On evaluating the state of steel specimens using dynamic spectrograms of acoustic signals

The main purpose of this work was to study the possibility of estimating the changes in properties of bearing steel during heat treatment by visualization using dynamic spectrograms (sonograms) that display changes in the spectral characteristics of acoustic signals within time. The changes of spectral characteristics of the point impact acoustic response in the steel specimens were studied depending on the microstructure changes generated in the specimens using heat treatment with various parameters. The analysis and comparison of acoustic response signals were performed using software spectrum analyzer. The informativeness of the applied technique aimed at estimating changes in the microstructure of steel as a result of heat treatment is shown. The analysis of dynamic spectrograms allowed revealing indicative spectral features, such as shifting in the resonant frequencies and change in the damping decrement. A clear dependence of the shift of the resonant frequencies and the damping decrement of the signal on the tempering temperature of the quenched ball-bearing steel specimens was found. After the tempered specimens were re-quenched, reverse changes in the resonant harmonics frequencies and the damping decrement were observed. Changes in the observed indicative spectral characteristics correspond to changes in hardness and microstructure of the steel. This technique can be used for development of nondestructive testing express methods to evaluate changes in the state of steel structural elements and perform quality control during heat treatment.