Transition from upper to lower bainite in Fe–C–Cr steel

Abstract

An analytical evaluation of transition temperature from upper to lower bainite in Fe – 0·38C – 0·93Cr (wt-%) steel was carried out. Calculations were based on the model constructed by Takahashi and Bhadeshia, which involves a comparison between the time tθ needed to precipitate cementite within the bainitic ferrite plates with the time tθ required to decarburise supersaturated ferrite plates. It was found that the distribution of lath widths, shown by histograms, of the bainitic ferrite varies with isothermal transformation temperatures and holding times. 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 td and tθ it was found that a transition temperature from upper to lower bainite reaction LS of about 350°C could be predicted if the thickness of bainitic ferrite laths is set as wo = 0·1 μm and the volume fraction of cementite set as ξ = 0·01. Calculated differences in the relative behaviour of td and tθ revealed the occurrence of upper and lower bainite in steel Fe – 0·38C – 0·93Cr consistent with the results of transmission electron microscopy investigation.