Heterogeneous changes in microstructure and degradation behaviour of 9Cr-1Mo-V-Nb steel during long term creep

Creep deformation behaviour and microstructural change have been investigated on 9Cr-1Mo-V-Nb steel (ASME SA-213 T91) in order to understand a degradation mechanism at the elevated temperatures, especially at the long term condition. Remarkable decrease in creep rupture strength at 873K and 923K have been observed at the long term region longer than about 10,000h. Under the higher stress and short term region, homogeneous progress in recovery of tempered martensitic structure, such as increase in lath width and coarsening of subgrain, has been observed. However, remarkable progress in such recovery of the microstructure has been observed only at the vicinity of prior austenite grain boundary under the lower stress and long term region. It has been found that such preferential recovery along a prior austenite grain boundary promotes the beginning of tertiary creep. It has been concluded that preferential recovery at the vicinity of prior austenite grain boundary is a main factor of degradation during long term creep deformation.