We investigated the evolutions of two nonlinear acoustic characterizations: resonant frequency shift and three-wave mixing, with electromagnetic acoustic resonance (EMAR) throughout the creep life in the welded joints of ASME Grade 91, one of high Cr ferritic heat resisting steels. EMAR is a combination of the resonant acoustic technique with a non-contact electromagnetic acoustic transducer (EMAT). We used bulk- shear-wave EMAT, which transmits and receives shear wave propagating in thickness direction of a plate specimen. Creep tests of thick welded joints specimens were carried out at 873 K, and 90 MPa and interrupted at several time steps. The two nonlinear acoustic parameters and ultrasonic attenuation decreased from the start to 30% of creep life. After that they rapidly increased from 40% of creep life to rupture. We interpreted these phenomena in terms of dislocation recovery, recrystallization, and restructuring and the initiation and growth of creep void, with support from the SEM and TEM observation.
