Formation of Stable Dislocation Network in Austenite and Its Effect on Nucleation of BWING in Bainitic Steels

Whether dislocation network could be introduced into austenite (γ) even during austenitization or not was analyzed by optical microscopy. Austenitization at 1450°C for 300s dissolves MnS completely into γ. It is clarified that isothermal holding at 900°C for 1200s after austenitization forms MnS along small-angle boundary within γ grain. It suggests that dislocation network forming the small-angle boundary is introduced and stable even in the high temperature region of γ. Serial sectioning observations of bainite laths with optical microscope reveals that BWING (bainite lath within γ grain) could nucleate from the dislocation network. Dislocation network forms by reaction of dislocations on each slip system. Each dislocation in a network would have different Burgers vector. Therefore, climbing rate of each dislocation in a network due to absorbing or generating vacancies might be different, resulting in bowing-out of dislocation belonging to a network and in increase in self-energy of a dislocation network. It could be concluded that dislocation network is stable even in γ and the network acts as nucleation site for BWING. Based on the conclusion, nucleation of BWING is enhanced through introduction of dense dislocation networks into γ.