高温用合金鋼ボルト材のクリープ損傷にともなう超音波減衰特性と材料微視組織の変化

抄録

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, dislocations’ recovery. This is supported by TEM observations for dislocation structure. EMAR exhibited a potential for the assessment of damage advance and the prediction of the remaining creep life of metals.