Discriminating pores from inclusions in rolled steel by ultrasonic echo analysis

The quality of steel is degraded by inclusions and pores which form during production. When exposed to external stress, large inclusions initiate cracks that weaken the mechanical strength of steel components. In this study, an ultrasonic immersion pulse-echo setup, which is currently used for quality control, was employed for signal-based inclusion and pore discrimination. The results from the used wavelet and short-time Fourier transform methods were verified with optical microscopy and scanning electron microscopy. The ultrasonic method was tested on 22 rolled bearing steel 100Cr6 samples featuring a total of 24 inclusions and 16 pores. Based on the differences in the echo peak frequency, it was possible to discriminate all pores from inclusions with the wavelet transform method. Sulfide and oxide inclusions also featured differences within the 9-14 MHz frequency range. The developed ultrasonic method was found to be capable of discriminating inclusions from pores in rolled bearing steel 100Cr6.