Acoustic Damping Characterization and Microstructure Evolution during Creep of a High Temperature Bolting Steel
Toshihiro OHTANI, Hirotsugu OGI and Masahiko HIRAO
Abstract:Electromagnetic acoustic resonance (EMAR) is a contactless resonant method with an electromagnetic acoustic transducer (EMAT). This method is free from extra energy losses, resulting in the measurement of intrinsic ultrasonic attenuation in solids. In this study, the EMAR was applied to detect the creep damage of a Cr-Mo-V steel, JIS-SNB16 which is used for high temperature bolt. The material was exposed to the temperature of 923K at various stresses. We measured ultrasonic attenuation for 1-7-MHz frequency range as the creep advanced. The attenuation coefficient exhibits much larger sensitivity to the damage accumulation than the velocity. The attenuation experiences a peak at around 30% and a minimum value at 50 % of the creep life, being independent of the applied stress. This novel phenomenon is interpreted as resulting from microstructure changes, especially, dislocationsf recovery. This is supported by TEM observations for dislocation structure and it can be explained by the string model of dislocation vibration. EMAR exhibited@a potential for the assessment of damage advance and the prediction of the remaining creep life of metals. Key Words:Creep damage, Cr-Mo-V steel, EMAT, Ultrasonic attenuation, Non-contact evaluation, Dislocation