New Concepts of Bainitic and Martensitic Transformations in Steels Based on Multistep γ-α Transformation

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The features of the γ–α transformation under isothermal conditions and during continuous cooling for a wide range of rates (0.1–500000 K/s) were studied on Fe–C alloys, on chromium steels containing up to 0.4%C and 4.3; 6.6 and 9.4% Cr and on some low–carbon alloyed steels. It is established that there are discrete, staged kinetics of the γ–α transformation in Fe–Cr alloys and alloyed steels. The transition from a diffusionless transformation to a bainite transformationin low–carbon chromium steels is examined.All the results on the kinetics of the γ–α transformation are obtained for iron alloys which show that, there are four stages of the transformation in steels and suggest a connection between the formation of upper and lower bainite and the transformation at stages II and III observed in pure Fe, respectively.A theory is developed which explains the two–stage dependence of the starting point of the martensitic transformation in structural steels on the cooling rate by the change in the interaction of the moving γ/α boundary with carbon atoms. The upper temperature stage of the transformation is realized in conditions where growth is blocked by carbon atoms, the lower temperature stage–when the interphase boundary separates from the impurity atmosphere.
社団法人日本鉄鋼協会の論文
1995-08-15