Change in Void Number Density with Creep Damage on SUS304 Stainless Steel Specimen with Multiple Round Notches

Abstract

304 stainless steels are widely used for high temperature components such as boiler tubes in thermal power plants. Therefore, it is important to clarify creep damage evolution process under multiaxial stress states in assessment of creep damage for actual components. In this study, creep tests were performed by using smooth specimens and notched specimens having multiple round notches with notch tip radius of 0.5mm(R0.5) and 2.0mm(R2.0) on SUS304 steel. Creep rupture times of the notched specimens were longer than those of the smooth specimens, and those of R0.5 were longer than those of R2.0. Creep damaged specimens were produced by interrupting the creep tests. Higher void number density was observed in the creep damaged smooth specimens compared with that of the CrMoV forging steel. The void number density takes the maximum value at notch root surface in R0.5, and decreased rapidly toward center of the specimen. On the other hand, distribution of the void number density almost uniform with a scatter of the values from notch root surface to center of the specimen in R2.0. Stresses and accumulated creep strain distributions at the notch root sections were clarified by finite element creep analysis. It was indicated that the distribution of the void number density depends not only on the maximum principal stress but also on accumulated creep strain. Then, a void number density prediction equation as a function of an axial stress and creep strain rate was derived. Increase of void number densities with time in the smooth and notched specimens were well predicted by using the prediction equation.