Effect of initial microstructure on long-term creep strength properties of2.25Cr-1Mo steel

Effect of initial microstructure on the long term creep deformation propert ies of 2.25Cr-1 Mo steel has been investigated and a correlation between cr eep strength and changes in microstructure during creep deformation has bee n discussed. Three different heat treatments of (1) quenching and tempering (QT), (2) normalizing and tempering (NT) and (3) annealing (Ann) were empl oyed to obtain different initial microstructure. There was not clear differ ence in both creep rupture strength and microstructure of the creep rupture d 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 a t higher stress condition and that is distinctly different from those of th e creep ruptured NT and, Ann steels. In the lower stress conditions less th an about 100 MPa however, no difference in creep rupture strength and micro structure of those steels was observed. The differences in creep deformatio n behaviour of these steels were explained by the differences in the parame ter alpha of a modified theta projection method. It has been shown that the magnitude of or depends on the stability of microstructure during creep de formation. Disappearance of the differences in creep rupture strength of three 2.25Cr- 1Mo steels in the long term region is caused by decrease in creep strength due to microstructural change, and the common long term creep rupture stren gth 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 microstr ucture at the elevated temperature is very important to evaluate a long ter m creep strength property. It has been proposed that cu parameter of a modi fied a projection method is a candidate indicator to describe a stability o f microstructure.