Session: Research Posters

Paper Number: 119214

119214 - Creep Void Nucleation and Growth Simulation Under Multiaxial Stress for Modified 9cr-1mo Forging Steel 

New modified 9Cr-1Mo forging steel has been produced by the latest forging method as an advanced heat resistant material for high temperature components such as steam turbine rotors. Creep damage preferentially extends at a stress concentration portion in high temperature components. Therefore, it is necessary to predict creep damage evolution at the portion under multiaxial stress to maintain reliable operation. In this study, creep tests using a plain specimen and round notch bar specimens with three kinds of notch radius (notch tip radius of 0.1mm (R0.1), 0.5mm (R0.5) and 2.0mm (R2.0)) on the Modified 9Cr-1Mo forging steel have been conducted to clarify effect of multiaxial stress on creep damage extension process. Different levels of creep damaged specimens were produced by interrupting the creep tests at certain test periods. 

As a result of creep tests, creep rupture times of round notch bar specimens were longer than that of plain specimens under the same nominal stress showing notch strengthening. Creep rupture times of notch specimens increased in order of higher elastic stress concentration coefficient. In order to clarify creep damage extension process, creep void observation was made at around center of the creep damaged plain specimens and through notch root section of the creep damaged notch specimens by a scanning electron microscope. Distribution pattern of measured void number density from notch root surface to specimen center was different depending on the notch radius. The void number density near notch surface is higher than that around the specimen center in R0.1 and R0.5, while it shows the minimum value at notch root surface and gradually increases toward specimen center in R2.0. The void number density becomes higher as creep damage increases in both plain and notch specimens.

Stress distribution of the notch specimens was calculated by finite element creep analysis to clarify relationship between stress and creep damage. Distribution pattern of the maximum principal stress from notch root surface to specimen center in the notch specimens corresponds to that of the void number density. Although applied loading direction is uniaxial along to specimen axis, triaxial tension stress occurred through notch root section resulting in higher void number density in the notch specimens than that in the plain specimens at the same creep damage rate. Axial creep strain distribution through notch root in the notch specimens is smaller than that in the plain specimen due to triaxial tensile stress state causing longer rupture time of the notch specimens than that of the plain specimens.

The void growth simulation method developed by the author was applied to predict the void number density under multiaxial stress state in the notch specimens. Although change in void number density with time for the plain specimen was successfully predicted by the simulation, overestimation was made for the notch specimens by the simulation. Therefore, a calculation equation of void nucleation period in the simulation was modified by considering influence of triaxial 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.

Presenting Author: Teppei Noguchi Chiba institute of technology

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

Authors:

Teppei Noguchi Chiba institute of technology
Takashi Ogata Chiba institute of technology