Effects of the Austenite Grain Size and Deformation in the Unrecrystallized Austenite Region on Bainite Transformation Behavior and Microstructure

The effects of austenite grain size and deformation in the unrecrystallized austenite region on bainite transformation behavior and microstructural characteristics were investigated under continuous cooling condition and compared with those in ferrite-pearlite transformation and martensite transformation. Austenite grain size does not affect the transformation temperature and resulting microstructure of bainitic ferrite except that nucleation site of bainitic ferrite is increased with the decrease in equiaxed austenite grain size. But the volume fraction of the transformed products depends on the temperature during continous cooling and does not change with the prior austenite grain size. These characteristics are quite different from those in ferrite-pearlite transformation. The deformation in the unrecrystallized austenite region raises the transformation temperature and decreases the hardness of bainitic ferrite. Kurdjumov-Sachs relationship holds between austenite and bainitic ferrite irrespective of prior austenite microstructure. No contribution of austenite grain boundary for the growth of bainitic ferrite seems to indicate the intensive involvement of the displacive mechanism in this transformation.