New Concepts of the Nature of Bainitic Transformations in Steels

Special features of theγ → transformation occurring in a wide range of cooling rates (0.1 – 500,000 K/sec) in pure iron and in carbon and chromium steels are considered. It is shown that four stages of the γ → transformation are possible in the steels, the first of which has a diffusion or massive nature, and the other three can be characterized as displacement transformations. The genetic tie between the upper and lower bainite and transformations occurring through stages II and III, respectively, in pure iron is shown. The difference between the upper and lower bainite is determined by the difference in the mechanisms of motion of the phase boundary, which can be accompanied by redistribution of carbon atoms. Stage IV corresponds to athermal martensitic transformation. __________ Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 7, pp. 24 – 29, July, 2005.