Session: Research Posters

Paper Number: 119690

119690 - Effect of Multiaxial Stress State on Creep Rupture Strength of Cc and Ds Ni-Based Superalloy Rene80 

 Conventional Casting (CC) and Directional Solidification Casting (DS) Ni based superalloys Rene80 are used for advanced gas turbine blades in combined-cycle power plants. Creep damage preferentially proceeds at stress concentration portions of the blades under multiaxial stress condition resulting in crack initiation. Therefore, it is important to clarify creep damage extension process under multiaxial stress state, and to establish a life prediction method for reliable operation.

In this study, CC and DS Rene80s were used as tested materials, and creep test specimens, which are smooth bar specimens and round notch bar specimens with notch tip radius of 0.5 mm (R0.5) and 2.0 mm (R2.0), were machined from CC and DS. Creep tests for these specimens were carried out until failure as well as interrupted at certain time to produce creep damaged samples whose microstructures were observed by a scanning electron microscope. Stress and strain distribution of the notch specimens under creep condition were calculated by finite element analysis. Crystal miss-orientation measurement for the creep damaged samples was also conducted by using an Electron Backscatter Diffraction (EBSD) method. Based on these findings, the damage process and the relationship between the stress states and creep damage were discussed.      

Creep rupture times of plain specimens of DS were longer than those of CC due to lack of grain boundaries normal to loading direction in DS showing higher rupture ductility. In both CC and DS, creep rupture time became longer in order of smooth, R2.0, R0.5 under the same nominal stress condition. A finite element analysis was carried out to clarify the factors showing such a tendency. From the finite element analysis, notch specimens are subjected to triaxial tensile stress through notch root section even under uniaxial tension applied loading causing suppression of axial creep strain compared to the smooth specimen. In the creep damaged samples of CC, creep voids were observed on grain boundaries causing grain boundary failure with relatively lower failure ductility. Location of the maximum void length in creep damaged samples of CC notch specimens corresponded to the maximum stress portion in the notch root section. On the other hand, no creep void was observed in creep damaged samples of DS because of alignment of grain boundaries parallel to the loading direction giving higher failure ductility. Therefore, since DS specimens have higher ductility under high temperature than CC specimens, it is important to clear the progression of creep deformation in order to evaluate the creep damage state under multiaxial stress.

 From EBSD measurement of creep damaged samples, Grain Reference Orientation Deviation (GROD) values increased with increasing creep damage showing higher GROD values at around notch root surface in both CC and DS. Change in GROD average value through the notch root section corresponded to axial creep strain distribution obtained from the analysis indicating that accumulated creep strain in both CC and DS is possibly predicted by GROD average measured by EBSD.

  Creep rupture life prediction methods of notch specimens were also studied based on stress analysis results. As Mises equivalent stress and the maximum principal stress distributed unevenly along notch root section, the maximum values were used to predict rupture time through stress rupture data obtained by smooth specimens. However accurate life prediction was not made by these stresses. Then area average creep damage concept was proposed. In this concept, creep rupture time is predicted based on average creep damage through notch root section calculated from finite element analysis results of a notch specimen. It was demonstrated that creep rupture times of both CC and DS were successfully predicted by the proposed method.

Presenting Author: Toshiki Kamada Chiba institute of technology

Presenting Author Biography: Undergraduate degree in mechanical engineering at Chiba Institute of Technology

Authors:

Toshiki Kamada Chiba institute of technology
Takashi Ogata Chiba institute of technology