Yoshinori Fujimura1 and Katsumi Muramatsu2
1Faculty of Engineering, Shizuoka University, Hamamatsu
The effect of austenitizing conditions on the lower bainite transformation (at 250°C) in a commercial eutectoid steel is reported as a part of the studies investigating the effect of austenitizing conditions on the transformation behavior in steels. The effect of austenitizing time after heat treated at 1200°C for 20 minutes on the bainite transformation was investigated mainly for the case keeping the time more than 60 minutes at an constant temperature. The principal findings are as follows.
(1) The amount of bainite formed during a certain transformation period increases as the time increases, and after reaching a maximum, it decreases. This tendency is irrespective of the austenitizing temperature.
(2) The higher the austenitizing temperature the shorter becomes the time to reach the maximum bainite formation. On the other hand the lower is the temperature the larger is the maximum amount.
(3) Bainite starts to nucleate at the grain boundaries and grows toward to inside of the grains in the case of shorter austenitizing time. As the austenitizing time becomes longer this tendency of nucleation at the grain boundaries decreases and bainite also nucleates inside the grains. In this case bainite plates at the grain boundaries as well as inside the grains are fine structures compared to those formed in a shorter austenitizing time.
(4) The value of activation energy obtained by assuming that a process to show the maximum amount of transformation is an thermally activated one is 34.7 kcal/mol.
(5) Nucleation of bainite inside the grains is caused by the nuclear action by AlN.
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