Paper Title:

Application of Advanced Experimental Techniques for the Microstructural Characterization of Nanobainitic Steels

Periodical Solid State Phenomena (Volumes 172 - 174)
Main Theme Solid-Solid Phase Transformations in Inorganic Materials
Edited by Yves Bréchet, Emmanuel Clouet, Alexis Deschamps, Alphonse Finel and Frédéric Soisson
Pages 1249-1254
DOI 10.4028/
Citation Ilana Timokhina et al., 2011, Solid State Phenomena, 172-174, 1249
Online since June 2011
Authors Ilana Timokhina, Hossein Beladi, Xiang Yuan Xiong, Yoshitaka Adachi, Peter D. Hodgson
Keywords APT, Bainitic Ferrite, High Strength Steel (HSS), Nano-Structured Bainitic Steel, Retained Austenite, TEM, TRIP Effect
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A 0.79C-1.5Si-1.98Mn-0.98Cr-0.24Mo-1.06Al-1.58Co (wt%) steel was isothermally heat treated at 350°C bainitic transformation temperature for 1 day to form fully bainitic structure with nano-layers of bainitic ferrite and retained austenite, while a 0.26C-1.96Si-2Mn-0.31Mo (wt%) steel was subjected to a successive isothermal heat treatment at 700°C for 300 min followed by 350°C for 120 min to form a hybrid microstructure consisting of ductile ferrite and fine scale bainite. The dislocation density and morphology of bainitic ferrite, and retained austenite characteristics such as size, and volume fraction were studied using Transmission Electron Microscopy. It was found that bainitic ferrite has high dislocation density for both steels. The retained austenite characteristics and bainite morphology were affected by composition of steels. Atom Probe Tomography (APT) has the high spatial resolution required for accurate determination of the carbon content of the bainitic ferrite and retained austenite, the solute distribution between these phases and calculation of the local composition of fine clusters and particles that allows to provide detailed insight into the bainite transformation of the steels. The carbon content of bainitic ferrite in both steels was found to be higher compared to the para-equilibrium level of carbon in ferrite. APT also revealed the presence of fine C-rich clusters and Fe-C carbides in bainitic ferrite of both steels.