Crystallography of carbide-free bainite in a hard bainitic steel

摘要

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 (111)_A0.70 ± 0.45° from(101)_B, [101]_A2.5 ± 1.5° from [111]_B However, the habit plane of upper bainite is different from that of lower bainite. The former has habit plane that is either within 5° of (221)_A or of (259)_A. The latter only corresponds with a habit plane that is within 5° of (259)_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.