Evolution of Microstructure and Mechanical Properties in Steels during Isothermal Holding in the Region of Bainitic Transformation Temperature in Dependence on Silicon Content

Abstract

Under isothermal treatment conditions, bainite transformation involves decomposition of austenite into a nonequilibrium structure consisting of needles of super-saturated bainitic ferrite and carbide precipitates. Similarly to martensitic transformation, bainitic ferrite forms by shear mechanism. Owing to relatively low temperature, only interstitial elements, predominantly carbon, can migrate by diffusion. Depending on the transformation temperature, carbon migrates from ferrite and forms carbides, either within bainitic ferrite needles and at the interphase interface between bainitic ferrite and austenite, or only within bainitic ferrite needles. In conventional steels, bainite transformation continues until the decomposition of austenite phase is almost complete. If the steel contains enough silicon, carbide precipitation may be suppressed or even prevented altogether. In such case, carbon which diffuses from the needles of bainitic ferrite may enrich adjacent austenite areas. Depending on heat treatment conditions, the carbon-enriched austenite may become sufficiently stable to resist decomposition and remain in the microstructure.