Effects of the initial microstructure and its changes on creep deformation behaviour of a 1Cr-0.5Mo steel have been investigated, in order to understand a mechanism of complex creep deformation behaviour which shows several local minima in creep rate. Sigmoidal inflections are observed on stress vs. time to rupture curves and stress vs. minimum creep rate curves at 823 and 873K. Under the stress conditions lower than 100MPa, creep rate vs. time curves indicate inflection at the same time of a tertiary creep stage.
Significant decrease in creep rupture strength due to prior ageing for 500h at 873K has been observed at stress conditions higher than 100MPa. However, no difference in creep rupture strength between pre-aged and un-aged steels has been observed in the stress conditions lower than 100MPa. The inflection of the creep rate vs. time curves has disappeared by ageing prior to creep test.
High density of dislocations and many fine carbide particles are observed within ferrite grain of the un-aged steel. On the other hand, coarse needle-like carbides and very low density of dislocations are observed within ferrite grain of the pre-aged steel. Because of the similar decrease in the number of dislocations and precipitate occurring in the un-aged steel during creep deformation, no difference in microstructural morphology is observed for the un-aged and pre-aged steels after creep for 200h at 873K-88MPa. It has been considered that the effect of precipitation strengthening have disappeared during ageing for 500 h at 873K prior to creep test and creep deformation for 200h at 873K-88MPa.
It has been concluded that complex creep deformation behaviour and sigmoidal inflection of stress vs. time to rupture curve are caused by decrease in creep strength due to microstructural change followed by advent of inherent creep strength.