Effect of the Austenitizing Condition on the Upper Bainite and Isothermal Pearlite Transformations in an Eutectoid Carbon Steel

Abstract

As has previously been reported on the lower bainite transformation, the effects of austenitizing temperature and time (longer than 60 min) on the upper bainite transformation (at 380°C) and the isothermal pearlite transformation (at 675°C) are investigated for a commercial eutectoid carbon steel heat treated at 1200°C for 20 min like those on the lower bainite transformation which were reported earlier. The principal findings are as follows.(1) The amount of bainite formed during a certain transformation period increases to a maximum with increasing time and then starts to decrease. On the other hand, the amount of pearlite formed decreases to a minimum as the time increases, after which it increases to a maximum and again decreases.(2) The higher the austenitizing temperature the shorter is the time to reach the maximum, and the lower the temperature the higher is the maximum in both bainite and pearite transformations. Besides, the higher the temperature the shorter is the time to reach the minimum pearlite formation, and the lower the temperature the higher is the minimum.(3) Bainite or pearlite starts to nucleate at grain boundaries in the case of a short austenitizing time. However, this tendency of nucleation becomes a little weaker as the time increases and the nucleation also starts inside the grains. Furthermore, in the case of a long austenitizing time, bainite plates and pearlite nodules at grain boundaries or inside the grains are fine compared to those at grain boundaries in the case of a short austenitizing time.(4) Nucleation of bainite or pearlite inside the grains is caused by the nuclear action by AlN. The nuclear action by AlN seems to be stronger in the bainite tranformation than that in the isothermal pearlite transformation.(5) The value of activation energy obtained by assuming that a process to show the maximum amount of transformation is associated with that for thermal activation can be taken as 33.8 kcal/mol in all the upper bainite, lower bainite and pearlite transformations.