Correlation of metallurgical analysis and higher harmonic ultrasound response for long term isothermally aged crept FM steels

Abstract

Higher harmonic ultrasound response of modified 9-12Cr Ferritic-Martensitic (FM) steel, long term isothermally aged and crept up to 25,000 hours near the operating temperature of Ultra-Super Critical (USC) Thermal Power Plants (TPP), was studied. Metallurgical analysis, including quantitative measurements of dislocation density and second phase distribution, revealed that the lath conversion to subgrain, Laves phase development and coarsening of pre-existing second phase MX and $M_{23}C_{6}$, and increased ultrasound attenuation are dominant effects on $beta$ parameter evolution with thermal aging. The possibility of a higher harmonic ultrasound technique to monitor the creep damage is quite obvious, since a substantial increase of roughly 2~300% was observed of the $beta$ parameter measurement results on long term crept material in this study. The major features of $beta$ parameter variation observed of crept material was (1) roughly linear correlation with Larson Miller Parameter (LMP), (2) large offset at low LMP and (3) gradual approach of two measurement group, material degradated with and without creep stress, at high LMP. Theoretical approximation for the contribution to $beta$ parameter and measurement results of ultrasound attenuation successfully verified that the increased ultrasound attenuation is one of the major contributors to $beta$ parameter variation of the material in question, in addition to the previously well studied dislocation density and second phase distribution.