Rationalisation of Austenite Transformation to Upper or Lower Bainite in Steels

The paper presents an analytical evaluation of transition temperature from upper to lower bainite in Fe-C-Cr steel. The calculations was based on the model constructed by Matas and Hehemann which involves a comparison between the times needed to precipitate cementite within the bainitic ferrite plates (t_θ), with the time required to decarburise supersaturated ferrite plates (t_d). The transition between upper and lower bainite is found to occur over a narrow range of temperatures (350-410°C) and depends on the thickness of bainitic ferrite laths and the volume fraction of precipitated cementite. On comparing the td and tθ times it was found that the transition temperature from upper to lower bainite reaction (LS) of about 350oC could be predicted if the thickness of bainitic ferrite laths is set as wo = 0.1 μm and volume fraction of cementite is set as ξ = 0.01