Application of acoustic emission to monitor bainitic and martensitic transformation

The paper concerns acoustic emission (AE) measurements during isothermal martensitic and bainitic heat treatments in 100CrMnSi6-4 bearing steel. A distribution of the total RMS voltage in arbitrary unit (AU) during austempering together with the M-S temperature results was determined. M-S represents the temperature at which martensite starts being formed. This temperature was determined with the use of the dilatometric method. The Short Time Fourier Transform algorithm was used for displaying the signal values on spectrogram graphs. AE signals were attributed to a midrib. Midrib is a thin-plate isothermal martensite formed before lower bainite nucleation. The evolution of midribs in remaining austenite was investigated with AE measurements at the temperature near M-S. Macroscopic effects of elongation changes and their division into various intensity stages related to a martensite transformation were analyzed. A comparison of the AE curves and dilatometric results suggests that the AE method determines a relatively initial stage of the martensite transformation, during which additional processes of austempering and carbon partitioning occur. Lower bainite containing the midrib was found to evolve in two stages.