The effects of austenite grain size on banite transformation kinetics in AISI 4340 steel

In order to study the kinetics of bainite continuous transformation, AISI 4340 steel was austenitized in a temperature range of 950～1150℃ to obtain different austenite grain sizes(AGS) and then step-cooled to room temperature.The results show that AGS has a great effect on bainite transformation kinetics.The larger the AGS,the higher the bainite start temperature(B_s).Transmission electron microscope(TEM) observation shows that the existence of particles such as AlN,TiC etc.on prior austenite grain boundaries promotes the start bainite transformation.The effect of AGS on bainite finish temperature(B_f) is not obvious.The other aspect of kinetics is bainite transformation rate,the higher the AGS,the lower the rate.The transformation rates of all specimens change obviously with increasing bainite volume fraction,and three stages can be found: the rapidly increasing stage,the slowly decreasing stage and the rapidly decreasing stage.