Micro-Structural Monitoring of 2.25Cr-1Mo Steel by Surface SH Wave

The feasibility of SH (shear horizontal) waves for monitoring thermally degraded materials is explored. It turns out that the use of SH waves leads to a promising nondestructive technique for the purpose of microstructure evaluation and material characterization of such materials. This is because SH modes can interact with entire surface and subsurface area of specimen that is mainly exposed to severe thermal environment, while a conventional point-by-point technique is confined to just local investigations, underneath a transducer. Consequently, SH modes’ data can show a better sensitivity and provide us with various features for thermal damage evaluation, compared to ones of local inspection, which results in the enhancement of experimental reliability. 2.25Cr-1Mo steel specimens for various degradation levels were prepared by isothermal aging heat treatment at 650° and evaluated by the present technique to investigate the influence of thermal damage to the SH wave feature based on the attenuation coefficients. Discussed is monitoring for thermally degraded structural materials used at high temperature by the surface SH-wave propagating along near surface and subsurface layer with horizontal displacement polarization. Because of the carbide precipitation increase and spheroidization near the grain boundary of a microstructure to aging degradation, the attenuation coefficient had a tendency to increase as degradation proceeded. It was identified possibly to evaluate degradation using the characteristics of the attenuation coefficient and amplitude spectra through Fast Fourier Transform (FFT) analysis. Frequency dependence of the ultrasonic attenuation coefficient to aging degradation appeared large, which made sure that the attenuation coefficient can be an important parameter for the evaluation of aging.