Degradation of Mod.9Cr-1Mo steel during long-term creep deformation

Long term creep strength property of a Mod.9Cr-1Mo steel has been investiga ted in conjunction with changes in microstructure, in order to understand a degradation mechanism at the long term conditions. The predicted creep rup ture lives from short term creep deformation data whose rupture lives are l ess than 1000 h, are consistent with the experimental data up to about 1000 0h, In the long term region longer than about 10000 h, however, creep ruptu re lives become shorter than the predicted one with increase in time to rup ture. In the specimens creep ruptured at short term region less than about 10000 h, homogeneous progress in recovery of tempered martensitic microstru cture, such as increase in lath width and coarsening of subgrain, has been observed. On the other hand, remarkable progress in such recovery of the mi crostructure has been observed at the vicinity of prior austenite grain bou ndary for the specimen creep ruptured at long term region under the low str esses, Such preferential recovery along a prior austenite grain boundary ca n promote the beginning of tertiary creep under the low stress condition. I t has been concluded that preferential recovery along a prior austenite gra in boundary is a main factor of a degradation of the Mod.9Cr-1Mo steel duri ng long term creep deformation.