Abstract
Effect of initial microstructure on the long term creep deformation properties of 2.25Cr-1Mo steel has been investigated and a correlation between creep strength and changes in microstructure during creep deformation has been discussed. Three different heat treatments of(1)quenching and tempering (QT), (2) normalizing and tempering (NT) and (3) annealing (Ann) were employed to obtain different initial microstructure. There was not clear difference in both creep rupture strength and microstructure of the creep ruptured specimens for NT and Ann steels. Creep strength of the QT steel is higher than those of NT and Ann steels at high stress and short term conditions. Very high dislocation density was observed on the creep ruptured QT steel at higher stress condition and that is distinctly different from those of the creep ruptured NT and Ann steels. In the lower stress conditions less than about 100 MPa, however, no difference in creep rupture strength and microstructure of those steels was observed. The differences in creep deformation behaviour of these steels were explained by the differences in the parameterα of a modified θ projection method. It has been shown that the magnitude of α depends on the stability of microstructure during creep deformation.
Disappearance of the differences in creep rupture strength of three 2.25Cr-1Mo steels in the long term region is caused by decrease increep strength due to microstructural change, and the common long term creep rupture strength for three steels is the inherent creep strength of the 2.25Cr-1Mo steel. It has been concluded that a proper assessment on a stability of microstructure at the elevated temperature is very important to evaluate a long term creep strength property. It has been proposed that α parameter of a modified θ projection method is a candidate indicator to describe a stability of microstructure.
