Acoustic phenomena near MS in hypereutectoid steels

Phenomena preceding bainitic transformation in the range of swing back in kinetics were investigated using acoustic methods and microscope analyses. The tests were performed using specimens made of a high carbon steel that indicates a swing back in kinetics at temperatures near MS. The acoustic effect tests were carried out during austempering at temperatures between 100 °C and 160 °C. It was found that a distinct peak in the root mean square data appeared near MS due to the midrib formation. The acoustic emission is discussed, focusing on martensitic midrib transformations. In addition to a lower bainite with midrib (B1), conventional lower bainite (B2) is formed, near the MS range of temperatures. Differences in morphology between conventional and with the midrib lower bainite were found and discussed. In order to calculate the volume fractions of two different structural components of steel, the formulas were established showing a relationship between the growth of real volumes of the components and their extended volumes.