.91 steel than in 12Cr steel.