1992 Volume 78 Issue 3 Pages 477-484
The spheroidization treatment for medium carbon steels has been usually performed by annealing just above Al temperature and slow cooling.
In this study, the spheroidization behavior of cementites in the steel was in-situ observed by ultra-high voltage electron microscopy, in order to investigate the effect of initial microstructure on the spheroidization behavior. Two microstructures, i. e. fine ferrite-pearlite and bainite, were selected as the initial microstructure for the treatment, and were prepared by thermo-mechanical treatments.
By continuous observation, following results were obtained. Re-austenitization occured at colony and grain boundaries in ferrite-pearlite structure. In the case of bainite, it occured in the vicinity of small cementite particle which distributed homogeneously in ferrite grain. Furthermore, substructures were formed by re-arrangement of dislocations.
During cooling, coalescence of austenite grains occured. Based upon the observation of this behavior, discussion was focused on the role of internal stress caused by eigen strain due to transformation.