| The microstructural relationship among carbide, ferrite and austenite in a medium carbon bainitic steel containing Si and Mn was studied. Si was added to inhibit the formation of carbide, Mn was added to stabilize the untransformed austenite at room temperature. The steel was annealed both in the fully austenite region and in the ferrite + austenite region. The 1.5%Si-0.5%Mn-0.45 %C steels examined in the present investigation was found to exhibit upper and lower bainites transformed through different transformation mechanism respectively. In the case of the upper bainite, the bainite reaction was thought to be shear transformation in the stage of nucleation, but to be diffusional one in the stage of growth of bainitic ferrite. But in the case of the lower bainite, transformation was thought to be basically shear transformation in all stages of reactions. The variation of the bainite transformation with annealing temperature was found to be that the reaction was slower in the case of intercritical annealing, but no change of transformation mechanism was investigated in both upper and lower bainites in comparison with full austenitizing. |
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