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The paper discusses the use of ultrasonic spectral analysis for microstructural characterization in AISI type 316 stainless steel and modified 9Cr–1Mo ferritic steel (T91/P91). Several specimens of AISI type 316 stainless steel have been heat treated at different temperatures varying from 1373 to 1623K. for different time durations (from 15 to 120min) to obtain specimens with different grain sizes. Spectral analysis has been carried out on the rf signal corresponding to the first back-wall echo obtained with a 25 MHz delay line longitudinal beam transducer. The shift in the peak frequency and the change in the full width at half-maximum of the autopower spectrum have been correlated with the average grain size and yield stress. Various specimens of modified 9Cr–1Mo ferritic steel have also been subjected to a series of heat treatments consisting of soaking for 5min at different temperatures in the range 1073 …