Morphology Map of Bainites in 2% Si Steels

Abstract
A metallographic study of isothermal transformation products formed at the early stage of transformation was made in silicon alloyed high-carbon steels (C wt%=0.6, 0.8, 1.1 and 1.4) in the temperature range between 700 and 200 °C. Observations are summarized as a "morphology map" showing the formation temperature-carbon content fields of isothermal transformation products. In all steels investigated, the proeutectoid ferrite, upper bainite and lower bainite are formed at temperatures below the extrapolated A 3 line and above 600 °C, between 550 and 400 °C, and below 350 °C and above the Ms temperature, respectively. The microstructural-Bs temperature and the transition temperature from upper bainite to lower bainite are independent of the bulk carbon content, and around 550 and 350 °C, respectively. It is shown that above results can be explained by a diffusional model for the proeutectoid ferrite formation and a displacive model for the bainite formation.