改良9Cr-1Mo鋼鍛造材のクリープボイド発生に及ぼす多軸応力の影響

抄録

Creep damage preferentially extends at a stress concentration portion in high temperature components. Therefore, it is necessary to predict creep damage accurately at a stress concentration portion under multiaxial stress to maintain reliable operation. In this study, creep tests using a plain and round bar notch specimens with different notch radius (notch tip radius of 0.1mm (R0.1), 0.5mm (R0.5) and 2.0mm (R2.0)) of Modified 9Cr-1Mo forging steel have been conducted to clarify influence of multiaxial stress on creep damage extension process. Stress analyses of the notch specimens were carried out by finite element method. Distribution pattern of the maximum principal stress from notch root surface to specimen center was different depending on the notch root radius. Creep rupture times of round notch bar specimens were longer than that of plain specimen under the same nominal stress. Void number density decreased from notch surface to specimen center in R0.1 and R0.5, while it increased toward specimen center in R2.0. These distribution pattern of void number density corresponded to the distribution of the maximum principal stress. Although change in void number density with time for the plain specimen was successfully predicted by the former proposed simulation, overestimation was made for the notch specimens by the simulation. Therefore, calculation of void nucleation period in the simulation was modified by considering influence of multiaxial stress state on void nucleation. Eventually, it was demonstrated that distribution patterns of the void number density through notch root section in all notch specimens were quantitatively predicted by the modified simulation procedure.