Bainite reaction in Fe-Ni-C alloys

Abstract

In order to clarify the mechanism of bainite reaction, a metallographic study of isothermal-transformation products formed at various temperatures was made in Fe-9% Ni-0.08% C, -0.23% C, and -0.28% C alloys (mass%) where the solute drag-like effect does not occur. TTT diagrams were also examined. In all the alloys, the TTT diagram consists of two C-curves. The kinetic-Bs temperature is observed at 760 K in both of the 0.23 and 0.28% C alloys. In the 0.08% C alloy, the kinetic-Bs is not seen, but a 'bay' appears around 750 K. The microstructure changes markedly below the kinetic-Bs in the 0.23 and 0.28% C alloys. Proeutectoid ferrite and degenerate pearlite form at temperatures above the kinetic-Bs, whereas upper bainite characterized by ferrite laths forming in parallel becomes dominant below the kinetic-Bs. The upper critical temperature for the formation of bainite (i.e., the microstructural-Bs) corresponds to the kinetic-Bs and is hardly affected by carbon content. TEM microstructure of bainitic ferrite is similar to that of the lath martensite. These results strongly suggest that the bainite reaction has its own C-curve and occurs by a displacive mechanism. A model for the bainite reaction as a displacive transformation is proposed; the bainite reaction is the martensitic transformation of which nucleation is assisted by the formation of carbon poor zones in the parent austenite as a result of statistical fluctuation of carbon concentration. This model deduces a prediction that the Bs temperature of Fe-M-C alloys (M: a substitutional element) is independent of carbon content and corresponds to the Ms temperature of the Fe-M binary alloy. It is demonstrated that the Bs temperature of 760 K in the Fe-9% Ni-C alloys is in very good agreement with the Ms temperature of Fe-9% Ni alloys. The incomplete transformation and the Bf temperature of bainite reaction are also discussed.