Effects of transformation temperature on variant pairing of bainitic ferrite in low carbon steel

The effects of transformation temperature on the variant pairing tendency of bainitic ferrite (BF) and the orientation relationship (OR) between BF and austenite matrix are investigated quantitatively in low-carbon, low-alloy steel by means of electron backscatter diffraction analysis. By numerical reconstruction of the austenite orientation it was found that BF holds a near Kurdjumov-Sachs OR with respect to the austenite matrix. The angular deviation of the close-packed planes (CPPs) becomes smaller with decreasing transformation temperature, while the small misorientation angle between the close-packed directions (CPDs) remains nearly the same. BF variants with small misorientations are formed side by side at high transformation temperatures by sharing the same Bain correspondence. In contrast, BF variants are formed adjacently at lower transformation temperatures by sharing the same parallel relations for CPP. In particular, pairing of twin-related variants is dominant in this case. Such a variant pairing tendency with respect to transformation temperature is discussed from the viewpoint of self-accommodation of transformation strain and inter-variant boundary energy.