Crystallography of carbide-free bainite in a hard bainitic steel

The convergent beam Kikuchi line diffraction technique has been used to accurately determine the orientation relationships between bainitic ferrite and retained austenite in a hard bainitic steel. A reproducible orientation relationship has been uniquely observed for both the upper and lower bainite. It is(1 1 1)A0.70±0.45° from (1 0 1)B,  [1¯ 0 1]A2.5±1.5° from [1¯ 1 1]BHowever, the habit plane of upper bainite is different from that of lower bainite. The former has habit plane that is either within 5° of (2 2 1)A or of (2 5 9)A. The latter only corresponds with a habit plane that is within 5° of (2 5 9)A. The determined orientation relationship is completely consistent with reported results determined using the same technique with an accuracy of ±0.5° in lath martensite in an Fe–20 wt.% Ni–6 wt.% Mn alloy and in a low carbon low alloy steel. It also agrees well with the orientation relationship between granular bainite and austenite in an Fe–19 wt.% Ni–3.5 wt.% Mn–0.15 wt.% C steel. Hence it is believed that, at least from a crystallographic point view, the bainite transformation has the characteristics of martensitic transformation.